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Object-Oriented Programing in Java

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1 Object-Oriented Programing in Java
Cheng-Chia Chen

2 The contents of an object/class Creating and initializing Objects
Object and Class The contents of an object/class Creating and initializing Objects Accessing object data and methods Destroying and finalizing objects Subclass and inheritance Interfaces Java modifiers summary

3 The properties of an object constitutes its current state.
What is an Object ? Real-world objects: Concrete objects: Apple1, Car1, TV2, Teacher2, Student3, … Conceptual Objects: 1, 2.3, Date1, Meeting2, point2, … Objects have: Properties (attributes): color, weight, height, sex, name, speed, position,… Capabilities (behaviors): can receive commands(, request, query) and respond (do actions) based on its internal states to change its internal state and/or external environment. The properties of an object constitutes its current state.

4 What is a Software object ?
a software bundle of data and functions used to model real-world objects you find in everyday life. In OOP, Software objects are building block of software systems program is a collection of interacting objects objects cooperate to complete a task to do this, they communicate by sending “messages” to each other Software objects can model tangible things: School, Car, Bicycle, conceptual things: meeting, date Processes: finding paths, sorting cards Note: Software objects are only abstraction of real-world objects; irrelevant properties and behavior will not be modeled in software objects.

5 In Java, an object consists of 0 or more fields and 0 or more methods.
What is a Java Object? In Java, an object consists of 0 or more fields and 0 or more methods. Fields are used to model properties. Methods are used to model capabilities. Fields are variables. like the fields of a C struct. An object without methods is equivalent to a C struct. A method is similar to a C function. Normally, it will operate on the fields of the object. These fields are accessible by name in the method. Java variables can not hold objects, only references to them. Object do not have a names. Object are created only at runtime. Given a reference r to an object, the syntax for accessing a field is: r.field_name, the syntax for accessing a method is: r.method()

6 Disadvantage: impractical to work with objects this way
Classes and Objects Current conception: a java/software object  a real-life object, e.g., a Java car  a real car Disadvantage: impractical to work with objects this way may be indefinitely many (i.e., modeling all atoms in the universe) do not want to describe each individual separately, because they may have much in common Classifying objects into classes of similar properties/behaviors factors out commonality among sets of similar objects lets us describe what is common once then “stamp out” any number of copies later Ex: Student: { S1, S2, S3 } Course:{C1,C2,C3} Teacher:{ T1,T2} but not {s1, t1}, {s2, t2}, {c1,c2,c3,s3} Analog: stamp 印章 (class) Imprints 戳印 (objects)

7 What is a Java Class? In Java, a class is a template (textual description) of a set of similar objects. All objects in the class have the same types of properties and the same set of capabilities. It defines the fields and methods that all objects in that class will have. Classes have names. Class appear in the text of your program. A java program consists of a set of classes. A defined class is a Java Type, so you can have objects or variables of that type.

8 Class diagrams

9 An Example: the class of Circles:
Properties: a circle can be described by the x, y position of its center and by its radius. Methods: Some useful operations on Circles: compute circumference, compute area, check whether points are inside the circle, etc.

10 By defining the Circle class (as below), we create a new data type.
// The class of circles (partially defined) class Circle { // Fields double x, y; double r; // Methods double circumference() { return 2 * * r; } double area() { return * r * r ; } void scale(double multiplier) { r *= multiplier; } void print() { System.out.println("circle of radius "+ r + " with center at (" + x + "," + y + ")“ ); } }

11 In Java, objects are created by the new operator. For example:
Creating Objects In Java, objects are created by the new operator. For example: //define a variable to refer to Circle objects;no objects yet. Circle c ; // c is null now // create a circle object and make the variable refer to it c = new Circle(); // define variable and create Circle object all at once Circle d = new Circle(); Note: the fields in these objects are given default values at creation (0 for numbers; null for object references)

12 Accessing Object Data We can access data fields of an object (subject to visibility restrictions -- see later). For example: // create a new Circle Circle c = new Circle(); //initialize our circle to have center (2, 5) and radius 1.0. c.x = 2.0; c.y = 5.0; c.r = 1.0; // create another circle Circle d = new Circle(); //initialize this circle to have center (10,7)and double the radius of circle c. d.x = 10.0; d.y = 7.0; d.r = c.r * 2.0;

13 a = c.area(); // Not a = area(c); Notes
Using Object Methods To access the methods of an object, use same syntax as accessing the data of an object: Circle c; double a; ... a = c.area(); // Not a = area(c); Notes Each method has a signature, which is defined by: The method name and The sequence of all types of arguments Each class can define several methods with same name and different types of arguments (overloading).

14 Circle c = new Circle();
Constructors Every class in Java has at least one constructor method, which has the same name as the class. The purpose of a constructor is to perform any necessary initialization for new objects. Java provides a default constructor that takes no arguments and performs no special initialization (i.e. gives objects default values). Note: Java compiler provide the default constructor: className() only if you do not provide any constructor at your class definition. For example: Circle c = new Circle();

15 Defining a Constructor
Can define additional constructors for initialization. // The circle class, with a constructor public class Circle { public double x, y, r; // Constructor method public Circle(double x, double y, double r) { this.x = x; this.y = y; this.r = r; } // Other methods ... as above ...

16 Defining a Constructor (cont.)
With the new constructor, we can create and initialize a Circle object as: Circle c = new Circle(2.0, 5.0, 1.0); A constructor is like a (static) method whose name is the same as the class name. The return value is an instance of the class. No return type is specified in constructor declarations, nor is the void keyword used.

17 Multiple Constructors
A class can have any number of constructor methods. public class Circle { public double x, y, r; // Constructors public Circle(double x, double y, double r) { this.x = x; this.y = y; this.r = r; } public Circle(double r) { x = 0.0; y = 0.0; this.r = r; } public Circle(Circle c) { this.x = c.x; this.y = c.y; this.r = c.r; } public Circle() { x = 0.0; y = 0.0; r = 1.0; } // Other methods ... as above }

18 Multiple Constructors (cont.)
With the new constructors, we can initialize circle objects as follows: Circle c1 = new Circle(2.0, 5.0, 1.0); // c1 contains (2.0, 5.0, 1.0) Circle c2 = new Circle(3.5); // c2 contains (0.0, 0.0, 3.5) Circle c3 = new Circle(c2); // c3 contains (0.0, 0.0, 3.5) Circle c4 = new Circle(); // c4 contains (0.0, 0.0, 1.0) All uninitialized data receives default values.

19 Invoking one constructor from another
We can use this(…) in a constructor to invoke other constructor. public class Circle { public double x, y, r; // Constructors public Circle(double x, double y, double r) { this.x = x; this.y = y; this.r = r; } public Circle(double r) //{x =0.0; y = 0.0; this.r = r;} replaceable by {this(0.0,0.0,r); } public Circle() //{x = 0.0; y = 0.0; r = 1.0;} replaceable by {this(1.0); } } Note: do not result in recursion.

20 Object and Object References
A java object is a memory structure containing both data and methods An object reference holds the memory address of an object Rather than dealing with arbitrary addresses, we often depict a reference graphically as a “pointer” to an object ChessPiece bishop1 = new ChessPiece(); bishop1

21 int num1 = 5, unm2 = 12; num2 = num1;
Assignment Revisited The act of assignment takes a copy of a value and stores it in a variable For primitive types: int num1 = 5, unm2 = 12; num2 = num1; Before num1 5 num2 12 After num1 5 num2

22 For object references, assignment copies the memory location:
Reference Assignment For object references, assignment copies the memory location: bishop2 = bishop1; Before bishop1 bishop2 After bishop1 bishop2

23 One object (and its data) can be accessed using different variables
Aliases Two or more references that refer to the same object are called aliases of each other One object (and its data) can be accessed using different variables Aliases can be useful, but should be managed carefully Changing the object’s state (its variables) through one reference changes it for all of its aliases Ex: Circle c = new Circle(1.0, 2.0, 3.0); Circle d = c; // c and d are aliases. c.r = 4.0; // d.r == 4.0 now.

24 How can references to objects '' go away'‘ ?
Destroying Objects When an object no longer has any valid references to it, it can no longer be accessed by the program It is useless, and therefore called garbage Java performs automatic garbage collection periodically, returning an garbage object's memory to the system for future use Hence it is needless to explicitly destroy objects. How can references to objects '' go away'‘ ? Re-assigning object variables (a = b; a = null) or object variables (locals or parameters) going out of scope. no more malloc/free bugs

25 Garbage collection only help freeing up memory.
Object Finalization A constructor method performs initialization for an object; a Java finalizer method performs finalization for an object. Garbage collection only help freeing up memory. But there are other resources needed to be released. file descriptors, sockets, lock, database connection.

26 Example: A Finalizer Method from the Java FileOutputStream class.
/** * Closes the stream when garbage is collected. * Checks the file descriptor first to make sure it is not already closed. */ protected void finalize() throws IOException { if (fd != null) close(); }

27 Notes about finalize()
invoked before the system garbage collects the object. no guarantees about when a finalizer will be invoked, or in what order finalizers will be invoked, or what thread will execute finalizers. After a finalizer is invoked, objects are not freed right away. because a finalizer method may "resurrect" an object by storing the this pointer somewhere. may throw an exception; If an uncaught exception actually occurs in a finalizer method, the exception is ignored by the system. No ‘class Finalization’ method defined.

28 exist at compile time exists at runtime only
Classes v.s. Objects two of the most frequently occurring terms in the OO programmer's vocabulary. A class An object... exist at compile time exists at runtime only a template/pattern created/instantiated from for objects a class' specification "only exists once" can be created many times from one class a .java file returned by the new operator dress pattern dress architectural plans house stamp imprints

29 Types of variables and methods in a Java class
There are two main types of variables/fields: instance variables class (or static) variables Instance variables store information pertaining to one particular object's state Class variables store information pertaining to all objects of one class Likewise, there are two types of methods: Instance methods Class (static) methods Instance methods belong to individual objects; whereas class methods belongs to the whole class. Note: In class method, you cannot use this and instance variables.(why?)

30 Class Name Instance Methods Class Methods
Class diagram of an Account class. Class Name Instance Methods ShowNumberOfAccount Class Methods

31 Declare class field/method with the ‘static’ Modifier
makes methods and variables belong to the class rather than instances of the class. Example: counting how many circles: public class Circle { public double x, y, r; // instance variables // ncircles is class variable public static int ncircles = 0; // Constructors public Circle(double x, double y, double r) { this.x = x; this.y = y; this.r = r; ncircles++; } public Circle(double r) { x = 0.0; y = 0.0; this.r = r; ...}

32 The static Modifier (cont.)
In the above example, there's only one instance of the ncircles variable per Circle class but one instance of x, y and r per Circle object. Diff. ways to reference ncircles: Circle.ncircles // ClassName.classVarName ncircles; this.ncircles, // used inside the Circle class definition only c.ncircles // where c is a Circle variable Similar approach for static methods. Examples of static methods (or called class method): Math.cos(x) Math.pow(x,y) Math.sqrt(x)

33 Must be declared with the static keyword
Notes on class methods Must be declared with the static keyword Also called static method Can only operate on class variables (e.g., static) Cannot use ‘this’ Cannot use instance variables To access a class method: same as to access class vars: Circle.countCircles() // ClassName.classVarName countCircles(); this.countCircles(), // legal only when inside the Circle class definition c.countCircles() // where c is a Circle variable Lots of examples of class methods in the JDK (e.g., String)

34 Example: instance member v.s. class member
Class B { int x; static int y; static int b1() { … } int b2 () { … } static int b3() { … // class method int c; c = x; c = this.y //error c = y; c = B.y; //ok! A a = new A(); B b = new B(); c = a.a1(); //ok! c = a.a2() ; // ok! c = A.a2() ; // error! c = A.a1() ; // ok! c = b.b1() ; //ok! c = b.b2() ; // ok! c = B.b2() ; // error! c = B.b1() ; // ok! c = b1(); // ok c = b2(); // error } Int b4() { // instance method c = x; c = this.x //ok! c = y; c = B.y; c = this.y //ok! c = b1(); c = this.b1() // ok c = b2(); c=this.b2() // ok } Class A { public static int a1(){…} public int a2() {…} … }

35 Class and Instance initializers
Both class and instance variables can have initializers attached to their declarations. static int num_circles = 0; float r = 1.0; Class variables are initialized when the class is first loaded. Instance variables are initialized when an object is created. Sometimes more complex initialization is needed. For instance variables, there are constructor methods ,and instance initializer. For class variables static initializers are provided

36 An example static/instance initializer
public class TrigCircle { // Trigonometric circle // Here are our static lookup tables, and their own simple initializers. static private double sin[] = new double[1000]; static private double cos[] = new double[1000]; // Here's a static initializer "method" that fills them in. // Notice the lack of any method declaration! static { double x, delta_x; int i; delta_x = (Circle.PI/2)/(1000-1); for(i = 0, x = 0.0; i < 1000; i++, x += delta_x) { sin[i] = Math.sin(x); cos[i] = Math.cos(x); } … // The rest of the class omitted.

37 An example static/instance initializer (continued)
// instance field and methods Private int[] data = new int[100]; // data[i] = i for i = 0..99 // instance initializer as an unnamed void method { for(int I = 0; I <100; I++) data[I] = I; } }

38 can have any number of static/instance initializers;
Notes on initializers can have any number of static/instance initializers; can appear anywhere a field or method can appear. Static initializer behaves like class method and cannot use this keyword and any instance fields of the class The body of each instance initializers (alone with field initialization expressions) is executed in the order they appear in the class and is executed at the beginning of every constructor. The body of each static initializers (alone with static field initialization expressions) is executed in the order they appear in the class and is executed while the class is loaded.

39 Inheritance relationships form tree-like hierarchical structures.
Inheritance in OOP Inheritance is a form of software reusability in which new classes are created from the existing classes by absorbing their attributes and behaviors. Instead of defining completely (separate) new class, the programmer can designate that the new class is to inherit attributes and behaviours of the existing class (called superclass). The new class is referred to as subclass. Programmer can add more attributes and behaviors to the subclass, hence, normally subclasses have more features than their superclasses. Inheritance relationships form tree-like hierarchical structures.

40 Subclasses and Inheritance
An important aspect of OO programming: the ability to create new data types based on existing data types Example ... a class of drawable Circles: we'd like to be able to draw the circles we create, as well as setting and examining their properties. for drawing, we need to know the color of the circle's outline and its body In Java, we implement this by defining a new class that extends the behavior of the Circle class. This new class is a subclass of Circle.

41 The class GraphicCircle: public class GraphicCircle extends Circle
Subclass Example The class GraphicCircle: public class GraphicCircle extends Circle { // Extra fields Color outline, fill; // Extra constructors public GraphicCircle(Color edge, Color fill) { x = 0.0; y = 0.0; r = 1.0; outline = edge; this.fill = fill; } public GraphicCircle(double r, Color edge, Color fill) { super(r); outline = edge; this.fill = fill; } // Extra methods public void draw(Graphics g) { g.setColor(outline); g.drawOval(x-r, y-r, 2*r, 2*r); g.setColor(fill); g.fillOval(x-r, y-r, 2*r, 2*r); } }

42 A subclass inherits fields and methods from its parent class.
Subclass Inheritance A subclass inherits fields and methods from its parent class. A subclass method overrides a superclass method if they have the same signature. A subclass field shadows a superclass field if they have the same name. Refer to the superclass field via super.field Note: you can also use super.method(…) to refer to overridden superclass method.

43 Subclasses are just like ordinary classes:
Using Subclasses Subclasses are just like ordinary classes: GraphicCircle gc = new GraphicCircle(); ... double area = gc.area(); We can assign an instance of GraphicCircle to a Circle variable. Example: GraphicCircle gc; Circle c = gc; //widening conversion is // always safe; explicit cast is not needed.

44 Superclasses, Objects, and the Class Hierarchy
Every class except Object has a superclass. If a class has no extends clause, it extends the Object class. Object Class: the only class that does not have a superclass methods defined in Object can be called by any Java object

45 Abstract class allows us to declare methods without implementation.
Abstract Classes Abstract class allows us to declare methods without implementation. the unimplemented method is called an abstract method. Subclasses can extend abstract class and provide implementation of all or portion of abstract methods. The benefit of an abstract class is that methods may be declared such that the programmer knows the interface definition of an object, however, methods can be implemented differently in different subclasses of the abstract class.

46 Abstract Classes (cont.)
Any class containing an abstract method is automatically abstract itself But an abstract class need not have abstract methods in it! an abstract class can not be instantiated a subclass of an abstract class can be instantiated if it overrides each of the abstract methods of its superclass and provides an implementation for all of them if a subclass of an abstract class does not implement all of the abstract methods it inherits, that subclass is itself abstract

47 Abstract Classes (cont.): an example
public abstract class Shape { public abstract double area(); // abstract methods // to be implemented by subclasses public abstract double circumference(); } public class Circle extends Shape { protected double r; protected static final double PI= ; public double Circle() { r = 1.0; } public double Circle(double r) { this.r = r; } // implementation of two abstract methods of shape class public double area(){ return PI*r*r; } public double circumference() { return 2*PI*r; } public double getRadius() { return r; }

48 Abstract Classes : an example (cont.)
public class Rectangle extends Shape { protected double w,h; // two constructors public Rectangle() { w=1.0; h=1.0; } public Rectangle(double w, double h) { this.w = w; this.h = h; } // implementation of two parent methods public double area() { return w*h; } public double circumference() { return 2*(w + h); } // methods for this class public double getWidth() { return w; } public double getHeight() { return h; } }

49 Inheritance revisited
the concept of inheritance the protected modifier adding and modifying methods through inheritance creating class hierarchies 1

50 The derived class is called the child class or subclass.
Inheritance Inheritance allows a software developer to derive a new class from an existing one The existing class is called the parent class, or superclass, or base class The derived class is called the child class or subclass. The child class inherits characteristics (data & methods) of the parent class That is, the child class inherits the methods and fields defined for the parent class 2

51 Inheritance Inheritance relationships are often shown graphically in a class diagram, with the arrow pointing to the parent class Inheritance relationships: base class: Vehicle derived class: Car Car inherits data & methods from Vehicle Vehicle Car Inheritance should create an is-a relationship, meaning the child is a more specific version of the parent

52 class Car extends Vehicle { // class contents } Example: next slide
Deriving Subclasses The reserved word extends is used to establish an inheritance relationship class Car extends Vehicle { // class contents } Example: next slide 4

53

54 class Book { protected int pages = 1500; public void pageMessage () { System.out.println ("Number of pages: " + pages); } // method pageMessage } // class Book class Dictionary extends Book { private int definitions = 52500; public void definitionMessage () { System.out.println ("Number of definitions: "+definitions); System.out.println ("Definitions per page: " definitions/pages); // inherited var } // method definitionMessage } // class Dictionary

55 class Main { // Test Driver public static void main (String[] args) {
Inheritance Example class Main { // Test Driver public static void main (String[] args) { Dictionary webster = new Dictionary (); webster.pageMessage(); // inherited method webster.definitionMessage(); } } // class Words

56 Controlling Inheritance by the protected Modifier
The protected (and public) visibility modifier allows a member of a parent class to be inherited into the child But protected visibility provides more encapsulation than public does However, protected visibility is not as tightly encapsulated as private visibility Note: Unlike C++, Inheritance does not change the visibility of the parent class members when used through instances of child classes. 5

57 Visibility modifiers and their usage
The visibility modifiers determine which class members can be referenced from where and which cannot. public members: all classes protected members all classes in the same package + all subclasses (and subsubclasses …) Note: Java has no notions of public, private or protected inheritance as in C++; all inheritances are public. package members [default visibility] all classes in the same package private members can only be used in the same class where the member is defined.

58 Visible regions for members of class A
visible region for pubic members [of class A] visible region for protected members visible region for package members package a.b.c; public class A { public int p1… protected int p2… int p4; //package scope private int p3 … } package a.b.c package a.b.c … extend A visible region for private members of class A … extend A … extend A … extend A

59 Protected members are accessible to subclass (and package) code
public class B extends A{ // p1 and p2 of A can // be used } public class A { public int p1; protected int p2; int p3 private int p4; … } class C extend B { int p; B b = new B(); p = b.p1; // ok since p1 is public p = b.p2 ; // ok p = p2; // ok ! protected p2 is inherited A a = new A(); p = a.p2 // error!! // protected field (p2) can be accessed [from subclasses] only through subclass instances } class D { B b = new B(); int i = b.p2; //error p2 is not visible within D class

60 The super Reference Constructors are not inherited, even though they are declared to have public visibility Yet we often want to use the parent's constructor to set up the "parent's part" of the object The super reference can be used to refer to the parent class, and is often used to invoke the parent's constructor 6

61 class Book { protected int pages; public Book (int pages) { this.pages = pages; } public void pageMessage () { System.out.println ("Number of pages: " + pages); } } // class Book class Dictionary extends Book { private int definitions; public Dictionary (int pages, int definitions) { super (pages); // construct Book part of a Dictionary this.definitions = definitions; } // constructor Dictionary public void definitionMessage () { System.out.println ("Number of definitions: " + definitions); System.out.println ("Definitions per page: " + definitions/pages); } // method definitionMessage } // class Dictionary

62 class Main2 { public static void main (String[] args) { Dictionary webster = new Dictionary (1500, 52500); webster.pageMessage(); webster.definitionMessage(); System.out.println(webster); // try println object } // method main } // class Words2

63 Single vs. Multiple Inheritance
Java supports single inheritance, meaning that a child class can have only one parent class Multiple inheritance allows a class to be derived from two or more classes, inheriting the members of all parents Collisions, such as the same variable name in two parents, have to be resolved In most cases, the use of interfaces gives us the best aspects of multiple inheritance without the overhead

64 Types of Objects and Variables
The type of an object is the class that is referred to when it is created via new construct. Student s1 = new Student(“name”, age, sex); Two types associated with variables ( including method parameters/fields): The declared type of a variable is the type referred to in its declaration (also: declared class; compile-time type) Person p1, p2 = new Person(…) ; The actual type of a variable is the type of the object bound to the variable at a specific moment during program execution (also: run-time type) p1 = s1 ; Note: the declared type of a variable is static(i.e., unchangable ) once it is declared, while the actual type of a variable is dynamic (i.e., can be changed by assignment of new object of different type. p2 = s1;

65 Example: Declared type and actual type

66 Example Actual type

67 Overriding Methods What happens if both parent and child class contains members of the same signature ? fields => variable shadowing; constructor : impossible! methods => methods overriding A child class can override the definition of an inherited method in favor of its own A child can redefine a method it inherits from its parent Overriding method: has the same signature as the parent's method has different code in the body The actual type (not casted type) of an object determines which method is invoked See Messages.java 8

68 class Messages { public static void main (String[] args) { Message m = new Message(); Advice a = new Advice(); m.message(); a.message(); (Message a).message() // same as a.message() } } // class Messages class Message { public void message() { System.out.println (”Message"); } } // class Thought class Advice extends Message { public void message() { // overriding method System.out.println (”Advice"); } } // class Advice

69 Overloading vs. Overriding
Don‘t confuse the concepts of overloading (多載) and overriding(覆蓋) Overloading deals with multiple methods in the same class with the same name but different signatures Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature Overloading lets you define a similar operation in different ways for different data Overriding lets you define a similar operation in different ways for different object types 9

70 The super Reference Revisited
Inherited parent methods/fields can be explicitly invoked using the super reference If a method/field is declared with the final modifier, it cannot be overridden The concept of overriding can be applied to data (called shadowing variables), but shadowing behaves quite differently from overriding. The syntax of super is: super.method (parameters) super.var See Firm.java 10

71 Shadowing superclass fields vs overriding superclass methods
class A { int x ; int m() …} class B extends A { int x; int m() …} class C extends B { int x; // x in B and A are shadowed // by this.x int m() {…} // m() overrides m() in A & B C c = new C(); … x, this.x // field x in C super.x, ((B) this).x // field x in B ((A) this).x // filed x in A super.super.x // syntax error!! c.x // field in C ((B)c).x // fields in B ((A)c).x // fields in A ((A) c).m() ; // m() in A ? no !! super.m(); // m() in B ((B) this).m(); // m() in C ((A) this).m(); // m() in C ((A) c).m(); ((B) c).m(); m(); // m() in C

72 public static void main (String[] args) {
class Firm { public static void main (String[] args) { Manager sam = new Manager ("Sam", "123 Main Line", " ", " ", ); Employee carla = new Employee ("Carla", "456 Off Line", " ", " ", ); Employee woody = new Employee ("Woody", "789 Off Rocker", " ", " ", ); woody.print(); System.out.println ("Paid: " + woody.pay()); System.out.println(); carla.print(); System.out.println ("Paid: " + carla.pay()); sam.print(); sam.awardBonus (2000); System.out.println ("Paid: " + sam.pay()); System.out.println(); } }

73 class Employee { protected String name, address, phone, ID; protected double salary; public Employee (String name, String address, String phone, String ID, double salary) { this.name = name; this.address = address; this.phone = phone; this.payRate = payRate; this.ID = ID; } // constructor Employee public double pay () { return salary; } // method pay public void print () { System.out.println (name + " " + ID); System.out.println (address); System.out.println (phone); } } // class Employee

74 class Manager extends Employee {
private double bonus; public Manager (String name, String address, String phone, String ID, double pay) { super (name, nddress, phone, ID, pay); // call parent’s constructor bonus = 0; // bonus yet to be awarded } public void awardBonus (double bonus) { this.bonus = bonus; } public double pay () { // managers need special way to count pay! double pay = super.pay() + bonus; // call parent’s method bonus = 0; return pay; } }

75 Class Hierarchies A child class of one parent can be the parent of another child, forming class hierarchies Animal Mammal Bird Horse Bat Parrot

76 Two children of the same parent are called siblings
Class Hierarchies Two children of the same parent are called siblings Good class design puts all common features as high in the hierarchy as is reasonable Class hierarchies often have to be extended and modified to keep up with changing needs There is no single class hierarchy that is appropriate for all situations See Accounts2.java 12

77 class Accounts2 { public static void main (String[] args) { SavingsAccount savings = new SavingsAccount (4321, , 0.02); BonusSaverAccount bigSavings = new BonusSaverAccount (6543, , 0.02); CheckingAccount checking = new CheckingAccount (9876, , savings); savings.deposit (148.04); bigSavings.deposit (41.52); savings.withdrawal (725.55); bigSavings.withdrawal (120.38); checking.withdrawal (320.18); } // method main } // class Accounts2

78 class BankAccount { protected int account; protected double balance; public BankAccount (int accountNum, double initialBalance) { account = accountNum; balance = initialBalance; } public void deposit (double amount) { balance += amount; } // method deposit public boolean withdrawal (double amount) { boolean result = false; if (amount > balance) System.out.println ("Insufficient funds."); else { balance -= amount; System.out.println ("New balance: " + balance); result = true; } return result; } } // class BankAccount

79 class SavingsAccount extends BankAccount {
protected double rate; public SavingsAccount (int accountNum, double initialBalance, double interestRate) { super (accountNum, initialBalance); rate = interestRate; } // constructor SavingsAccount public void addInterest () { balance += balance * rate; } // method addInterest } // class SavingsAccount

80 class BonusSaverAccount extends SavingsAccount {
private final int PENALTY = 25; private final double BONUSRATE = 0.03; public BonusSaverAccount (int accountNum, double initialBalance, double interestRate) { super (accountNum, initialBalance, interestRate); } // constructor SuperSaverAccount public boolean withdrawal (double amount) { return super.withdrawal (amount+PENALTY); } // method withdrawal public void addInterest () { balance += balance * (rate + BONUSRATE); } // method addInterest

81 class CheckingAccount extends BankAccount {
private SavingsAccount overdraft; public CheckingAccount (int accountNum, double initialBalance, SavingsAccount protection) { super (accountNum, initialBalance); overdraft = protection; } // constructor CheckingAccount public boolean withdrawal (double amount) { boolean result = false; if ( ! super.withdrawal (amount) ) { System.out.println ("Using overdraft..."); if ( ! overdraft.withdrawal (amount - balance) ) System.out.println ("Overdraft source insufficient."); else { balance = 0; System.out.println ("New balance on account " + account + ": " + balance); result = true; } } return result; } } // class CheckingAccount

82 All objects are derived from the Object class
A class called Object is defined in the java.lang package of the Java standard class library All objects are derived from the Object class If a class is not explicitly defined to be the child of an existing class, it is assumed to be the child of the Object class The Object class is therefore the ultimate root of all class hierarchies The Object class contains a few useful methods, such as toString(),equals(), hashCode() which are inherited by all classes You may choose to override equals and/or toString to define equality/toString in your way. 13

83 import java.awt.Point; class TestToString { public static void main (String[] args) { Integer n = new Integer (25); Point p = new Point (0, 0); A a = new A(); System.out.println ( n.toString() ); System.out.println ( p.toString() ); System.out.println ( a.toString() ); } // method main } // class TestToString class A { public String toString() { return "I am AnyClass"; } // method toString } // class AnyClass

84 Abstract Classes revisted
An abstract class is a placeholder in a class hierarchy that represents a generic concept An abstract class cannot be instantiated We use the modifier abstract on the class header to declare a class as abstract An abstract class often contains abstract methods (like an interface does), though it doesn’t have to

85 Abstract Classes The child of an abstract class must override the abstract methods of the parent, or it too will be considered abstract An abstract method cannot be defined as final (because it must be overridden) or static (because it has no definition yet) The use of abstract classes is a design decision; it helps us establish common elements in a class that is too general to instantiate

86 References and Inheritance
An object reference can refer to an object of its class, or to an object of any class related to it by inheritance For example, if the Holiday class is used to derive a child class called Christmas, then a Holiday reference could actually be used to point to a Christmas object Holiday Christmas Holiday day; day = new Christmas();

87 References and Inheritance
Assigning a descendant class instance to an ancestor reference is considered to be a widening conversion, and can be performed by simple assignment Assigning an ancestor object to a subclass reference can also be done, but it is considered to be a narrowing conversion and must be done with a cast The widening conversion is the most useful

88 Now consider the following invocation: day.celebrate();
Polymorphism A polymorphic reference is one which can refer to one of several possible methods. Same invocation expression(say o.m(…)), different methods actually called. Suppose the Holiday class has a method called celebrate, and the Christmas class overrode it Now consider the following invocation: day.celebrate(); If day refers to a Holiday object, it invokes Holiday's version of celebrate; if it refers to a Christmas object, it invokes that version 16

89 Polymorphism In general, it is the type of the object being referenced, not the reference type, that determines which method is invoked Note that, if an invocation is in a loop, the exact same line of code could execute different methods at different times Polymorphic references are therefore resolved at run-time, not during compilation 17

90 A Vector is designed to store Object references
Polymorphism Note that, because all classes inherit from the Object class, an Object reference can refer to any type of object A Vector is designed to store Object references The instanceOf operator can be used to determine the class from which an object was created See Variety.java 18

91 import java.awt.Point; import java.util.Vector;
class MyVariety { public static void main (String[] args) { Vector collector = new Vector(); Integer num1 = new Integer (10); collector.addElement (num1); Point origin = new Point (0, 0); collector.addElement (origin); Integer num2 = new Integer (37);collector.addElement (num2); Point corner=new Point (12, 45);collector.addElement (corner); int temp; Object something; for (int count=0; count < collector.size(); count++) { something = collector.elementAt (count); if (something instanceof Integer) { temp = ((Integer)something).intValue() + 20; System.out.println (something + " + 20 = " + temp); } else System.out.println ("Point: " + something); } } }

92 Polymorphism via Inheritance
Consider the following class hierarchy: StaffMember Volunteer Employee Executive Hourly

93 class Firm2 { public static void main (String[] args) { Staff personnel = new Staff(); personnel.payday(); }

94 StaffMember[] staffList = new StaffMember[6]; public Staff() {
class Staff { StaffMember[] staffList = new StaffMember[6]; public Staff() { staffList[0] = new Executive ("Sam", "123 Main Line", " ", " ", ); staffList[1] = new Employee ("Carla", "456 Off Line", " ", " ", ); staffList[2] = new Employee ("Woody", "789 Off Rocker", " ", " ", ); staffList[3] = new Hourly ("Diane", "678 Fifth Ave.", " ", " ", 8.55); staffList[4] = new Volunteer ("Norm", "987 Suds Blvd.", " "); staffList[5] = new Volunteer ("Cliff", "321 Duds Lane", " "); ((Executive)staffList[0]).awardBonus (5000); ((Hourly)staffList[3]).addHours (40); } // constructor Staff

95 public void payday() { double amount; for (int count=0; count < staffList.length; count++) { staffList[count].print(); amount = staffList[count].pay(); if (amount == 0.0) System.out.println ("Thanks!"); else System.out.println ("Paid: " + amount); System.out.println ("**********************"); } } // method payday } // class Staff

96 class StaffMember { protected String name, address, phone; public StaffMember (String empName, String empAddress, String empPhone) { name = empName; address = empAddress; phone = empPhone; } // constructor StaffMember public double pay() { return 0.0; } // default pay method public void print() { System.out.println ("Name: " + name); System.out.println ("Address: " + address); System.out.println ("Phone: " + phone); } } // class StaffMember

97 class Volunteer extends StaffMember {
public Volunteer (String empName, String empAddress, String empPhone) { super (empName, empAddress, empPhone); } // constructor Volunteer public double pay() { return 0.0; } // method pay } // class Volunteer

98 class Employee extends StaffMember {
protected String ID; protected double payRate; public Employee (String empName, String empAddress, String empPhone, String empSsnumber, double empRate) { super (empName, empAddress, empPhone); this.ID = ID; payRate = empRate; } // constructor Employee public double pay () { return payRate; } // method pay public void print () { super.print(); System.out.println (“ID number: " + ID); System.out.println ("Pay rate: " + payRate); } // method print } // class Employee

99 class Executive extends Employee {
private double bonus; public Executive (String name, String addr, String phone, String ID, double pay) { super (name, addr, phone, ID, pay); bonus = 0; // bonus yet to be awarded } // constructor Executive public void awardBonus (double bonus) { this.bonus = bonus; } // method awardBonus public double pay () { double pay = super.pay() + bonus; bonus = 0; return pay; } // method pay public void print () { super.print(); System.out.println ("Current bonus: " + bonus); } // method print }

100 class Hourly extends Employee {
private int hoursWorked; public Hourly (String name, String addr, String phone, String ID, double hrRate) { super (name, address, phone, ID, hrRate); hoursWorked = 0; } public void addHours (int moreHours) { hoursWorked += moreHours; } // method addHours public double pay () { return payRate * hoursWorked; } // method pay public void print () { super.print(); System.out.println ("Current hours: " + hoursWorked); } // method print } // class Hourly

101 Summary for inheritance
Inheritance: reuse the existing objects (is-a relation) Protect modifier: better encapsulation Use super to invoke parent’s methods. Overriding methods and overloaded methods All Java classes inherit from object class Polymorphism: which overriding method is invoked based on the object’s type Widening & narrowing

102 A Java interface is a collection of abstract methods and constants
Interfaces A Java interface is a collection of abstract methods and constants An abstract method is a method header without a method body (i.e., no implementation) An abstract method in an interface can be declared using the modifier abstract, but because all methods in an interface are abstract, it is usually left off. cf: abstract methods in an abstract class must be declared explicitly using the abstract modifier. An interface is used to formally define a set of methods that a class will implement

103 interface is a reserved word
Interfaces interface is a reserved word None of the methods in an interface are given a definition (body) public interface Doable { public void doThis(); public int doThat(); public void doThis2 (float value, char ch); public boolean doTheOther (int num); } A semicolon immediately follows each method header

104 An interface cannot be instantiated
Interfaces An interface cannot be instantiated Doable d = new Doable(); // error Like a class, a user-defined interface can be used as the type of variables. Doable a, b; Methods in an interface have public visibility by default A class formally implements an interface by stating so in the class header providing implementations for each abstract method in the interface If a class asserts that it implements an interface, it must define all methods in the interface or the compiler will produce errors.

105 public class CanDo implements Doable { public void doThis ()
Interfaces public class CanDo implements Doable { public void doThis () // whatever } public void doThat () // etc. implements is a reserved word Each method listed in Doable is given a definition

106 A class can implement more than one interfaces
See Speaker.java (page 236) See Philosopher.java (page 237) See Dog.java (page 238) The interfaces are listed in the implements clause, separated by commas The class must implement all methods in all interfaces listed in the header

107 An interface can be implemented by multiple classes
Interfaces An interface can be implemented by multiple classes Each implementing class can provide their own unique version of the method definitions An interface is not part of the class hierarchy A class can be derived from a base class and implement one or more interfaces 9

108 Interface constants Unlike interface methods, interface constants require nothing special of the implementing class Constants in an interface can be used in the implementing class as if they were declared locally This feature provides a convenient technique for distributing common constant values among multiple classes 10

109 Note that the interface hierarchy and the class hierarchy are distinct
Extending Interfaces An interface can be derived from another interface, using the extends reserved word The child interface inherits the constants and abstract methods of the parent Note that the interface hierarchy and the class hierarchy are distinct Unlike class hierarchy, an interface can extend more than one interfaces. public interface Transformable extends Scable, Translatable, Rotatable { } A class that implements an interface must define also all methods in all ancestors of the interface. 11

110 public String print(); // public can be omitted
An interface Example interface Printable { public String name(); public String print(); // public can be omitted } // interface Printable class PrintLogger { public void log (Printable file) { System.out.println (file.name() + " : " + file.print()); } // method log } // class PrintLogger

111 class File { protected String id; protected int size; public File (String id, int size) { this.id = id; this.size = size; } // constructor File public String name() { return id; } // method name } // class File class TextFile extends File implements Printable { protected String text; public TextFile (String id, int size, String contents) { super(id, size); text = contents; } // constructor TextFile public String print() { return text; } } // class TextFile

112 class BinaryFile extends File {
protected byte[] data; public BinaryFile (String id, int size, byte[] data) { super(id, size); this.data = data; } // constructor BinaryFile } // class BinaryFile class ImageFile extends BinaryFile implements Printable { public ImageFile (String id, int size, byte[] data) { super(id, size, data); } // constructor ImageFile public String print() { return new String (data); } } // class Image_File

113 public class Printer { public static void main (String[] args) { byte[] logoData = {41, 42, 49, 44 }; TextFile report = new TextFile (“Reprot 1", 1024, "One two three …"); ImageFile logo = new ImageFile(“Picture 1", 4, logoData); PrintLogger daily = new PrintLogger(); daily.log (report); daily.log (logo); }

114 An interface without including any method.
Marker interface An interface without including any method. useful for providing additional information about an object. EX: java.lang.Serializable java.lang.Cloneable java.rmi.Remote Ex: Object obj; Object copy; copy = o.clone() // may raise CloneNotSupportedExceptionexception if(obj instanceof Cloneable) copy = o.clone(); else copy = null;

115 Polymorphism via Interfaces
An interface name can be used as the type of an object reference variable Doable obj; The obj reference can be used to point to any object of any class that implements the Doable interface The version of doThis that the following line invokes depends on the type of object that obj is referring to: obj.doThis();

116 Polymorphism via Interfaces
That reference is polymorphic, which can be defined as "having many forms" That line of code might execute different methods at different times if the object that obj points to changes See PrinterLogger.java(slide 106) Note that polymorphic references must be resolved at run time; this is called dynamic binding Careful use of polymorphic references can lead to elegant, robust software designs

117 Some interfaces used in core java classes
The Java standard class library contains many interfaces that are helpful in certain situations The Comparable interface contains an abstract method called compareTo, which is used to compare two objects pubilc iterface Comparable { public abstract int comparedTo(Object); } Ex: int rlt = x.comparedTo(y); if(rlt < 0) {… } // x < y else if (rlt>0) { …} // x > y else {…} // rlt = 0 means x is equal to y. The String class implements Comparable which gives us the ability to put strings in alphabetical order

118 The Iterator and Enumeration interface
The java.util.Iterator/Enumeration interface contain methods that allow the user to move through a collection of objects easily public interface Iterator { public abstract boolean hasNext(); public abstract Object next(); public abstract void remove(); } pubic interface Enumeration { public boolean hasMoreElements(); pubic Object nextElement(); } Ex: Object obj ; // obj is an object implementing Iterator for(Iterator i = (Iterator)obj; i.hasNext(); ) processing(i.next());

119 Often events correspond to user actions, but not always
Events [skipped] An event is an object that represents some activity to which we may want to respond For example, we may want our program to perform some action when the following occurs: the mouse is moved a mouse button is clicked the mouse is dragged a graphical button is clicked a keyboard key is pressed a timer expires Often events correspond to user actions, but not always

120 Other objects, called listeners, respond to events
The Java standard class library contains several classes that represent typical events Certain objects, such as an applet or a graphical button, generate (fire) an event when it occurs Other objects, called listeners, respond to events We can write listener objects to do whatever we want when an event occurs

121 This object waits for and
Events and Listeners Event Source This object may generate an event Listener This object waits for and responds to an event When an event occurs, the source calls the appropriate method of the listener, passing an Event object that describes the event

122 Listener Interfaces We can create a listener object by writing a class that implements a particular listener interface The Java standard class library contains several interfaces that correspond to particular event categories For example, the MouseListener interface contains methods that correspond to mouse events After creating the listener, we add the listener to the component that might generate the event to set up a formal relationship between the generator and listener

123 The following are mouse events:
mouse pressed - the mouse button is pressed down mouse released - the mouse button is released mouse clicked - the mouse button is pressed and released mouse entered - the mouse pointer is moved over a particular component mouse exited - the mouse pointer is moved off of a particular component Any given program can listen for some, none, or all of these See Dots.java (page 246) See DotsMouseListener.java (page 248)

124 The following are called mouse motion events:
mouse moved - the mouse is moved mouse dragged - the mouse is moved while the mouse button is held down There is a corresponding MouseMotionListener interface One class can serve as both a generator and a listener One class can serve as a listener for multiple event types See RubberLines.java (page 249)

125 The following are called key events:
key pressed - a keyboard key is pressed down key released - a keyboard key is released key typed - a keyboard key is pressed and released The KeyListener interface handles key events Listener classes are often implemented as inner classes, nested within the component that they are listening to See Direction.java (page 253)

126 The speed of a Java animation is usually controlled by a Timer object
Animations An animation is a constantly changing series of pictures or images that create the illusion of movement We can create animations in Java by changing a picture slightly over time The speed of a Java animation is usually controlled by a Timer object The Timer class is defined in the javax.swing package

127 The ActionListener interface contains an actionPerformed method
Animations A Timer object generates an ActionEvent every n milliseconds (where n is set by the object creator) The ActionListener interface contains an actionPerformed method Whenever the timer expires (generating an ActionEvent) the animation can be updated See Rebound.java (page 258)

128 Summary of Java Modifiers
Modifiers used in java: for accessibility: public, [package], protected, private abstract, final, static native, strictfp synchronized transient volatile

129 usage of accessibility modifiers
used on accessible to code from private member (i.e. field, constructor or method) the containing class none[package] class, interface, member the containing package protected the containing package or subclasses of the containing class public anywhere

130 usage of abstract, final and static modifiers
used on Meaning abstract class + interface method contains abstract methods the body is not implemented final class field + variable cannot be extended cannot be overridden cannot be changed static class+interface field+method initializer the nested class (interface) is top-level class field(method) run when class loaded

131 usage of native, synchronized, transient, strictfp and volatile
modifier used on meaning native method implemented by non-java code. no method body. synchronized lock this.class or this before executing method transient field non-persistent data; need not be serialized volatile (rarely used) updated value on thread WM must be reflected on MM immediately. strictfp FP operations must strictly conform to IEEE754

132 Example of transient fields
class Point { int x, y; transient float rho, theta; } // rho and theta are not persistent data

133 Example of synchronized method and volatile field
class Test { static int i = 0, j = 0; static void one() { i++; j++; } static void two() { System.out.println("i=" + i + " j=" + j); } } Volatile modifier guarantees that any thread that read a field will get the most recently written value.

134 Example of synchronized method and volatile field
public class Main { pubic static void main(String[] args){ new Thread1().start(); new Thread2().start(); }} class Thread1 extend Thread { public void run(){ for(;;) Test.one();} } class Thread2 extend Thread { public void run(){ for(;;) {Test.two(); sleep(500); } } // it is possible that Thread2 prints a result with // j > i, since i,j may be updated out of order in MM.

135 Example of synchronized method and volatile field
class Test { static int i = 0, j = 0; static synchronized void one() { i++; j++; } static synchronized void two() { System.out.println("i=" + i + " j=" + j); } } // i and j must be equal static volatile int i = 0, j = 0; static void one() { i++; j++; } static void two() { } } // i always >= j.

136 Nested Classes In addition to a class containing data and methods, it can also contain other classes A class declared within another class is called a nested class (or called inner class) Outer Class Nested Class

137 Nested classes can be hidden from other classes in the same package.
Why Nested Classes A nested class has access to the variables and methods of the outer class, even if they are declared private Nested classes can be hidden from other classes in the same package. Anonymous classes are handy when defining callbacks on the fly. Convenient when writing event-driven programming

138 A nested class produces a separate bytecode file
Nested Classes A nested class produces a separate bytecode file If a nested class called Inside is declared in an outer class called Outside, two bytecode files will be produced: Outside.class Outside$Inside.class Nested classes can be declared as static, in which case they cannot refer to this, instance variables or methods of outer classes. A nonstatic nested class is called an inner class

139 Kinds of Java classes /interfaces
Top-level classes /interfaces Non-nested top-level classes/interfaces are ordinary classes/interfaces that are direct members of a package. Nested top-level classes / interfaces are static members of other top-level classes/interfaces nested interfaces are implicitly static (hence top-level). Inner classes: Member classes are non-static nested classes Local classes are classes defined inside method body Anonymous classes are classes defined within method body without given a class name

140 static A1 { // I am a nested top-level class …}
Example class A { … static A1 { // I am a nested top-level class …} A2 { // w/o static modifier, so I am a member class void m1(…) { class A3 { // A3 is declared inside a method, so is a local class …} void m2(…) { // A4 is an interface or class A4 a = new A4() { void m3() {…} m4() … }; // a is an object of an anonymous subclass/implementation of A4.

141 Nested top-level classes /interfaces
also called static member classes/interfaces behave like an ordinary top-level class/interface except that it can access the static members of all of its direct or indirect containing classes. can be public, protected, package or private. Should use the name A.B.C t o reference to a class C enclosed by class B enclosed by class A. A A B B C

142 Example of a static member interface
public class LinkedStack { public interface Linkable { // interfaces are static by default. public Linkable getNext(); public void setNext(Linkable node); } // The top of the stack is a Linkable object. Linkable top; public LinkedStack() {}; pubic boolean empty() { return (top == null) ;} public void push(Linkable node) { node.setNext(top); top = node; } public Object pop(Linkable node) throw EmptyStackException { if(empty()) throw new EmptyStackException(); Object r = top; top = top.getNext(); return r; } } Linkable getNext() top null

143 // This class defines a type of node that we'd like to
// use in a linkedStack. class IntegerNode implements LinkedStack.Linkable { // Here's the node's data and constructor. private int i; public IntegerNode(int i) { this.i = i; } // implementation of LinkedStack.Linkable. private LinkedList.Linkable next; public LinkedList.Linkable getNext() { return next; } public void setNext(LinkedList.Linkable node) { next = node; } }

144 public class test { pubic static void main(String[] args) { // declare an array of 10 IntergerNodes IntergerNode[] n = new IntergerNode[10]; LinkedStack s = null; for (int i = 0; i < n.length; i++) { n[i] = new IntergerNode(i); s.push(n[i]) ; } while(! s.empty()) System.out.println(s.pop());

145 Features of static member classes
obey the same rules of static methods: can access only static members (using simple or full name) accessible to other classes according to the used visibility modifier. note: useful for compiler only. as to interpreters: pubic or protected nested/member classes => visible to all classes, package or private nested/member classes => visible to containing package

146 How is a nested class referenced ?
How to reference a nested static class C inside [static] class A of [static] class B of package a.b : outside package a.b => a.b.A.B.C if import a.b.A.B.C or a.b.A.B.* => C // not recommended if import a.b.A.B or a.b.A.* => B.C // not recommended inside package a.b => A.B.C (or a.b.A.B.C) inside class A => B.C (or A.B.C or a.b.A.B.C) inside class B => C ( or any of the above) Note: All static fields, methods, and classes of a top level class are accessible to all code [even inside a static class] within the class no matter they are private or not.

147 import LinkedStack.Linkable;
// or import LinkedStack.*; class IntegerNode implements Linkable { // Here's the node's data and constructor. private int i; public IntegerNode(int i) { this.i = i; } // implementation of LinkedStack.Linkable. private LinkedList.Linkable next; public Linkable getNext() { return next; } public void setNext(Linkable node) { next = node; } }

148 access references in nested classes
public class A { static int a; static private int ma(); static class B1 { static int y1; static private int mb1(); static class C1 { static int c1; static int mc1(); }} static class B2 { static class B1 { … } static int b2; static private int mb2(); static class C2 { // various ways to access other members // a, A.a, ma(), A.ma(), B1, A.B1 reference A’s members // b2, B2.b2, A.B2.b2, mb2, B2.mb2(), A.B2.mb2(), B2.B1, B1. // A.B1.y1, A.B1.C1.c1, B1.C1.c1, C1.c1 }}

149 static class behaves like static fields/methods
Member classes static class behaves like static fields/methods member class  behaves like instance fields/methods beside referring to all static fields/methods, can also reference this, instance field/method of all enclosing classes, even they are private. associated with an instance of each of the enclosing classes Member class v.s. Static class static class and its enclosing classes are static class-class relationship member class and its enclosing classes are instance-instance relationship. 1. Each member class instance must be created/accessed through instances of the containing class. 2. Each member class instance is associated with an unique instance of each of its containing classes.

150 Example: A LinkedList Enumerator, as a Member Class
import java.util.Enumeration; public class LinkedStack { // those from old LinkedStack public interface Linkable { ... } private Linkable top; public void push(…) { ... } public Object pop() { ... } // This method returns an Enumeration object for this inkedStack. // Note: no LinkedStack object is explicitly passed to the // constructor. public Enumeration enumerate() { return new Enumerator(); }

151 Example: A LinkedList Enumerator, as a Member Class
// the implementation of the Enumeration interface. protected class Enumerator implements Enumeration { Linkable current; public Enumerator() { current = top; } public boolean hasMoreElements() { return (current != null); } public Object nextElement() { if (current == null) throw new NoSuchElementException("LinkedStack"); Object value = current; current = current.getNext(); return value; } } } Note: Enumerator is only accessible to subclasses or the package of LinkedStack.

152 Restrictions on member classes
A member class cannot have the same name as any containing class or package. Member classes cannot contain any static fields, methods or classes (with the exception of constant fields). since member class is associated with object instances, it is nonsense/needless to have static members. Interfaces cannot be defined as member classes. since interfaces cannot be instantiated, there is no way for an object to create an interface instance. A nested interface is by default static, even if the modifier ‘static’ is not given in the header.

153 New syntax for member classes
member class can access instance field/method of containing class. public Enumerator() { current = top; } How to make the reference explicit ? public Enumeration() { this.current = this.top;} // this.current ok ; but this.top err!! // since there is no top in class Enumeration Solution: public Enumeration() { this.current = LinkedStack.this.top;} New syntax: C: a containing class name C.this is used to reference the associated C instance. needed only when using this incurs ambiguity.

154 accessing superclass members of the containing class
Recall that we use super.f (or super.m(…)) to reference shadowed or overridden member of parent class of this. Likewise, we use C.super.f to reference the f field of the parent class of C, which is a containing class of this, and use C.super.m() to reference the method m() of the parent class of C.

155 using containing class instance to invoke constructors of member class
Every instance of a member class is associated with an instance of its containing class. pubic Enumeration enumerate(){return new Enumeration();} can also be written as pubic Enumeration enumerate() { return this.new Enumeration;} More useful case: LinkedStack stack = new LinkedStack(); Enumeration e1 = stack.enumerate(); // could create one without invoking enumerate()!! Enumeration e1 = stack.new Enumeration(); syntax: C: a containing class of member class D with constructor D(…); s : this or var of type C s.new D(…) will invoke D(…) of class D in instance of C referenced by s.

156 It is possible that a class extends a member class.
Some special case It is possible that a class extends a member class. public class A { … public class B { …} …} class C extend A.B { pubic C( … ) { ??? } … } problem: what is the instance of the containing class A of the parent class B of C Solution: pubic C( A a, … ) { a.super(…); }

157 Containing Hierarchy vs Inheritance Hierarchy
class A extends A1 { int x; class B extends B1 { int x; class C extends C1 { int x; …} } } class A1 { int x; …} class B1 { int x; …} class C1 extends C2 { int x; …} class C2 { int x; … } Problem: how to reference different x in class C. this.x, C.this.x // x in C B.this.x // x in B A.this.x // x in A super.x, ((C1)this).x // x in C1 ((C2)this).x // x in C ((B1) (B.this)).x, B.super.x // x in B1 ((A1) (A.this)).x, A.super.x // x in A1 Problem: how about overridden methods ?

158 class declared locally within a block of Java code.
Local Classes class declared locally within a block of Java code. within method body within instance/static initialization block Feature of Local class: Local class is to member class what local variable is to instance variable. Properties similar to that of local variables: 1. invisible outside the containing block. 2. cannot use accessibility or static modifiers Properties similar to member classes 1.can access any member of any containing classes. 2. no local interfaces can use any final local variables or method parameters that are visible from the scope in which they are defined.

159 Example: Defining and using a Local Class
// This method creates and returns an Enumeration object for this LinkedStack. public Enumeration enumerate() { // Here's the definition of Enumerator as a local class. class Enumerator implements Enumeration { Linkable current; public Enumerator() { current = top; } public boolean hasMoreElements() { return (current != null); } public Object nextElement() { … } // omitted } // Create and return an instance of the Enumerator class defined here. return new Enumerator();

160 Fields and variables accessible to a local class
class A { protected char a = 'a'; } class B { protected char b = 'b'; } pubic class C extends A { public static void main(String[] args) { // Create an instance of the containing class, and invoke the // method that defines and creates the local class. C c = new C(); c.createLocalObject('e'); // pass a value for final parameter e. }

161 private char c = 'c'; // Private fields visible to local class.
public static char d = 'd'; public void createLocalObject(final char e) { final char f = 'f'; int i = 0; // i not final; not usable by local class. class Local extends B { char g = 'g'; public void printVars() { // All of these fields and variables are accessible. System.out.println(g); // (this.g) g is a field of this class. System.out.println(f); // f is a final local variable. System.out.println(e); // e is a final local argument. System.out.println(d); // (C.this.d) d -- field of containing class. System.out.println(c); // (C.this.c) c -- field of containing class. System.out.println(b); // b is inherited by this class. System.out.println(a); // a is inherited by the containing class } } Local l = this.new Local(); // Create an instance of the local class l.printVars(); // and call its printVars() method. }

162 Typical uses of local classes [TBA]
used to implement adapter classes

163 a local class without a name very commonly used as adapter classes.
Anonymous classes a local class without a name very commonly used as adapter classes. created through another extension to the syntax of the new operator. defined by a Java expression, not a Java statement.

164 Example: Implementing an Interface with an Anonymous Class
import java.io.*; // A simple program to list all Java source files in a directory public class Lister { public static void main(String[] args) { File dir = new File(args[0]); // f represents the specified directory. // List the files in the directory, using the specified filter object. // The anonymous class is defined as part of a method call expression. String[] list = dir.list( new FilenameFilter() { public boolean accept(File f, String name) { return name.endsWith(".java"); }}); for(int i = 0; i < list.length; i++) // output the list System.out.println(list[i]); }}

165 Anonymous class vs local class
when to use anonymous class: The class has a very short body. Only one instance of the class is needed. The class is used right after it is defined. The name of the class does not make your code any easier to understand. Restrictions on anonymous classes: An anonymous class has no name and hence cannot be used to create more than one instance for each execution. It is not possible to define constructors for anonymous classes.

166 Example: Enumeration implemented as an anonymous class
public Enumeration enumerate() { // Instantiate and return this implementation. return new Enumeration() { Linkable current = top; // This used to be in the constructor, but // anonymous classes don't have constructors. public boolean hasMoreElements() { return (current != null); } public Object nextElement() { if (current == null) throw new NoSuchElementException("LinkedList"); Object value = current; current = current.getNext(); return value; } }; // Note the required semicolon. It terminates the return statement. }

167 Additional Java Syntax for Anonymous Classes
new class-name ( [ argument-list ] ) { class-body } new interface-name () { class-body } Syntax 1 return an instance of an anonymous subclass of class-name the subclass can provide additional (helper ) methods/fields and/or overrides or implements existing super class methods. Syntax 2 return an instance of a class implementing interface-name.

168 use initializer to help construct anonymous class instances
public class InitializerDemo { public int[] array1; // This is an instance initializer. // It runs for every new instance, after the superclass constructor // and before the class constructor, if any. { array1 = new int[10]; for(int i = 0; i < 10; i++) array1[i] = i; } // another instance initializer. The instance initializers run in the order in // which they appear. int[] array2 = new int[10]; { for(int i=0; i<10; i++) array2[i] = i*2; } static int[] static_array = new int[10]; // By contrast, the block below is a static initializer. Note the static // keyword. It runs only once, when the class is first loaded. static { for(int i = 0; i < 10; i++) static_array[i] = i; } }

169 Some other notes Class Literals: Each user or system defined class is represented by an object of the type java.lang.Class at runtime. Given a fully qualified class name a.b.C, there are two ways to refer to the Class object representing a.b.C: 1. Class.forName(“a.b.C”) 2. a.b.C.class (2.) is useful for inner classes as well; as to (1.) it requires special knowledge of how inner classes is translated into top level classes. Class.forName(a.b.C$D); // a.b.C.D.class

170 Default value of variables
Kinds of variables: class/static variables instance variables local variables formal method parameters formal constructor parameters array component : int[ ] ar = new int[10] ; // => ar[0] .. ar[9] exception-handler parameters Notes about default value of uninitalized variables: 1. Default values are given only to array components or class/instance variables which are not final. 2. Local variables are not assigned default values.

171 { final int y = 10 ; // y is final but not blank final.
Blank finals a field or local variable declared final but without given an initial value in the declaration. Recall that a final variable cannot be reassigned values (for primitive type) or references (for array or object type) once it has been given a content. public class Test { final int y = 10 ; // y is final but not blank final. int z ; // z is not final and has default value 0 final int x ; // x is blank final, has no value here { x = 1; } // x assigned value for the 1st time. ok!

172 public void setX(int nx, final int ny) {
Blank final public void setX(int nx, final int ny) { x = nx; //error, final field x reassigned value. // still error even removing {x=1;} why ? // no way to determine if x = nx will be executed once only. nx = nx + 10 ; // ok, nx is not final and has value ny = ny + 10 ; // error, final ny reassigned value final int nx10 = nx; // ok final int nx11 ; // blank final int nz // local var has no default value nx = nz + 1; // error nz has no value here nz = nx11 = nx; // ok nx11 = nx; // error }}

173 Object of final variable can change state.
class Person { final Person wife = findAWife(..); final Person[] children = new Person[5] ; public void m() { wife.name = newName(); // ok, final object changes state wife = FindAnotherWife(…) // error! final var changes reference children[0] = bearOne(…); // ok! final object can change state children = FindChilren(..) // error! }


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