Java simple callback

Question

It is basic problem. I have class A which gives some task to class B. When class B finish the tast it must notify class A. I want register A class method as callback in class B.

I really want do it in this way, not by observer pattern with interface Observable.

public class A
{
    public void A()
    { 
        B b = new B()
        b.registerCallback(callback);
    }

    private void callback()
    {

    }

}

public class B
{
    private ???? callbackoNotify;

    public class registerCallback(??? callback)
    {
        callbackoNotify = callback;
    }

    public void notify()
    { 
        callback();
    }
}

Imesha Sudasingha · Accepted Answer · 2017-02-26 02:19:42Z

4

You can define an interface for the callback.

interface Callback{
    void call();
}

Then, let class A implement it.

class A implements Callback{
    private B b;

    public A(){
        b = new B();
        b.registerCallback(this);
    }    

    // Implementation of the callback interface
    public void call(){

    }
}

Then, let class B to handle the callback.

public class B
{
    private Callback callbackoNotify;

    public class registerCallback(Callback callback)
    {
        callbackoNotify = callback;
    }

    public void notify()
    { 
        callbackNotify.call();
    }
}

But in the above scenario, callbackNotify can be null. Therefore, it is better if you can pass that callback in the constructor to B.

Hope you got the idea.

answered Feb 26, 2017 at 2:19

Imesha Sudasingha

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3 Comments

aQuu Over a year ago

Then class A Has public method 'call' which everyone can call, although only B should can.

ninnemannk Over a year ago

So, make A an abstract class that extends Callback. Make B a concrete class that extends A with a protected method that is "call". Maybe if you're having these issues in the first place, you should look into an asynchronous framework such as Ratpack.

Imesha Sudasingha Over a year ago

@aQuu That is the use of interfaces. In OOP, if we are to call this call method, we should have an object of type Callback. If you are doing a.call() where a is of type A, then it is a bad practice. Even in class B, we keep the reference to the callback in the type Callback

john16384 · Accepted Answer · 2017-02-26 09:56:43Z

3

You can do this without callbacks as well. Here's an example:

A a = ...
B b = ...

Let's say b has a doTask method like this:

public void doTask(Runnable task);

A can now call it like this:

b.doTask(() -> {System.out.println("Hi There");});

However, A will not be informed when the task gets completed. You could simply change the task like this:

Runnable taskForB = () -> {System.out.println("Hi There");};
Runnable wrapperForTaskWithCallback = () -> {
    taskForB.run();
    taskWasFinished();
};

And then run the wrapper task instead:

b.doTask(wrapperForTaskWithCallback);

And give A a method:

public void taskWasFinished();

answered Feb 26, 2017 at 9:56

john16384

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