1Objects and ClassesCS211Fall 20002Much of Java Looks Like C■ Goal was to make a programming language that people would pick up easily■ There are lots of C and C++ programmers, so make it much like C■ Arithmetic & relational operators are the same:+, -, *, / and <, >, <=, >=■ Assignment is the same: a = b;■ Conditional & looping statements are the same:if/else, while, for, do, break, continue, switch■ Arrays are the same:a[i] and b[i][j]3What’s Different?■ Java allows method overloading● C++ does this, but C does not● C++ also allows operator overloading; Java does not■ The Java numeric types all conform to IEEE standards● C numeric types can vary depending on platform■ Java does not have explicit pointers■ In Java, there is a separate String class● A String is not the same as an array of characters and it is notterminated by the NUL character■ Java does automatic Garbage Collection■ Many other differences…■ Java is claimed to be safer, more portable, and easier to use than C++4Object Oriented Programming■ This is a style of programming based on the ideas of● Objects● Classes● Inheritance■ Java is based on these ideas■ Currently, this is the best of known programming styles■ An object is a software bundle of data and related operations (the operations are called methods in Java)■ A class is a template that defines objects of a certain kind■ Using one class, I can create several objects, where each is an instanceof this class5Simple Inheritance■ Classes can be defined in terms of other classes● If a new class B is based on a previous class A then▲ B is a subclass of A▲ A is a superclass of B■ In general, the variables and operations of a class are available to its subclasses■ Some classes in Java● String● Vector● Stack● Hashtable■ Stack is a subclass of Vector which is a subclassof Object■ All Java classes are subclasses of Object6Java Programs■ A Java program consists of a number of interacting classes● All methods and all variables reside within some class■ When an application runs● You specify a class● The “system” looks for and runs the method that looks like▲ public static void main(String[ ] args)27Java Programs: Applets■ When a Java Applet runs● The web page specifies a class ● The “system” looks for these methods▲ public void init()▼ Runs when Applet is first loaded▲ public void start()▼ Runs when Applet appears on screen▲ public void stop()▼ Runs when Applet is off screen▲ public void destroy()▼ Runs when Applet is terminating8Object Basics■ Primitive types in Java:● byte, short, int, long● float, double● char● boolean■ Everything else is an Object● Each object is an instance of a Java class● There are manypredefined Java classes■ Operators (with one exception) work only on primitive types● What’s the exception?■ Each Java variable holds one of two things:● a primitive type or● a reference to an object9A Simple Example Classpublic class Thing {private int value;public static int count;public void setValue (int v) { value = v; }public int getValue ( ) { return value; }// Plus other methods}WhenThing t = new Thing( );is executed, an object is created that looks like thisThingvaluecountgetValue( )setValue( )class nameattributesmethodsWarning: The picture suggests that each object gets its own copy of each method. This provides some good intuition, but is not really true…10■ private?■ static?■ static members vs. instance members?■ function vs. procedure?■ accessor methods vs. modifier methods?Some Terminologypublic class Thing {private int value;public static int count;public void setValue (int v) { value = v; }public int getValue ( ) { return value; }// Plus other methods}11Objects vs. ReferencesWhenThing t = new Thing ( );is executed, the variable s does not contain the object■ Instead, it contains a reference to the object■ In other words, t contains the address of the place in memory where the object is stored■ Pictorially, we represent this as■ In fact, it’s more like thisThe objecttThe objecttaddress 225432254312What happens?Thing t1;Thing t2;t1 = new Thing ( );t2 = t1;t2.setValue(4);System.out.println(t2.getValue( ));t2 = new Thing ( );System.out.println(t1.getValue( ));System.out.println(Thing.count);Object vs. Reference Examplepublic class Thing {private int value;public static int count;public void setValue (int v) { value = v; }public int getValue ( ) { return value; }// A constructorpublic Thing ( ) { count++; }// Plus other methods}313Null■ What happens after the declaration, but before the assignment?Thing t1;// What has happened here?t1 = new Thing( );■ The variable t1 exists, but it contains no reference● It holds the special value null● null can be assigned to any object variable● null can be used in “==” tests14Equality■ The “==” operator in Java tests whether two variables contain the same value● For primitive types, this is what we want● For objects, this compares “addresses”■ Need an “equals( )” method that compares the contents of the objectWhat happens?Thing t1 = new Thing ( );Thing t2 = new Thing ( );t1.setValue(44);t2.setValue(44);System.out.println( t1 == t2 );15■ Every class automatically has an equals( ) method● The default equals( ) method is inherited from Object▲ This is usually not what you actually want▲ You often need to write your own equals( )An Improved Thing classpublic class Thing {private int value;public static int count;public void setValue (int v) { value = v; }public int getValue ( ) { return value; }// A constructorpublic Thing ( ) { count++; }// Equality testpublic boolean equals (Thing other) {return value == other.value;}// Plus other methods}16Assignment vs. Copying (Cloning)■ What happens if we really want to make a copy of an object?■ Can’t do it this way:Thing t1 = new Thing( );// Do stuff with t1; now make a copyThing t2 = new Thing( );t2 = t1;■ Instead we use the “clone( )” method:Thing t2 = t1.clone( );■ Can use inherited (from Object) clone( ) if class Thing implements Cloneablepublic class Thing {private int value;public static int count;public void setValue (int v) { value = v; }public int getValue ( ) { return value; }public Thing ( ) { count++; }public boolean equals (Thing other) {return value == other.value;}public Thing clone ( ) {Thing thing = new Thing( );thing.value = getValue(
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