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javainheritanceabstract-classmultiple-inheritance
Gian Laager
Gian Laager

Reputation: 514

Alternative to Multiple Inheritance to Extend Library

I'm currently trying to extend WPILib. Unfortunately I would need multiple inheritance.

I've drawn a uml diagram of the problem (forgive me my crappy handwriting) enter image description here

As you can see with the red arrows I would need to extend from two classes.

I can't change any thing inside the WPI box and the requires method from ICommand needs some functions from CommandBase so I can't make it default in ICommand.

How can I solve this?

If you want to try some thing out here's my github repo.

Upvotes: 2

Views: 112

Answers (2)

michal3377
michal3377

Reputation: 1462

If I understood correctly, you want to have your custom classes (SequentialCommandGroup, ParallelCommandGroup) to extend from library classes, but also to have your own implementation for that "Command" part.

You cannot have multiple inheritance in Java of course, but I think you could utilize Proxy pattern here.

Your classes would still inherit from Library classes, but the implementation of ICommand interface could be delegated to Command object.

Example:

public class MyParallelCommandGroup extends ParallelCommandGroup implements ICommand {

    private Command commandProxy;
    
    public MyParallelCommandGroup() {
        // instantiate commandProxy here, or inject it as constructor param
    }
    
    @Override
    public void requires() {
        commandProxy.requires();
    }
    
}

You could even extract whole Command class to an interface and implement it in your proxy class. I'm not sure whether this covers your use-case, but maybe it will help you to find a proper solution.

Upvotes: 3

Monu Rohilla
Monu Rohilla

Reputation: 665

Use Interface because java is not supports Multiple Inheritance. Interfaces are abstract types that specify behaviors for classes to implement. Because they are abstract, interfaces do not contain detailed instructions for their behaviors. Instead, the classes provide concrete implementations of interface behaviors.

Interfaces have many defining characteristics:

  • Unlike classes, you don't instantiate interfaces. Instead, classes implement interfaces

  • Interfaces contain only public constant definitions and method headers

  • Interfaces can only extend other interfaces, not classes

  • Interfaces can extend multiple interfaces, and classes can implement multiple interfaces

Read more about Inheritance

Upvotes: 0

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