Person class. A class can be derived from an existing class by using the form

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1 Person class //Person.java - characteristics common to all people class Person { Person(String name) { this.name = name; void setage(int age) { this.age = age; void setgender(gender gender) { this.gender = gender; void setname(string name) { this.name = name; int getage() { return age; Gender getgender() { return gender; String getname() { return name; static enum Gender {M, F private String name; private int age; private Gender gender; A class can be derived from an existing class by using the form class class-name extends superclass-name { The new class is a subclass of the class from which it is derived, its superclass. The keyword extends defines this relationship. The subclass being defined automatically inherits all the data members and methods of its superclass. The keyword super is used to refer to the superclass object that is implicitly part of the subclass object.

2 Student class //Student.java - an example subclass class Student extends Person { Student(String name) { super(name); void setcollege(string college) { this.college = college; void setgpa(double gpa) { this.gpa = gpa; void setyear(yearinschool year) { this.year = year; String getcollege() { return college; double getgpa() { return gpa; YearInSchool getyear() { return year; static enum YearInSchool { frosh, sophmore, junior, senior private String college = "Unknown"; private YearInSchool year; // frosh, sophomore, private double gpa; //0.0 to 4.0 //StudentTest.java class StudentTest { public static void main(string[] args) { Student student = new Student("Jane Programmer"); student.setage(21); student.setgender(person.gender.f); student.setcollege("ucsc"); student.setyear(student.yearinschool.frosh); student.setgpa(3.75f); System.out.println(student.toString()); Example import java.awt.color; class Rectangle { private int x, y, height, width; public void draw() { public void move(int dx, int dy){ class ColoredRectangle extends Rectangle { private Color FillColor; public Color getfillcolor() { return this.fillcolor; public void setfillcolor(color c) { this.fillcolor = c; public void move(int dx, int dy){ Sub class can override the methods defined by the super class. Overridden methods should have same signature and same return type. Java implements Run Time Polymorphism by method overriding. Call to overridden methods is resolved at run time. Call to a overridden method is not decided by the type of reference variable. Rather it is decided by type of the object referred to.

3 class A { System.out.println("Hello This is show() in A"); B class overrides show() class B extends A { method from super class A System.out.println("Hello This is show() in B"); class override { public static void main(string args[]) { // super class reference variable // can point to sub class object A a1 = new A(); a1 = new B(); Call to show() of A class Call to show() of B class class A { void show(int a) { System.out.println("Hello This is show() in A"); class B extends A { System.out.println("Hello This is show() in B"); class override1 { public static void main(string args[]) { // A a1 = new B(); // A a1 = new A(); a1.show(10); B b1 = new B(); b1.show(10); b1.show(); Is this Method Overriding NO OUTPUT Hello This is show() in A Hello This is show() in A Hello This is show() in B Super class reference variable can refer to a sub class object. Super class variable if refers to sub class object can call only overridden methods. Call to an overridden method is decided by the type of object referred to. A a1 = new B(); // call to show() of B a1 = new C(); // call to show() of C a1 = new D(); // call to show() of D A B C D Assume show() Method is Overridden by sub classes class A { System.out.println("Hello This is show() in A"); class B extends A { System.out.println("Hello This is show() in B"); class C extends A { System.out.println("Hello This is show() in C"); class D extends A { System.out.println("Hello This is show() in D");

4 class override2 { public static void main(string args[]) { A a1 = new A(); a1 = new B(); a1 = new C(); a1 = new D(); Hello This is show() in A Hello This is show() in B Hello This is show() in C Hello This is show() in D class A {. class B extends A {. void show(int x) { void print() { A a1 = new B(); a1.show() ; // Valid a1.show(10); // Error!! a1.print(); // Error!! When a super class variable points to a sub class object, then it can only call overridden methods of the sub class. When you create an object of the derived class, it contains within it a subobject of the base class. It s essential that the base-class subobject be initialized correctly. perform the initialization in the constructor by calling the base-class constructor Java automatically inserts calls to the base-class constructor in the derived-class constructor. class Art { Art () { System.out.println("Art constructor"); class Drawing extends Art { Drawing () { System.out.println( Drawing constructor"); public class Cartoon extends Drawing { public Cartoon() { System.out.println("Cartoon constructor"); public static void main(string[] args) { Cartoon x = new Cartoon(); <Output> java Cartoon Art constructor Drawing constructor Cartoon construtor

5 If your class doesn t have default arguments, or if you want to call a base-class constructor that has an argument, you must explicitly write the calls to the base-class constructor using the super keyword and the appropriate argument list. class Game { Game (int i) { System.out.println( Game constructor"); class BoardGame extends Game { BoardGame (int i) { super (i); System.out.println( Game constructor"); public class Chess extends BoardGame { public Chess () { super (11); System.out.println( Chess constructor"); public static void main(string[] args) { Chess x = new Chess (); In Java, groups of related classes can be placed in a package. The first reason in for package access. A field or method that is defined without any of the access modifiers public, protected, or private can be accessed only from a method in a class from the same package. The second reason for using packages is to provide separate name spaces. Example: com.coolsoft.containers.stack, java.util.stack If you don t call the base-class constructor in BoardGame( ), the compiler will complain that it can t find a constructor of the form Game( ). 17 Whenever you want to use a class from an existing package, you need to use an import statement to import the class. Example: import java.util.*; import java.util.scanner; Every Java program can use classes from one unnamed package. Any classes in a single directory on your computer s disk that don t specify a package name are part of the same unnamed package. To place the classes in a named package, you need to do two things: (1) add a package statement to the top of each class in the package, and (2) place the classes in an appropriately named location on your computer s disk.

6 protected Example: Person and Student classes into a package called jbd.database //Person.java - characteristics common to all people package jbd.database; class Person { Person(String name) { this.name = name; //Student.java - an example subclass package jbd.database; class Student extends Person { Student(String name) { super(name); We need to place the source files in a subdirectory where the path to the subdirectory from the current directory is the same as the package name replacing any dots in the package name with the pathname separator for the local file system. Linux: jbd/databases/person.java, jbd/databases/student.java Compile: javac jbd/databases/*.java Access to a protected member from other classes is permitted only if the accessing class is a subclass of the class with the protected member. package jbd.database; public class Person { protected String name; protected int age; protected Gender gender; import jbd.database.*; class Driver extends Person { public boolean needsdrivered() { if (age < 25) return true; else return false; (1) Private Use this level whenever possible. You can make changes to private members by examining only the class containing the member. This restriction provides maximum information hiding. (2) Package Use this level when one class needs to be aware of some implementation details of another class in the same package. You may use package access for fields that are used heavily by other classes in the same package. (3) Protected Use this level when designing a class to be reused via inheritance from outside the package in which it is defined. You should carefully consider any decision to export a protected member. Such exporting seriously weakens information hiding and makes program maintenance significantly more difficult and error prone. (4) Public Reserve use of this level for the required interface elements to a class or package. You should never expose unnecessary implementation details with public access. Exporting a field with public access is very unusual, unless the field is also declared to be final. In Java, everything inherits from class Object. Object is generic, in that any operation applicable to an instance of the class Object is applicable to an instance of any class. Generic methods tostring () equals () Return true if elements have the same value. It returns true if and only if the two references refer to the same object. By overriding equals() we can write a generic methods that need to check if two objects are equal.

7 An abstract class is used to derive other (concrete) classes. An abstract class usually provides some complete method definitions, but leaves some undefined, requiring all subclasses to provide definitions for the undefined methods. abstract class class-name { abstract type method id(argument-list); //other concrete and abstract methods An abstract method is used to defer the implementation decision of the computation to a subclass. For example, we could create a collection of different types of counter classes, all with some common features. abstract public class AbstractCounter { protected int value; abstract public void click(); public int get() { return value; public void set(int x) { value = x; public class Counter extends AbstractCounter { public void click() { value = (value + 1) % 100; public class CounterByTwo extends AbstractCounter { public void click() { value = (value + 2) % 100; public class CounterTest { public static void main (String args[]) { // AbstractCounter absctr = new AbstractCounter (); // -> Error!! You cannot create an object of an abstract class. AbstractCounter absctr = new Counter (); absctr.click (); absctr.click (); System.out.println ( value = + absctr.get ()); absctr = new CounterByTwo (); absctr.click (); absctr.click (); System.out.println ( value = + absctr.get ()); Output : value = 2 value = 4 An interface in Java, is essentially an pure abstract class - all methods are abstract. An interface can contain only abstract methods, and constants. There are no instance variables. interface Instrument { // Compile-time constant: int I = 5; // static & final // Cannot have method definitions: void play(note n); String what(); void adjust(); An interface is allowed to have only public methods, and by convention the keyword public is omitted. Because all methods in an interface are abstract, the keyword abstract is also omitted.

8 Implementing an interface is like extending an abstract class. A new class implementing the interface uses the keyword implements. The class that implements the interface must provide an implementation for all methods in the interface. class Percussion implements Instrument { public void play (Note n) { System.out.println("Percussion.play() " + n ); public String what() { return "Percussion"; public void adjust() { class Wind implements Instrument { public void play (Note n) { System.out.println("Wind.play() " + n); public String what() { return "Wind"; public void adjust() { Just as with abstract classes, you can declare variables and parameters to be of an interface type, but you cannot create objects of an interface type. Using the declarations on the previous slides, new Instrument() would be illegal. You could declare a variable or parameter to be of type Instrument. This variable would always either be null or refer to an instance of some class that implements the interface Instrument. Because an interface has no implementation at all that is, there is no storage associated with an interface there s nothing to prevent many interfaces from being combined. This is valuable because there are times when you need to say An x is an a and a b and a c. In C++, this act of combining multiple class interfaces is called multiple inheritance. It carries some rather sticky baggage because each class can have an implementation. In Java, you can perform the same act, but only one of the classes can have an implementation. 31 In Java, If you inherit from a non-interface, you can inherit from only one. All the rest of the base elements must be interfaces. 32

9 interface CanFight { void fight(); interface CanSwim { void swim(); interface CanFly { void fly(); class ActionCharacter { public void fight() { class Hero extends ActionCharacter implements CanFight, CanSwim, CanFly { public void swim() { public void fly() { public class Adventure { public static void t (CanFight x) { x.fight(); public static void u (CanSwim x) { x.swim(); public static void v (CanFly x) { x.fly(); public static void w (ActionCharacter x) { x.fight (); public static void main(string[] args) { Hero h = new Hero(); t(h); // Treat it as a CanFight u(h); // Treat it as a CanSwim v(h); // Treat it as a CanFly w(h); // Treat it as an ActionCharacter 33 You can easily add new method declarations to an interface by using inheritance. You can also combine several interfaces into a new interface with inheritance. 34 interface Monster { void menace(); interface DangerousMonster extends Monster { void destroy(); interface Lethal { void kill(); interface Vampire extends DangerousMonster, Lethal { void drinkblood(); class DragonZilla implements DangerousMonster { public void menace() { public void destroy() { class VeryBadVampire implements Vampire { public void menace() { public void destroy() { public void kill() { public void drinkblood() { public class HorrorShow { static void u (Monster b) { b.menace(); static void v (DangerousMonster d) { d.menace(); d.destroy(); static void w (Lethal l) { l.kill(); public static void main(string[] args) { DangerousMonster barney = new DragonZilla(); u (barney); v (barney); Vampire vlad = new VeryBadVampire(); u (vlad); v (vlad); w (vlad); 35 36

10 public abstract class Shape { protected int x, y; public abstract double area(); public abstract double circumference(); public void move (int x1, int y1) { x += x1; y += y1; public class Circle exrtends Shape { public Circle () { r = 0; public double circumference () { return Math.PI * 2 * r; public double area () { return Math.PI * r * r; public int getradius () { return r; public void setradius (int r) { this.r = r; import java.awt.graphics; interface Drawable { void paint(graphics g); import java.awt.*; public class DrawableCircle extends Circle implements Drawable { public DrawableCircle() { this(0, 0, 0); public DrawableCircle (int x, int y, int r) { super(r); this.x = x; this.y = y; public void paint(graphics g) { g.drawoval (x-r, y-r, 2*r, 2*r); g.drawstring (String.valueOf(area()), x-r, y ); 37 if (x intanceof Shape) // x is a Shape, act accordingly else // x is not a Shape, act accordingly Person person = new Student(); if (person instanceof Student) { Student student = (Student)person; // operate on student Some illegal casts can be determined at compile time, others can only be detected at runtime. Trying to cast a Person into a String is a compile time error. The compiler knows that String is not a subclass of Person and hence the cast is always illegal. Trying to cast a Person into a Student, when the Person is not a Student, is a runtime error - even if YOU can see that this person couldn't possibly be a Student. The following can only be detected at runtime, even though you can see it is clearly illegal. Person person = new Person(); Student student = (Student)person;