Proprietary© 1997

University of Houston-Clear Lake

Software Development

Summary of Chapter 3Summary of Chapter 3

• A class is a collection of data and methods that operate on that data.

• An object is a particular instance of a class.

• Object members (fields and methods) are accessed with a dot between

the object name and the member name.

• Instance (non-static) variables occur in each instance of a class.

• Class (static) variables are associated with the class. There is one copy

of a class variable regardless of the number of instances of a class.

• Instance (non-static) methods of a class are passed an implicit this

argument that identifies the object being operated on.

• Class (static) methods are not passed a this argument and therefore do

not have a current instance of the class that can be used to implicitly

refer to instance variables or invoke instance methods.

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Summary - ContinuedSummary - Continued

• Objects are created with the new keyword, which invokes a class constructor method with a list of arguments.

• Objects are not explicitly freed or destroyed in any way. The Java garbage collector automatically reclaims objects no longer used.

• If the first line of a constructor method does not invoke another constructor with a this() call, or a superclass constructor with a super( ) call,

• Java automatically inserts a call to the superclass constructor that takes no arguments. This enforces "constructor chaining."

• If a class does not define a constructor, Java provides a default constructor.

• A class may inherit the non-private methods and variables of another class by "subclassing"--i.e., by declaring that class in its extends clause

Proprietary© 1997

University of Houston-Clear Lake

Software Development Summary - ContinuedSummary - Continued

• java.lang.Object is the default superclass for a class. It is the root of the Java class hierarchy and has no superclass itself. All Java classes inherit the methods defined by Object.

• Method overloading is the practice of defining multiple methods which have the same name but have different argument lists.

• Method overriding occurs when a class redefines a method inherited from its superclass.

• Dynamic method lookup ensures that the correct method is invoked for an object, even when the object is an instance of a class that has overridden the method.

• static, private, and final methods cannot be overridden and are not subject to dynamic method lookup. This allows compiler optimizations such as inlining.

• From a subclass, you can explicitly invoke an overridden method of a superclass with the super keyword.

• You can explicitly refer to a shadowed variable with the super keyword.

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Summary - ConcludedSummary - Concluded

• Data and methods may be hidden or encapsulated within a class by specifying the private or protected visibility modifiers. Members declared public are visible everywhere.

• Members with no visibility modifiers are visible only within the package.

• An abstract method has no method body (i.e., no implementation).

• An abstract class contains abstract methods. The methods must be implemented in a subclass before the subclass can be instantiated.

• An interface is a collection of abstract methods and constants (static final variables).

• Declaring an interface creates a new data type.

• A class implements an interface by declaring the interface in its implements clause and by providing a method body for each of the

abstract methods in the interface.

Proprietary© 1997

University of Houston-Clear Lake

Software Development Java 1.1 - What’s New?Java 1.1 - What’s New?

Inner classes

Java Beans

A framework for defining reusable modular software components.

Internationalization

A variety of new features that make it possible to write programs that run around the globe.

New event model

A new model for handling events in graphical user interfaces that should make it easier to create those interfaces.

Other new AWT features

The Java 1.1 AWT includes support for printing, cut-and-paste, popup menus, menu shortcuts, and focus traversal. It has improved support for colors, fonts, cursors, scrolling, image manipulation, and clipping.

Applets

JAR files allow all of an applet's files to be grouped into a single archive. Digital signatures allow trusted applets to run with fewer security restrictions. The HTML <APPLET> tag has new features.

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Java 1.1 - More of What’s New?Java 1.1 - More of What’s New?

Object serialization

Objects can now be easily "serialized" and sent over the network or written to disk for persistent storage.

Security

Java 1.1 includes a new package that supports digital signatures, message digests, key management, and access control lists.

Java Database Connectivity (JDBC)

A new package that allows Java programs to send SQL queries to database servers. It includes a "bridge" that allows it to inter-operate with existing ODBC database servers.

Remote Method Invocation (RMI)

An interface that supports distributed Java applications in which a program running on one computer can invoke methods of Java objects that exist on a different computer.

Proprietary© 1997

University of Houston-Clear Lake

Software Development

23 Packages in the Core Java API23 Packages in the Core Java API

• java.applet

• java.awt, java.awt.datatransfer, java.awt.event,

java.awt.image, java.awt.peer

• java.beans, java.reflect

• java.io, java.lang, java.math, java.net

• java.rmi, java.rmi.dgc, java.rmi.registry, java.rmi.server

• java.security, java.security.acl, java.security.interfaces

• java.sql, java.text, java.util, java.util.zip

Proprietary© 1997

University of Houston-Clear Lake

Software Development

“Deprecated” Features“Deprecated” Features

• Compile using the -deprecation flag, and

javac provides a detailed warning about each

use of a deprecated feature

• For example, the old AWT event-handling

model has been deprecated in Java 1.1

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Input to a Java ProgramInput to a Java Program

• When a file is opened in Java, an object is created with an associated stream

• Three stream objects are created for you when your Java program begins executing

– System.in - the object representing the standard or default input stream

– System.out - the standard output stream

– System.err - the standard error output stream

• Normally input from the keyboard is read with

System.in.read(); // read a character

Proprietary© 1997

University of Houston-Clear Lake

Software Development Example of ReadingExample of Reading

import java.io.IOException;class InputDemo { public static void main(String args[ ]) { try { // Input a single character System.out.println("Type a character:"); char ch = (char)System.in.read( ); System.out.println("You entered: " + ch); ch = (char)System.in.read( ); // Ignore new line // Input a string System.out.println("Type a string:"); StringBuffer s = new StringBuffer( ); while ((ch = (char)System.in.read( )) != '\n') s.append(ch); System.out.println("You entered: " + s); } catch (IOException e) { System.out.println("Input error detected"); } }}

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Reading CharactersReading Characters

• System.in.read(); // read a character

• Characters can be stored in most integer data types because they are represented as 2-byte integers

• We can treat a character as either an integer or a character depending upon its use

• To skip a character, you may use read() or you may use System.in.skip(2)

Since 2 bytes need to be read in

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Reading Other TypesReading Other Types

• System.in.readBoolean();

• System.in.readByte();

• System.in.readChar();

• System.in.readShort();

• System.in.readInt();

• System.in.readLong();

• System.in.readFloat();

• System.in.readDouble();

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Reading Still Other TypesReading Still Other Types

• System.in.readUnsignedByte();

• System.in.readUnsignedShort();

• System.in.readUTF();

// Unicode formatted strings

• System.in.readFully();

// byte arrays

Proprietary© 1997

University of Houston-Clear Lake

Software Development

File StreamsFile Streams

Keyboard Program Display

Input File Program Output File

Standard Input Stream

Standard Output Stream

FileOutput Stream

File Input Stream

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Opening a FileOpening a File

FileInputStream inFile = new FileInputStream(“in.dat”);

StreamTokenizer tokns = new StreamTokenizer(inFile);

tokns.nextToken(); int y = (int) tokns.nval;

Sufficient to read a byte stream

Allows the reading of tokens from the stream

To get next token from the stream

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Getting Several TokensGetting Several Tokens

import java.io.*;

public class Demo {

public static void main( String argv[ ]) {

FileInputStream inFile = new FileInputStream(“in.dat”);

StreamTokenizer tokns = new StreamTokenizer(inFile);

while (tokns.nextToken() != tokns.TT_EOF) {

tokns.nextToken(); int x = (int) tokns.nval;

tokns.nextToken(); float y = (float) tokns.nval;

System.out.println(“x is “ + x + ” and y is “ + y);

}

inFile.close( );

}

Creates a a tokenizer from a file input stream

Proprietary© 1997

University of Houston-Clear Lake

Software Development Labels, Breaks, and ContinueLabels, Breaks, and Continue

class LabelExample { public static void main(String args[ ]) { int i, j; Outer: for ( i = 1; i < 100; i++) { System.out.println("\nOuter loop - " + i); Inner: for ( j = 1; j < 10; j++) { if ( j % 2 == 0) continue Inner; // Skip even j values if ( i > 4) break Outer; // Abort if i > 4 System.out.println( "j = " + j); } // end of inner for statement } // end of outer for statement System.out.println("Program exiting at OuterLoop:"); } // end of main( ) method} // end of class declaration

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Static Methods May not Reference Instance VariablesStatic Methods May not Reference Instance Variables

Class Goofy {

static int j; // a class variable

int k; // an instance variable

public static void main(String args[ ]) {

j = 3; // Okay

k = 2; // Oops - will not compile

}

}

Proprietary© 1997

University of Houston-Clear Lake

Software Development Finalizer ExampleFinalizer Exampleclass AnyClass { AnyClass( ) { System.out.println("Inside AnyClass( ) constructor"); } protected void finalize( ) { System.out.println("Inside AnyClass( ) destructor"); }}

class FinalDemo { public static void f( ) { System.out.println( "Start method f( )"); AnyClass obj1 = new AnyClass( ); System.out.println( "End method f( )"); } public static void main( String args[ ]) { System.out.println("Start method main( )"); f( ); AnyClass obj2 = new AnyClass( ); System.out.println( "End method main( )"); }}

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Executing the ExampleExecuting the Example

% javac FinalDemo.java

% java FinalDemo

Start method main( )

Start method f( )

Inside AnyClass( ) constructor

End method f( )

Inside AnyClass( ) constructor

End method main( )

%

Note: No output from the destructor calls! The memory wasn’t needed !

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Random NumbersRandom Numbers

class RandomDemo { public static void main(String args[ ]) { int rows, cols; StringBuffer buffer; for (rows = 1; rows <= 8; rows++) { buffer = new StringBuffer(128); for (cols = 1; cols <= 5; cols++) buffer.append(Math.random( ) + " \t"); System.out.println(buffer); } }}

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Random Number ExampleRandom Number Example

% javac RandomDemo.java% java RandomDemo

%

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Example AppletExample Applet

import java.awt.Graphics;

public class Howdy extends java.applet.Applet {

public void paint(Graphics g) {

g.drawString("Howdy", 100, 40);

}

}

Proprietary© 1997

University of Houston-Clear Lake

Software Development

Example HTMLExample HTML

<html><head><title>Howdy Example</title></head><body><hr><applet code=Howdy.class id=Howdy width=300 height=200 ></applet><hr><a href="Howdy.java">Source Code</a></body></html>