Delegate vs Callback in Java

Question

I have some misunderstanding about terms of delegates and callbacks in Java.

class MyDriver {

    public static void main(String[] argv){
        MyObject myObj = new MyObject();
        // definition of HelpCallback omitted for brevity
        myObj.getHelp(new HelpCallback () {
            @Override
            public void call(int result) {
                System.out.println("Help Callback: "+result);
            }
        });
    }
}

class MyObject {

    public void getHelp(HelpCallback callback){
        //do something
        callback.call(OK);
    }
}

Is it callback or delegate (Are delegates and callbacks the same or similar?)?

How to implement then another one?

Answer 1

I'd argue that "callback" is a name for a generic pattern where you provide the module you're calling with a a way for said module to call your code. A C# delegate, or an ObjC delegate object, (these two being entirely different beasts) or a Java class-implementing-the-callback-interface are different, platform-specific ways of implementing the callback pattern. (They can also themselves be considered patterns.) Other languages have more or less subtly different ways of doing so.

The above concept of "delegate" is also similar to the Strategy pattern, where the delegate can be thought of as one. Similarly, a Visitor is also a type of callback. (A visitor is also a strategy for processing each visited item.)

All this is using definitions that are intuitive to me, and might not be to anyone else, because neither "callback" or "delegate" are formal terms and it makes little sense to discuss them without referring to a previous definition thereof that's valid in your context. Consequently it makes little sense to ask what the definition is since, to the best of my knowledge, there isn't an authoritative one. To wit, the fact that other answers to this question are likely to say something entirely different.

My recommendation would be to focus on the merits of your design – whether it achieves what you need, doesn't introduce tight coupling, etc. – rather than on minutiae of semantics. When two design patterns appear similar, they probably can be used to achieve similar goals equally well.

Answer 2

This is a callback. According to Wikipedia:

In computer programming, a callback is a reference to a piece of executable code that is passed as an argument to other code.

So let's look at the executable code:

public void getHelp(HelpCallback callback){
    //do something
    callback.call(OK);
}

Here, the callback argument is a reference to an object of type HelpCallback. Since that reference is passed in as an argument, it is a callback.

An example of delegation

Delegation is done internally by the object - independent of how the method is invoked. If, for example, the callback variable wasn't an argument, but rather an instance variable:

class MyDriver {

    public static void main(String[] argv){
        // definition of HelpStrategy omitted for brevity
        MyObject myObj = new MyObject(new HelpStrategy() {
            @Override
            public void getHelp() {
                System.out.println("Getting help!");
            }
        });
        myObj.getHelp();
    }

}

class MyObject {
    private final HelpStrategy helpStrategy;

    public MyObject(HelpStrategy helpStrategy) {
        this.helpStrategy = helpStrategy;
    }

    public void getHelp(){
        helpStrategy.getHelp();
    }
}

... then it would be delegation.

Here, MyObject uses the strategy pattern. There are two things to note:

1. The invocation of getHelp() doesn't involve passing a reference to executable code. i.e. this is not a callback.
2. The fact that MyObject.getHelp() invokes helpStrategy.getHelp() is not evident from the public interface of the MyObject object or from the getHelp() invocation. This kind of information hiding is somewhat characteristic of delegation.

Also of note is the lack of a // do something section in the getHelp() method. When using a callback, the callback does not do anything relevant to the object's behavior: it just notifies the caller in some way, which is why a // do something section was necessary. However, when using delegation the actual behavior of the method depends on the delegate - so really we could end up needing both since they serve distinct purposes:

    public void getHelp(HelpCallback callback){
        helpStrategy.getHelp(); // perform logic / behavior; "do something" as some might say
        if(callback != null) {
            callback.call(); // invoke the callback, to notify the caller of something
        }
    }