Are Default Methods in JDK 8 Considered a Form of Multiple Inheritance in Java?

Do default methods in JDK 8 represent a form of multiple inheritance in Java?

JDK 8 introduced default methods in interfaces, allowing developers to add new methods with default implementations without breaking existing code. This feature can resemble multiple inheritance since a class can implement multiple interfaces that provide default methods. However, Java still enforces specific rules to manage method resolution and avoid ambiguity when conflicts arise.

public interface Attendance {
    default boolean present() { return true; }
}

public interface Timeline {
    default boolean present() { return false; }
}

public class TimeTravelingStudent implements Attendance, Timeline {
    @Override
    public boolean present() { return Attendance.super.present(); } // Resolving ambiguity
}

TimeTravelingStudent student = new TimeTravelingStudent(); 
System.out.println(student.present()); // Outputs: true

Causes

• Default methods allow interfaces to evolve without affecting existing implementations.
• A class can implement multiple interfaces, each potentially providing a default method with the same signature.

Solutions

• If a class implements multiple interfaces with conflicting default methods, the compiler will require an explicit override of the method.
• You can resolve ambiguity by providing a concrete implementation in the class that implements the interfaces.

Mistake: Failing to override a method when implementing multiple interfaces with the same default method signature.

Solution: Always provide an explicit implementation of the default method to resolve any conflicts.

Mistake: Assuming default methods are the same as instance methods.

Solution: Understand that default methods are still part of the interface and follow interface rules.