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Original Post: Java 8 Interface Static and Default Methods
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Java 8 introduced two new methods to declare they are 1.default methods 2.static methods
By this interfaces and abstract class are same but still having differences like abstract class can have a constructor etc will discuss more before that we need to know about these Java 8 features of interfaces.
Defaults methods are also called as defender methods can be implemented inside the interface
Like normal class now with java 8 we can declare static methods in side a interface.
Lets jump deep into Java 8 default and static methods
1.Interface Default Methods in Java 8
Before Java 8 in interfaces we can and able to declare only abstract methods only.
If we declare a method without abstract that will be treated as abstract by default.
As we know all methods in interfaces are by default abstract methods.
These methods wont have body means implementaions
The class which is implementing this interface need to provide body / implementation for this abstract methods.
Now with java 8 default methods we can add methods to interface without disturbing existing functionality.
So instead of overriding now we can inherit these default methods from interfaces
package com.instanceofjava;
interface Java8Interface{
abstract void show();
default void display(){
System.out.println("default method of interface");
}
}
package com.instanceofjava;
class Sample implements Java8Interface {
void show(){
System.out.print("overridden method ")
}
public static void main(String[] args){
Sample obj= new Sample();
obj.show(); // calling implemented method
obj.display(); // calling inherited method
Java8Interface.display(); calling using interface name
}
}
Output:
overridden method
default method of interface
default method of interface
How to call default methods:
We can all these default methods by using interface name and also by using object of the class which is implementing.
From above example
obj.show(); // calling implemented method
obj.display(); // calling inherited method
Java8Interface.display(); calling using interface name
Can we override java 8 default method
As we discussed above default methods in interfaces are implemented methods with bodies
Yes we can override same method in class which is implementing this interface.
Lets see one sample program how to override and what happens if we override
package com.instanceofjava;
interface InterfaceWithDefault{
default void defMethod(){
System.out.println("default method of interface");
}
}
package com.instanceofjava;
class Demo implements InterfaceWithDefault{
void defMethod(){
System.out.print("overridden method in class Demo ")
}
public static void main(String[] args){
Demo obj= new Demo();
obj.defMethod(); // calling overridden method
Java8Interface.defMethod(); calling using interface name : interface defMethod will be called
}
}
Output:
overridden method in class Demo
default method of interface
What happens if we implement two interfaces having same default methods
Now lets see if a class implementing two interfaces which are having same default methods
Whatever the implementation in the two interfaces defined if we implementing two interfaces which are having a default method in both then compilation error will come if two methods have same signature. works fine if two methods have same name with different arguments.
Check the below example programs to understand more.
package com.instanceofjava;
interface A{
default void defMethod(){
System.out.println("default method of interface: A");
}
}
package com.instanceofjava;
interface B{
default void defMethod(){
System.out.println("default method of interface: B");
}
}
package com.instanceofjava;
class Demo implements A, B{ // compilation error will come
public static void main(String[] args){
Demo obj= new Demo();
}
}
If we implement two interfaces which are having same method with same parameters then compilation error will occur.
Duplicate default methods named "defMethod" with the parameters () and () are inherited from the types A and B.
If we define two methods with different type of parameters then we can work with both interfaces.
package com.instanceofjava;
interface A{
default void defMethod(){
System.out.println("Default method of interface: A");
}
}
package com.instanceofjava;
interface B{
default void defMethod(String str){
System.out.println("Default method of interface: B");
System.out.println(str);
}
}
package com.instanceofjava;
class Demo implements A, B{ // compilation error will come
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