Re: multiple inheritance in Java

lipska the kat <"nospam at neversurrender dot co dot uk">
Wed, 03 Jul 2013 13:01:19 +0100
On 03/07/13 10:46, Steven Simpson wrote:

On 02/07/13 19:53, Lew wrote:

WIth Java 8 don't we get default methods? You can multiply-inherit

I don't see how default methods help here. You might define Position as
an interface:

   interface Position {
     int getX();
     int getY();
     void setX(int x);
     void setY(int y);

...but what default implementations can you give any of these methods
without adding some state to Position?

This is an interesting question
It is possible to add state to an interface so that it is *almost* a class

The following compiles and runs on 64 bit Ubuntu Linux with

java version "1.8.0-ea"
Java(TM) SE Runtime Environment (build 1.8.0-ea-b96)
Java HotSpot(TM) 64-Bit Server VM (build 25.0-b38, mixed mode)

javac 1.8.0-ea

I haven't figured out all the mechanics or ramifications of this yet but
it certainly looks like it is effectively possible to implement a form
of multiple inheritance in Java. Not exactly an edifying prospect.


/* public instance variables for brevity */
class State{
     public Integer a = 0;
     public Integer b = 0;

interface Foo{

    /* explicitly static and final */
    Integer i = 0;
    State state = new State(); /* final reference, mutable instance */

    default public void doFoo(Integer x){
        //i = 10; /* illegal */
        Integer y = x + i;
        state.a = y + y;
        System.out.println("doingFoo, i + x is " + y);
        System.out.println("doingFoo, accessing state state.a is " +

interface Bar{

    Integer i = 0;
    State state = new State();

    default public void doBar(Integer x){
        Integer y = x + i;
        state.a = y + y;
        System.out.println("doingBar, i + x is " + y);
        System.out.println("doingBar, accessing state state.a is " +

/* who owns the state? */
class FooBarImpl implements Foo, Bar{

public class Test{
     public static void main(String[] args){
         FooBarImpl fbi = new FooBarImpl();


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