InterfaceTest.java

import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

public class InterfaceTest {

    interface TestInterface {
        default Set<String> getRequiredStrings() {
            return Collections.emptySet();
        }
    }

    static class B implements TestInterface {}
    static class C extends B {}

    static class D extends C {
        @Override
        public Set<String> getRequiredStrings() {
            return new HashSet<>(Arrays.asList("Not an empty set"));
        }
    }

    public static void main(String[] args) {
        TestInterface d = new D();
        System.out.println("d.getRequiredStrings() = " + d.getRequiredStrings());
    }
}

`public class InterfaceTest {

interface TestInterface {
    default Set<String> getRequiredStrings() {
        return Collections.emptySet();
    }
}

public abstract class B implements TestInterface { // few method implementations and abstract methods}
public abstract class C extends B {//few method implementations}

public class D extends C {
    @Override
    public Set<String> getRequiredStrings() {
        return new HashSet<>(Arrays.asList("Not an empty set"));
    }
}

public static void main(String[] args) {
    TestInterface d = new D();
    System.out.println("d.getRequiredStrings() = " + d.getRequiredStrings());
}

}`