How to implement delegate pattern (like in objective-c ) in java

Question

I looked at an example where delegation pattern is explained for java. Didn't find much use for it (excuse the ignorance) as i feel it lacks the flexibility in objective-c. Is there way to dynamically set the delegate object as one can do in objective-c. Isn't that the whole point of delegation? My knowledge of java is very preliminary , so please explain a bit in detail.

Accepted Answer

I think there are many ways ways to implement delegation pattern in Java, but probably none which feels like a built-in.

Take a look at the Wikipedia example. Your basic option is to manually implement an interface, and then simply forward the calls to a concrete instance which you can change during run-time as much as you wish.

Now depending on what tools you have and can use, you can make this forwarding more automatic. One idea, is to use aspect-oriented programming, like AspectJ.

Having an aspect compiler (or runtime) you could utilize annotations and come up with a simple extension to the language:

```
class Person {
  @Delegate Animal animal;
  ...
}
```

You'd then have to write an aspect that finds `@Delegate`s and automatically adds forwarding methods in the class'es (eg. `Person`) interface.

If you are able to use a more groovy JVM language, then you wouldn't even have to write a single line of code, because such languages have delegates in the standard library:

You can have a look here, to see how it's done in Groovy. (essentialy exactly like the `Person` example syntax I came up with... but built-in!)

How to implement delegate pattern (like in objective-c ) in java

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