Java Classes methods and inheritance

Java Classes methods and inheritance

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  Classes, Methods and Inheritance SCJP / OCJP objectives : 1.1, 1.2, 1.3, 1.4, 5.5 By, www.JAVA9S.com Srinivas Reddy.S
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  Declaring Classes Syntax: package com.java9s.ocjp; import com.sun.sample; class Car{ int speed;  State void move(){  behaviour //code related to move } } Save the file with Car.java.  If there are multiple classes in a file, the file name should be the name of the class with public access modifier. www.JAVA9S.com
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  Declaring classes -Rules • The package statement should be the first statement in a file if the class belongs to a package. • Import statements comes next to package statement. • The order packageimportclass should be maintained. • Comments can come anywhere in the java file. • A java file can have any number of non public class files www.JAVA9S.com
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  Declaring classes -Rules • package and import statements declared in a file apply to all the classes defined in the file. • A separate .class file will be generated for each class defined in the java file. • Any name can be given to a file when there is no public class declared in it. www.JAVA9S.com
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  Creating Objects Car c = new Car(); Instantiation Declaration C is the reference which holds the Car Speed=50 memory address of the Car object C www.JAVA9S.com
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  Creating Objects Car a = new Car(); a Car b = new Car(); b Car c = new Car(); c Car d = new Car(); d Car e = d; e d.speed =60; System.out.println(e.speed) ;-> 60 www.JAVA9S.com
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  Methods • Methods are members of a class. • Methods have the behavior of an object. • Methods can be declared with or without arguments. • Two variants for a method: – Methods that return something – Methods that don’t return anything - void www.JAVA9S.com
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  Methods – Returntype Syntax: type methodName(arguments){ //code that decides the methods return x; } E.g., int addition(int a, int b){ int c = a+b; return c; } www.JAVA9S.com
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  Methods – voidtype Syntax: void methodName(arguments){ //Method code.. No need to return anything. } E.g., void saveToFile(String message){ //Code related to saving message to file.. } www.JAVA9S.com
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  Method – withoutargument Method with No arguments and with a return type: Date getCurrentDate(){ return Calender.get(Calender.DAY_OF_MONTH); } Method with no argument and no return type void printCurrentDate(){ System.out.println(Calendar.get(Calender.DAY_OF_MONTH)); } www.JAVA9S.com
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  Inheritance • Inheritance is a way to reuse code from already existing types or objects. • Inheritance is implemented between two classes using extends keyword. • When a class extends another class, extending class is called subclass and extended class is super class. www.JAVA9S.com
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  Inheritance class Car{ void move(){ •Car is super class System.out.println(“Moves”); •Ford is subclass. } } class Ford extends Car{ Ford f = new Ford(); f.moveFast(); void moveFast(){ f.move(); System.out.println(“Moves Fast”); } } www.JAVA9S.com
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  Inheritance • With inheritance, all the members of super class are available to the subclass objects. • When a class extends another class, it has an IS-A relation with its super class. • instanceof keyword can be used to confirm IS-A relationship. www.JAVA9S.com
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  Right or Wrong?? class Car{ } class Ford extends Car{ } class BMW extends Car{ } f instanceOf Car Ford f = new Ford(); f instanceOf BMW BMW b = new BMW(); Car c = new Car(); b instanceOf Ford b instanceOf Car c instanceOf Ford www.JAVA9S.com
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  super and thiskeywords • super is used to access the super class members. • this is used to access the currently executing objects members. www.JAVA9S.com
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  super - example class Car{ int speed; } class Ford extends Car{ int speed; void move(){ System.out.println(“Moving with car speed:”+super.speed) } } www.JAVA9S.com
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  this - example class Ford{ int price; int price; setFordPrice(int price){ setFordPrice(int price){ this.price = price; this.price = price; } } } a Ford a = new Ford(); a.setFordPrice(3000); ‘this’ is not mandatory. But if you have local variables declared inside a method, to avoid confusion, this can be used to refer to members of the object. www.JAVA9S.com
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  HAS-A relationship class Student{ Pen p = new Pen(); } class Pen{ } Student HAS-A pen. HAS – A relationship helps to reduce the complexity of the classes by the composition of the other classes. www.JAVA9S.com
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