Why do we need super keyword to call default interface methods?

Question

In the below code when I am having a class implementing two interfaces with same default method signature it ask me to override it. but in the overriden method why I have to

Answer 1

_{Based on comment from @MalteHartwig.}

Direct quote from Java tutorial:

If two or more independently defined default methods conflict, or a default method conflicts with an abstract method, then the Java compiler produces a compiler error. You must explicitly override the supertype methods.

Consider the example about computer-controlled cars that can now fly. You have two interfaces (OperateCar and FlyCar) that provide default implementations for the same method, (startEngine):

public interface OperateCar {
    // ...
    default public int startEngine(EncryptedKey key) {
        // Implementation
    }
}

public interface FlyCar {
    // ...
    default public int startEngine(EncryptedKey key) {
        // Implementation
    }
}

A class that implements both OperateCar and FlyCar must override the method startEngine. You could invoke any of the of the default implementations with the super keyword.

public class FlyingCar implements OperateCar, FlyCar {
    // ...
    public int startEngine(EncryptedKey key) {
        FlyCar.super.startEngine(key);
        OperateCar.super.startEngine(key);
    }
}

The name preceding super (in this example, FlyCar or OperateCar) must refer to a direct superinterface that defines or inherits a default for the invoked method. This form of method invocation is not restricted to differentiating between multiple implemented interfaces that contain default methods with the same signature. You can use the super keyword to invoke a default method in both classes and interfaces.