Implementing two interfaces with same default methods.

In java8 , a new concept is introduced wherein we can provide implementation for methods in an interface using default keyword. But what will happen if a class implements two interfaces with same default methods , which method the implementing class will inherit. Let us see this in the following working example.

I have one interface, TestInterface1 with a default method test()

package com.test;

public interface TestInterface1 {

public default void test() {
System.out.println(“hi”);
}
}

An another interface, TestInterface2 too has implementation for method test()

package com.test;

public interface TestInterface2 {

default public void test() {
System.out.println(“hello”);
}
}

Now, if I have a class Test that implements above two interfaces , the compiler gives me an error “Duplicate default methods named test with the parameters () and () are inherited from the types TestInterface2 and TestInterface1”

package com.test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import static java.util.stream.Collectors.*;

import java.lang.reflect.Method;

public class Test implements TestInterface1,TestInterface2{

public static void main(String[] args) {
Test test=new Test();
test.test();

}

However, I can remove the above error by providing the compiler which interface method implementation I need in my implementation class.

package com.test;

public class Test implements TestInterface1,TestInterface2{

public static void main(String[] args) {
Test test=new Test();
test.test();

}

@Override
public void test() {
// TODO Auto-generated method stub
TestInterface1.super.test(); }
}

So, as shown above in my implementation class method I need to call the method with the interface name whose method implementation I need.

Behaviour of static variable in inheritance

Parent and child instances share a same copy of static variable. This I concluded running the following example.

package com.test;

public class Fruit {

static int count=0;

Fruit(){
count++;
}

public static void main(String[] args) {
Chickoo chickoo=new Chickoo();
System.out.println(chickoo.count);
Fruit fruit=new Fruit();
System.out.println(fruit.count);

}
}

class Chickoo extends Fruit{
Chickoo(){
count++;
}
}

The above code prints value of count as “2” and “3”.