'Back-End/Java_1'에 해당되는 글 48건

 Super keyword

- The super keyword is a reference variable that is used to refer immediate parent class object

1. Usage

- super is used to refer immediate parent class instance variable

- super() is used to invoke immediate parent class constructor

- super is used to invoke immediate parent class method

2. parent class instance variable

class Vehicle{ int speed = 50; } public class Bike extends Vehicle{ int speed = 100; void display(){ System.out.println(super.speed); } public static void main(String[]args){ Bike a = new Bike(); a.display(); } }

- 50

3. To invoke parent class constructor

class Vehicle{ Vehicle(){ System.out.println("Vehicle"); } } public class Bike extends Vehicle{ Bike(){ super(); System.out.println("Bike"); } public static void main(String[]args){ Bike a = new Bike(); } }  

- vehicle, bike

- super() is added in each class constructor automatically by compiler

- If In the above example Bike doesn't have a super(), It will print out same result

4. To invoke parent class method

- It should be used in case subclass contains the same method as parent class

package stepByStep; class Vehicle{ void message(){ System.out.println("Vehicle"); } } public class Bike extends Vehicle{ void message(){ System.out.println("Bike"); } void display(){ message(); super.message(); } public static void main(String[]args){ Bike a = new Bike(); a.display(); } }  

- In case there is no method in subclass as parent, there is no need to use super.

class Vehicle{ void message(){ System.out.println("Vehicle"); } } public class Bike extends Vehicle{ void display(){ message(); } public static void main(String[]args){ Bike a = new Bike(); a.display(); } }  

 Covariant Return type

- The covariant return type specifies that the return type may vary in the same direction as the subclass

- Since Java5, it is possible to override method by changing the return type if subclass overrides and method whose return type is Non-primitive

1. Example

package forLogic; class A{ A get(){ return this; } } public class Test extends A{ Test get(){ return this; } void message(){ System.out.print("covariant"); } public static void main(String[]args){ new Test().get().message(); } }

- The return type of the get() of A class is A, but the return type of the get() of Test class is Test

- Both methods have different return type but it is overriding

Overloading vs Overriding

Method Overriding

- If subclass has the same method as declared in the parent class, it is known as method overriding

1. Usage

- Method overriding is used to provide specific implementation of a method that is already provided by its super class

- Method overriding is used for runtime polymorphism

2. Rule

- method must have same name as in the parent

- method must have same parameter

3. Problem without method overriding

class boss{ void run(){ System.out.println("I'm the boss"); } } public class Emp extends boss{ public static void main(String[]args){ Emp obj = new Emp(); obj.run(); } }

- Problem is that I have to provide a specific implementation of run() method

4. Example

package forLogic; class Vehicle{ String part(){ return "body"; } } class Bm extends Vehicle{ String part(){ return "tire"; } } class au extends Vehicle{ String part(){ return "window"; } } public class Test { public static void main(String[]args){ Bm t = new Bm(); au a = new au(); System.out.print(t.part()); System.out.print(a.part()); } }  

5. Question

1) Can we override static method?

- No, static method can't be overriden. It can be proved by runtime polymorphism

2) Why we can't overriden static method?

- Static method is bound with class whereas instance method is bound with object

- Static belongs to class area

- instance belongs to heap area