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B.Ramamurthy Copyright 1998 CSE Department
Jump Start : Java B.Ramamurthy Copyright 1998 CSE Department 9/18/2018 BR
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Topics for Discussion OO Design Principles Java Virtual Machine
Java Application Structure Class and objects Methods and Variables Access/Visibility Modifiers Debugging Inheritance Abstract classes, interfaces and implementation JDK Event Model Applets and Appletviewer Summary 9/18/2018 BR
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Object-Oriented Principles
OOP Inheritance -- Hierarchy -- Reusability -- Extensibility -- Expressive power -- Reflects many real-world problems Polymorphism -- Many forms of same function -- Abstract Methods -- Abstract Classes Encapsulation (class) -- Information Hiding -- Interface and Implementations -- Standardization -- Access Control mechanisms (private /public etc.) 9/18/2018 BR
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Conventional Compiled Languages
C++source Mac Compiler Mac Hardware PC Hardware PC Compiler Sun Compiler 9/18/2018 BR
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Java Virtual Machine Java compiler :javac JVM Mac Hardware C++source
Byte code C++source Mac Compiler Mac interpreter JVM PC Hardware Byte code C++source PC Interpreter JVM PC Hardware Byte code C++source Sun Interpreter 9/18/2018 BR
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“Run-anywhere” Capability
On any machine when you compile Java source code using javac byte code equivalent to the source is generated. Byte code is machine-independent. This enables the “run-anywhere” capability. You invoke java command which will feed the byte code to the machine-dependent interpreter. 9/18/2018 BR
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Java Application Program Interface (Java API)
Package of related classes : java.awt Random java.util Date Dictionary Java.io, java.beans,.. Etc.. package class 9/18/2018 BR
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Java API : A Simplistic View
packages classes methods and data declarations 9/18/2018 BR
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Java API Classes Unlike many other languages, you will referring to the classes in the API. Where is the API? Packages are in: /util/lang/jdk1.1.6/src/java “cd” to any package of interest: awt, lang, io, graphics….. You will see the methods and data defined in the classes in the package. 9/18/2018 BR
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Types of Programs Java is fully object-oriented.
Every “function” has to be attached to a class. You will mainly deal with three types of programs: class: methods and data (variables + constants) describing a collection (type) of object application: class that has a main method: represents a our regular program applet: class that is meant for execution using a appletviewer/browser 9/18/2018 BR
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Problem Solving Using Java
OO Design and Progamming in Java Write an applet class Identify classes needed Write an application class Reuse API classes Reuse your classes Design new classes Create and use objects 9/18/2018 BR
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What is an Object? Object-oriented programming supports the view that programs are composed of objects that interact with one another. How would you describe an object? Using its characteristics (has a ----?) and its behaviors (can do ----?) Object must have unique identity (name) : Basketball, Blue ball Consider a ball: Color and diameter are characteristics (Data Declarations) throw, bounce, roll are behaviors (Methods) 9/18/2018 BR
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Classes are Blueprints
A class defines the general nature of a collection of objects of the same type. The process creating an object from a class is called instantiation. Every object is an instance of a particular class. There can be many instances of objects from the same class possible with different values for data. 9/18/2018 BR
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Example objects Object References redRose class Rose blueRose class
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Instantiation : Examples
class FordCar defines a class name FordCar FordCar windstar; ---- defines a Object reference windStar windstar = new FordCar(); instantiates a windstar Object class HousePlan1 { color…. HousePlan1 blueHouse; blueHouse = new HousePlan1(BLUE); HousePlan1 greenHouse = new HousePlan1(GREEN); 9/18/2018 BR
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Operator new and “dot” new operator creates a object and returns a reference to that object. After an object has been instantiated, you can use dot operator to access its methods and data declarations (if you have access permissions). EX: redRose.bloom(); greenHouse.color 9/18/2018 BR
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Elements of a Class class data declarations (variables, constants)
methods header body header statements modifiers, type, name variables, constants parameters repetition others selection assignment 9/18/2018 BR
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Class Structure class variables constants methods 9/18/2018 BR
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Defining Classes Syntax: class class_name { data-declarations
constructors methods } Constructors are special methods used for instantiating (or creating) objects from a class. Data declarations are implemented using variable and constant declarations. 9/18/2018 BR
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Naming Convention Constants: All characters in uppercase, words in the identifier separated by underscore: EX: MAX_NUM Variables, objects, methods: First word all lowercase, subsequent words start with uppercase. EX: nextInt, myPen, readInt() Classes: Start with an uppercase letter. EX: Tree, Car, System , Math 9/18/2018 BR
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A complete example Problem Statement: You have been hired to assist in an secret encryption project. In this project each message (string) sent out is attached to a randomly generated secret code (integer) between 1 and 999. Design and develop an application program in Java to carry out this project. 9/18/2018 BR
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Identify Objects There are two central objects:
Message Secret code Is there any class predefined in JAVA API that can be associated with these objects? Yes , “string” of java.lang and “Random” of java.util 9/18/2018 BR
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The Random class Random class is defined in java.util package.
nextInt() method of Random class returns an integer between 0 and MAXINT of the system. 9/18/2018 BR
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Design Class Random Class String An instance of Random
An instance of string Class Random An instance of Random number generator Input and fill up message. Generate Random integer Attach (concatenate) Output combined message. For implementation see /projects/bina/java/secretMsg.java 9/18/2018 BR
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Debugging and Testing Compile-time Errors : Usually typos or syntax errors Run-time Errors : Occurs during execution. Example: divide by zero . Logic Errors: Software will compile and execute with no problem, but will not produce expected results. (Solution: testing and correction) See /projects/bina/java/Peets directory for an exercise. 9/18/2018 BR
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Class Components Class name (starts with uppercase), constants, instance variables, constructors definitions and method definitions. Constants: public final static double PI = 3.14; Variables: private double bonus; public string name; 9/18/2018 BR
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Method Invocation/Call
Syntax: method_name (values); object_name.method_name(values); classname.method_name(values); Examples: computeSum(); // call to method from within the class where it is located YourRose.paintIt(Red); Math.abs(X); 9/18/2018 BR
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Defining Methods A method is group of (related) statements that carry out a specified function. A method is associated with a particular class and it specifies a behavior or functionality of the class. A method definition specifies the code to be executed when the method is invoked/activated/called. 9/18/2018 BR
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Method Definition : Syntax
visibility return_type method_name (parameter_list) { statements } 9/18/2018 BR
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Return Type can be void, type or class identifier
void indicates that the method called to perform an action in a self-standing way: Example: println type or class specify the value returned using a return statement inside the method. 9/18/2018 BR
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Return Statement Syntax of return statement:
return; // for void methods return expression; // for type or class return value // the expression type and return type should be same 9/18/2018 BR
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Parameter List Parameter list specified in method header provides a mechanism for sending information to a method. It is powerful mechanism for specializing an object. The parameter list that appears in the header of a method specifies the type and name of each parameter and is called formal parameter list. The corresponding parameter list in the method invocation is called an actual parameter list. 9/18/2018 BR
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Parameter list : Syntax
Formal parameter list: This is like molds or templates (parm_type parm_name, parm_type parm_name, ....) Actual parameter list: This is like material that fit into the mold or template specified in the formal list: (expression, expression....) 9/18/2018 BR
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Method Definition : review
header body Visibility modifiers parameter list return type Name { statements } 9/18/2018 BR
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Method Definition : Example
Write a method that computes and returns the perimeter of a rectangle class. Analysis: Send to the method: Length and Width Compute inside the method: Perimeter Return from the method: Perimeter 9/18/2018 BR
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...Example (contd.) public int Perimeter (int Length, int Width) {
int Temp; // local temporary variable Temp = 2 * (Length + Width); // compute perimeter return Temp; // return computed value } 9/18/2018 BR
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What happens when a method is called?
Control is transferred to the method called and execution continues inside the method. Control is transferred back to the caller when a return statement is executed inside the method. 9/18/2018 BR
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Method Invocation : semantics
Operating System 1. OS to main method 2. Main method execution 3. Invoke Area 4. Transfer control to Area 5. Execute Area method 6. Return control back to main method 7. Resume executing main 8. Exit to OS 1 8 2 Main method Rect.Area(….) 3 7 4 8 Area method 5 6 9/18/2018 BR
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Constructors A Constructor is used to create or instantiate an object from the class. Constructor is a special method: It has the same name as the class. It has no return type or return statement. Typically a class has more than one constructor: a default constructor which has no parameters, and other constructors with parameters. 9/18/2018 BR
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Constructors (contd.) You don’t have to define a constructor if you need only a default constructor. When you want initializing constructors : 1. you must include a default constructor in this case. 2. You will use initializing constructors when you want the object to start with a specific initial state rather than as default state. 3. Example: Car myCar(Red); // initializing constructor for Car class with color as parameter 9/18/2018 BR
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Visibility Modifiers type Method/variable name public protected
“nothing” DEFAULT private static “nothing” DEFAULT To indicate class method/ variable To indicate object method/ variable 9/18/2018 BR
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..Modifiers (contd.) private : available only within class
“nothing” specified : DEFAULT: within class and within package protected : within inherited hierarchy (only to sub classes) public : available to any class. 9/18/2018 BR
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CLASSPATH Many times classes needed are available in a repository other than the Java API. Set the CLASSPATH environment variable to point to such repositories. One such is located in /projects/bina/cs114/structures setenv CLASSPATH .:/util/lang/jdk1.1.6/lib/classes.zip:/projects/bina/cs114/structures (all in a continuous line) 9/18/2018 BR
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CLASSPATH Now you may refer to any one of the classes in the structures directory. The source code is available in /projects/bina/cs114/sources Checkout ReadStream and Keyboard classes for nice inputs. 9/18/2018 BR
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Inheritance Inheritance is the act of deriving a new class from an existing one. A primary purpose of inheritance is to reuse existing software. Original class used to derive a new class is called “super” class and the derived class is called “sub” class. Inheritance represents “is a” relationship between the superclass and the subclass. 9/18/2018 BR
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Syntax class subclass extends superclass { class definition } Example:
class Windstar extends FordCar // meaning it inherits from class Fordcar{ ....} Windstar myCar(); In this example, FordCar is the super-class and Windstar is a sub-class and myCar is an object Windstar class. 9/18/2018 BR
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Representing the Relationship
BankClass has a has a has a Account [ ] MortgageSVC BrokerageSVC is a is a Savings Checking is a : use inheritance has a : use composition, or membership 9/18/2018 BR
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Modifers Visibility modifiers: private, public, protected
Protected modifier is used when the entity needs to be available to the subclass but not to the public. 9/18/2018 BR
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Example : Words Main class Book class Super class Uses is a
Dictionary Class subclass 9/18/2018 BR
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Example : School Student Main class uses Grad Student 9/18/2018 BR
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Abstract Class (ABC) and methods
An abstract class or method is defined using “abstract” modifier. An abstract method has no definition in the super class. ABC cannot be instantiated. Sole purpose of ABC is to provide an appropriate super class from which classed may inherit. Classes from which objects can be instantiated are called concrete classes. 9/18/2018 BR
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Example Food Abstract super class Pizza Hamburger HotDog subclasses
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Interface An abstract method is one that does not have a definition within the class. Only the prototype is present. An interface is collection of constants and abstract methods. Syntax interface interface_name { constant -declarations; abstract methods; } 9/18/2018 BR
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Example interface EPA { bool emissionControl();
bool pollutionControl(); … } class NYepa implements EPA { bool emissionControl () { details/semantics /statements how to implement it } class CAepa implements EPA { bool emissionControl () {…. // different details on implementation….} 9/18/2018 BR
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Inheritance and Interfaces
In Java class may inherit (extend) from only one class. (C++ allows multiple inheritance). But a Java class may implement many interfaces. For example, public class Scribble extends Applet implements MouseListner, MouseMotionListner { 9/18/2018 BR
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Types of Programs Console programs:
Control-driven: You start the program and the program code determines the sequence of events. Mostly the actions are predetermined. Windows-based programs: Event-driven: The operation of the program depends on what you do with the controls presented (usually by the GUI) Selecting menu items, pressing buttons, dialog interaction cause actions within the program. 9/18/2018 BR
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Events 1. Actions such as clicking a button, moving a mouse, are recognized and identified by the operating systems(OS). 2. For each action, OS determines which of the many currently running programs should receive the signal (of the action) 3. The signals that the application receives from the OS as result of the actions are called events. 9/18/2018 BR
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Event Generation User Interface Other event sources
Mouse actions, Keystrokes Operating Systems JAVA API Events Methods and handlers Methods and handlers Application 1 Application 2 9/18/2018 BR
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Event Handlers An application responds to the events by executing particular code meant for each type of event. Not all events need to be handled by an application. For example, a drawing application may be interested in handling only mouse movements. As a designer of an event-driven application you will write methods to handle the relevant events. 9/18/2018 BR
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Event Handling Process
Source of an event is modeled as an object. Ex: button click’s object is a button Type of the event: ActionEvent, WindowEvent, MouseEvent etc. Ex: An ActionEvent object is passed to the application that contains information about the action. Target of an event: Listener object of the event. Passing the event to a listener means calling the particular method of the listener object. 9/18/2018 BR
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Java Event Model Java event model has changed quite significantly between JDK1.0 and JDK 1.1. Many Web Browsers still use JDK1.0. JDK1.0 event model is good for understanding event-driven programming and for simple applications but does not scale well for large applications. It is impossible to take a in-depth look at any one. But we will learn enough to use some basic features. 9/18/2018 BR
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Java Class Hierarchy java.lang Object java.awt Component “others”
Label Button Container TextComponent java.applet package Window Panel Applet Dialog Frame 9/18/2018 BR
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JDK1.1 Event Model : System support
Many Event classes Low-level classes Semantic classes FocusEvent MouseEvent Etc. Etc. ActionEvent TextEvent Listener Interfaces : methods 9/18/2018 BR
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JDK1.1 Event Model : Application Support
Instantiate objects from component classes such as buttons and checkboxes which will serve as source of the events. Implement listener interfaces. Define methods to handle events of interest. Example: /projects/bina/java/Doodle.java 9/18/2018 BR
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Implementing Listeners
An application (class) becomes a listener when it implements the listener interface. Java 1.1 event model provides a variety of listeners. EX: If an application is interested in button click, it will have to implement ActionListener interface. actionPerformed() method of the ActionListener will have be implemented by the application. When the button click event occurs, actionPerformed() method will be called with event object as argument. 9/18/2018 BR
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Example: Doodle.java 1. Include “implements” at the header of the class : This application uses MouseListener and MouseMotionListener interfaces 2. Invoke “ methods” to add/activate required listeners. Here we have three: a) mouse click b) mouse motion c) window close 3. Set up board : use paint() and fillRect() methods. 9/18/2018 BR
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Example (contd.) 4. Implement all the methods (even if it is empty) of the interface (why?) 5. This application needs only MousePressed and MouseReleased for line color selection and MouseDragged for drawing lines. 6. Finally an adapter class that handles window closing. 9/18/2018 BR
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Applet An applet is a Java program that operates within a browser and hence can appear in a web page. It can be fetched from a remote computer and run on the local computer. Applet’s enabling technologies: Java Virtual Machine (JVM) Interpretation rather than compilation 9/18/2018 BR
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Requirements Besides the classes, an applet requires a .html file and a tool for viewing the applet (Ex: appletviewer of JDK) An applet extends Applet class of java.applet package. import java.applet.Applet; An applet should have at least an init() or a paint() method (just as an application should have main() method) 9/18/2018 BR
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Applet Class Applet class has four important methods: init() start()
stop() destroy() Besides these paint() method of component class is commonly used for drawing graphics in an applet window. 9/18/2018 BR
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Applet Runtime Structure
html file .html Applet file .java javac includes appletviewer Java runtime JVM Applet file .class 9/18/2018 BR
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Example 1: Using only paint()
//file : MyApplet.java import java.applet.Applet; // step 1 import java.awt.Graphics; public class MyApplet extends Applet //step 2 { public void paint( Graphics g) // step 3 { g.drawString(“To climb a ladder”, 20, 90); } } 9/18/2018 BR
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Example 1: interpretation
step 1: import Applet class step 2: inherit from Applet class step 3: Overload /redefine/customize paint() method 9/18/2018 BR
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html file 1. Create a html file:
<APPLET code=“MyApplet.class” width=300 height=200> <\APPLET> 2. Save as MyApplet.html 3. To create class: javac MyApplet.java 4. To execute: appletviewer MyApplet.html 9/18/2018 BR
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Example 1: execution semantics
Java run-time system will look for init(), then start() methods. Default definitions of init() and start() are executed. Then user-applet-defined paint() method is executed. Rest of the control follows default methods. 9/18/2018 BR
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More on Applets 1. main() is replaced by init() or paint() methods
2. An applet cannot access local programs or files 3. Very useful in creation of active/dynamic web pages 4. Implements “behavior streaming” 5. Popular component of Java language. 9/18/2018 BR
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On-line Information file:/util/lang/jdk1.1.6/api/API_users_guide.html
has user’s manual for Java API /util/lang/jdk1.1.6/demo has many demo programs and of course, Sun’s home page has the JDK distribution. 9/18/2018 BR
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Summary We studied Java classes and objects, object-oriented programming, applications and applets. A good understanding of the event-model is necessary to write event-driven programs. Graphics and awt features (buttons, panels etc.) will tremendously enhance the capabilities and presentation of the event driven programs. 9/18/2018 BR
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