returning a Void object

Question

What is the correct way to return a Void type, when it isn't a primitive? Eg. I currently use null as below.

interface B<E>{ E method(); }

class A implements B<Void>{

    public Void method(){
        // do something
        return null;
    }
}

Answer 1

The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void.

So any of the following would suffice:

• parameterizing with Object and returning new Object() or null
• parameterizing with Void and returning null
• parameterizing with a NullObject of yours

You can't make this method void, and anything else returns something. Since that something is ignored, you can return anything.

Answer 2

There is no generic type which will tell the compiler that a method returns nothing.

I believe the convention is to use Object when inheriting as a type parameter

OR

Propagate the type parameter up and then let users of your class instantiate using Object and assigning the object to a variable typed using a type-wildcard ?:

interface B<E>{ E method(); }

class A<T> implements B<T>{

    public T method(){
        // do something
        return null;
    }
}

A<?> a = new A<Object>();

Answer 3

If you just don't need anything as your type, you can use void. This can be used for implementing functions, or actions. You could then do something like this:

interface Action<T> {
    public T execute();
}

abstract class VoidAction implements Action<Void> {
    public Void execute() {
        executeInternal();
        return null;
    }

    abstract void executeInternal();
}

Or you could omit the abstract class, and do the return null in every action that doesn't require a return value yourself.

You could then use those actions like this:

Given a method

private static <T> T executeAction(Action<T> action) {
    return action.execute();
}

you can call it like

String result = executeAction(new Action<String>() {
    @Override
    public String execute() {
        //code here
        return "Return me!";
    }
});

or, for the void action (note that you're not assigning the result to anything)

executeAction(new VoidAction() {
    @Override
    public void executeInternal() {
        //code here
    }
});

Answer 4

May be this one will also help you: http://stackoverflow.com/questions/1177132/void-value-as-return-parameter