Multiple Inheritance in Java


2 min read

See the Java code.

Ever since Java 8 introduced default interface methods, I felt there had to be a way to use it for multiple inheritance. I have never needed it, but I was bored for a bit today, so decided to try the following idea:

  • Create a non-public class XData to hold the fields the interface needs to work with, and a public interface X in the same source file

  • The interface has one virtual method getXData() that returns the XData instance

  • The remaining interface methods are default methods that call getXData() to read and write the fields as necessary to perform some useful operations.

  • Create another interface Y and class YData in the same pattern

  • Create a class XY that implements both interfaces X and Y

  • The class XY has XData and YData fields, which are returned by getXData() and getYData(). These are the only two interface methods XY is required to inplement.

  • I didn't bother in my example, but XY would have to decide how to implement the Object methods hashCode, equals, and toString. These methods cannot be implemented by interfaces (but they could be implemented in XData and YData classes, if desired)

The end result is the XY class is an instance of both X and Y interfaces, and inherits the encapsulated behaviours of both X and Y default methods - multiple inheritance by any reasonable measure.

It is better than C++ multiple inheritance, in that all constructor logic and declared fields are in the XY class. The only problem you could get is if X and Y interfaces declare the same method that differs only by return type - but this is just a general problem of implementing multiple interfaces.