Jordan Anastasiade. All rights reserved. 1 Class In this lesson you will be learning about: Class. Inheritance. Polymorphism. Nested and Inner Classes. Interface. Packages in Java. Using multiple classes, packages in Java files.

Jordan Anastasiade. All rights reserved. 2 Creating Classes The fundamental unit of programming in Java is class. public class Sphere { static final double PI = 3.14; double radius; Point3D center; public Sphere(double r, Point3D c) { radius = r; center = c; } double volume() { double v = 4.0/3.0 * PI; return v * radius*radius*radius; } Class Declaration Class Body Constructor Method Fields 1 Modifiers : public,, abstract, final, strictfp 1

Jordan Anastasiade. All rights reserved. 3 Member Variables Member variables declaration appears within the class body but outside of any methods or constructors. public static Vector cubeVertices; private final int id = 0; initilizer 1 Access modifiers: public, protected, private, 2 static – variable is a class variable Attribues final – the value of the variable cannot be changed transient – marker used in object serialization volatile – prevent compiler from optimization typename

Jordan Anastasiade. All rights reserved. 4 Constructors - super(); this(); class AnimationThread extends Thread { int framesPerSecond; int numImages; Image[] images; AnimationThread(int fps) { super("AnimationThread"); super() superclass constructor invocation framesPerSecond = fps; } AnimationThread(int fps, int num) { this(fps); this() explicit constructor invocation numImages = num; } AnimationThread(int fps, int num, Image[] images ) { super("AnimationThread"); framesPerSecond = fps; numImages = num; this.images = images; } this current object reference }

Jordan Anastasiade. All rights reserved. 5 Constructor’s Modifiers One can specify how other objects can create instances of a given class by using an access modifier in the constructors' declaration: private No other class can instantiate the given class. The class may contain public class methods (sometimes called factory methods), and those methods can construct an object and return it, but no other classes can. protected Only subclasses of the given class can create instances of it. public Any class can create an instance of the given class. no modifiers gives package access Only classes within the same package as the class can construct an instance of it.

Jordan Anastasiade. All rights reserved. 6 Method Declaration The only required elements of a method declaration are the method’s name, its return type, and a pair of parentheses ( ). public abstract int doIt(Object o) throws Exception Return type Method nameParametes List Checked exceptions Method Signature 1 Access modifiers: public, protected, private, 2 static – method is a class method abstract – method has no implementation – abstract class final – method cannot be overridden by a subclass native – method implemented in a language other than Java synchronized – method has a thread-safe implementation Attribues strictfp – strict constrain for floating point arithmetic

Jordan Anastasiade. All rights reserved. 7 Overloading Defining more methods with the same name, but with different signature is called method overloading.  Java supports method name overloading so that multiple methods can share the same name. Ex: class ShapeRender { … void draw(Shape s) { …} void draw(Rectagle r) { …} void draw(Circle c, Point position) { …} … }

Jordan Anastasiade. All rights reserved. 8 Inheritance Definition: A subclass is a class that extends another class. A subclass inherits state and behavior from all its ancestor. The superclass refers to a direct ancestor. Subclass inherits all superclasses members and can access any member declared as public or protected or declared with no access specifier as long as the subclass is in the same package as the superclasses. public class SubClass extends SuperClass { … } public class SuperClass { … }

Jordan Anastasiade. All rights reserved. 9 Overriding Replacing the superclass’s implementation with a new method in a subclass is called overriding.  The signature and the return type should be identical.  Only accessible non-static method can be overridden.  Access modifier could be different in overridden method as long as the subclass modifier is less restrictive then the superclass.  A subclass can change whether a parameter in an overriddent method is final ( final is not part of method signature).  Each type in overriding method’s throws clause must be polymorphically compatible with a least one of the types listed in the throws clause of the supertype’s method.  Fields cannot be overridden; they can only be hidden. super acts as a reference accessing fields and method of superclass. Ex: super.superclassField;

Jordan Anastasiade. All rights reserved. 10 Example of Overriding class Base { public String str = “BaseString”; public void show() { System.out.println(“Base show() “ + str); } } class Extended extends Base { public String str = “ExtendedString”; public void show() { System.out.println(“Extended show() “ + str); } public static void main(String[ ] args) { Extended e = new Extended(); Base b = e; b.show(); e.show(); System.out.println(b.str + “ “ + e.str); } } Results: 1. b.show()Extended show() ExtendedString 2. e.show()Extended show() ExtendedString 3. b.str + e.strBaseString ExtendedString When invoke a method on an object, the actual class of the object governs which implementation is used. When access a field the declared type of the referenced is used.

Jordan Anastasiade. All rights reserved. 11 Polymorphism class Point { int x; int y; void clear() { x = 0; y = 0 } } class Pixel extends Point { Color color; public void clear() { super.clear(); color = null; } Pixel extends both data and behavior of its Point superclass. All the Point code can be used by any one with a Pixel in hand. A single object like Pixel could have many (poly) forms (-morph) and can be used as both a Pixel object and a Point object. Pixel ’s behavior extends Point ’s behavior. Point point = new Pixel(); Implicit casting – a reference of extended class is assigned to a reference of the base class – upcasting. Point object Pixel

Jordan Anastasiade. All rights reserved. 12 Constructor Order Each constructor has three phases:  1. Invoke a superclass’s constructor.  2. Initialize the fields using their initializers and any initialization block. zero for all numeric types, false for boolean, \u0000 for char, null for references.  3. Execute the body of constructor. Each class has a least one constructor  If class has no constructor the compiler adds the default constructor. Ex: class Bush { Bush(int i){ … } Bush(double d) { …} } An invocation Bush bushObj = new Bush(); will generate a compile error – there is no default constructor.

Jordan Anastasiade. All rights reserved. 13 Abstract Class A method that provides only the definition without an implementation is called abstract method. A class that defines only part of its methods is called and declared as an abstract class. An abstract class could contain abstract mehtods and concret methods Ex: abstract class Benchmark { public long repeat(int count) { long start = System.currentTimeMillis(); for (int k = 0; k < count; k++) long now = System.currentTimeMillis(); benchmark(); return now – start; } abstract void benchmark(); // definition without implementation }

Jordan Anastasiade. All rights reserved. 14 Interface A Java interface defines a set of methods but does not implement them. A class that implements the interface agrees to implement all of the methods defined in the interface.  Methods in an interface are implicitly abstract and public.  Fields in an interface are always static and final.  A class could implement more than one interface – multiple inheritance.  Interface definition creates type name just as class definition do. Ex: public interface X { int MAX_VAL_SIZE = 10; Object setObject(); } public class A extends B implements X { Object setObject() { Object o; … return o; } }

Jordan Anastasiade. All rights reserved. 15 Interface – a Type Name Interface definitions create type names just as class definition do. An interface defines a protocol of behavior. A class that implements an interface adheres to the protocol defined by that interface. Ex: public interface ConversionFactors { double INCH_TO_MM = 25.4; } public interface Conversions extends ConversionFactors { double inchToMM(double inches); } public class Transform implements Conversions { public double inchToMM(double inches) { return inches * INCH_TO_MM; } Conversions conv = new Transform();

Jordan Anastasiade. All rights reserved. 16 Packages Packages are collections of related classes, interfaces, and subpackages.  Interfaces and class can use normal public names without any conflict with similar common names.  Packages can have types and members that are available only within the package and are inaccessible to outside code. package com.magic.attributes; Ex: Some of the packages in JDK 1.2 java.applet;java.awt;java.awt.*; java.beans;java.io;java.lang; java.rmi;java.security;java.util; java.naming;javax.swing;org.omg.CORBA; Should be the first line in a prgramm

Jordan Anastasiade. All rights reserved. 17 Nested Class Definition: A nested class is a class that is a member of another class. One use nested classes to reflect the relationship between two classes. class EnclosingClass{... class ANestedClass {... } A static nested class is called just that: a static nested class. A nonstatic nested class is called an inner class. class public EnclosingClass {... public static class AStaticNestedClass {... } protected class InnerClass {... }

Jordan Anastasiade. All rights reserved. 18 Inner Class Definition : An inner class is a nested class whose instance exists within an instance of its enclosing class and has direct access to the instance members of its enclosing instance. public class Stack { private Vector items;...//code for Stack's methods and constructors not shown... public Enumeration enumerator() { return new StackEnum(); } class StackEnum implements Enumeration { int currentItem = items.size() - 1; public boolean hasMoreElements() { return (currentItem >= 0); } public Object nextElement() { if (!hasMoreElements()) throw new NoSuchElementException(); else return items.elementAt(currentItem--); } Instance of Enclosing Class Stack Instance of Inner Class StackEnum

Jordan Anastasiade. All rights reserved. 19 Anonymous Classes Definition : An inner class without a name is called anonymous class. public class Stack { private Vector items;...//code for Stack's methods not shown... public Enumeration enumerator() { return new Enumeration() { int currentItem = items.size() - 1; public boolean hasMoreElements() { return (currentItem >= 0); } public Object nextElement() { if (!hasMoreElements()) throw new NoSuchElementException(); else return items.elementAt(currentItem--); } Interface Enumeration public boolean hasMoreElements(); public Object nextElement();

Jordan Anastasiade. All rights reserved. 20 Conclusions After completion of this lesson you should know: How to write Java Program using Inheritance. How to write Java Program using Polymorphism. Creating and using Packages.