Are defaults in JDK 8 a form of multiple inheritance in Java?

A new feature coming in JDK 8 allows you to add to an existing interface while preserving binary compatibility.

The syntax is like

public interface SomeInterface() {
  void existingInterface();
  void newInterface() default SomeClass.defaultImplementation;
}

This way for all existing implementations of SomeInterface when they upgrade to this new version they don't all suddenly have compiles errors around newInterface().

While this is neat, what happens when you are implementing two interfaces which both have added a new defaulted method which you did not implement? Let me explain with an example.

public interface Attendance {
   boolean present() default DefaultAttendance.present;
}

public interface Timeline {
   boolean present() default DefaultTimeline.present;
}

public class TimeTravelingStudent implements Attendance, Timeline {

}

// which code gets called?
new TimeTravelingStudent().present();

Has this been defined as part of JDK 8 yet?

I found the Java gods talking about something similar here http://cs.oswego.edu/pipermail/lambda-lib/2011-February/000068.html, but its part of private mailing list and I cannot ask them directly.

See this for more details on how defaults are going to be used in JDK 8 and extending the Collection interface to support lambdas: https://oracleus.wingateweb.com/published/oracleus2011/sessions/25066/25066_Cho223662.pdf

The answer to the duplicate operation is:

> To solve multiple inheritance issue a class implementing two interfaces providing a default implementation for the same method name and signature must provide an implementation of the method. [Full Article]

My answer to your question is: Yes, it is a form of multiple inheritance, because you can inherit behavior from different parents. What's missing is to inherit states, i. e., attributes.

I know this is a old post, but as i'm working with this stuff...

You will have an error from the compiler, telling you that:

> class TimeTravelingStudent inherits unrelated defaults for present() from types Attendance and Timeline > reference to present is ambiguous, both method present() in Timeline and method present() in Attendance match.

There are two scenarios:

1. First, that was mentioned, where there is no most specific interface

   public interface A { default void doStuff(){ /* implementation */ } }

   public interface B { default void doStuff() { /* implementation */ } }

   public class C implements A, B { // option 1: own implementation // OR // option 2: use new syntax to call specific interface or face compilation error void doStuff(){ B.super.doStuff(); } }

2. Second, when there IS a more specific interface:

   public interface A {
       default void doStuff() { /* implementation */ } 
    }
    
    public interface B extends A {
       default void doStuff() { /* implementation */ } 
    }
    
    public class C implements A, B {
    // will use method from B, as it is "closer" to C
    }
   

In short: it's a compile time error, must override the method by hand in the implementation.

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Purpose of default method

The major purpose to introduce default method in Java 8, is to make interface extendable, without breaking existing implementations (there are so many 3rd party Java libraries).

And multiple inheritance like in C++ is actually intended to be avoided, that's definitely not the purpose of default method in Java.

---

How to override

2 options:

• Override the method, with its own logic.
• Override the method, call one of the interface's method via super, format: <interface_name>.super.<method_name>();

Tips:

• method from interface is default to public, so don't forget to add public keyword when override it.

> My answer to your question is: Yes, it is a form of multiple inheritance, because you can inherit behavior from different parents. What's missing is to inherit states, i. e., attributes.

Yes, but you can add getters and setters to your interface that the implementing classes then must implement. Nevertheless, the implementing classes don't inherit attributes. So, AFAICS, it's more like a trait-style solution rather than a multiple inheritance style solution.

If anyone's still looking for an answer, in case a class implements two interfaces with the same default method then the class needs to resolve the disambiguity by providing an implementation of its own. Look at this tutorial for more details on how inheritance in default methods work.

"How will we distinguish the methods" was a question that was put on Stackoverflow and referred to this question https://stackoverflow.com/questions/15968987/concrete-methods-in-interfaces-java1-8

The following is an example that should answer that question:

interface A{
default public void m(){
System.out.println("Interface A: m()");
}
}

interface B{
default public void m(){
System.out.println("Interface B: m()");
}
}

 class C implements A,B { 
     
 public void m(){
  System.out.println("Concrete C: m()");   
 }

public static void main(String[] args) {
   C aC = new C();
   aC.m();
   new A(){}.m();
   new B(){}.m();
}
}

Class C above must implement its own concrete method of the interfaces A and B. Namely:

 public void m(){
  System.out.println("Interface C: m()");   
 }

To call a concrete implementation of a method from a specific interface, you can instantiate the interface and explicitly call the concrete method of that interface

For example, the following code calls the concrete implementation of the method m() from interface A:

new A(){}.m();

The output of the above would be:

Interface A: m()

As far as I see it, it is no multiple inheritance because they are stateless. So virtual extension methods don't support full object or class functionality.