Overwriting methods: how to "inject" into the super-method?

Question

Assuming three classes, one being a subclass of the other. Each overwrite the parents' method.

public class BaseClass {

    public void doStuff() {
        performBaseTasks();
    }

}

public class MiddleClass extends BaseClass {

    // {BaseClass} Overrides

    public void doStuff() {
        performMiddleTasks();
        super.doStuff();
    }

}

public class FinalClass extends MiddleClass {

    // {BaseClass} Overrides

    public void doStuff() {
        performFinalTasks();
        super.doStuff();
    }

}

When calling new FinalClass().doStuff(), this would lead to a method invokation order as follows:

performFinalTasks();
performMiddleTasks();
performBaseTasks();

I want to bring the perfomFinalTasks() between performMiddleTasks() and performBaseTasks(). How can I do this?

performMiddleTasks();
performFinalTasks();
performBaseTasks();

Is this a trick question or something coming from an actual use case? What classes do you have access to I.e what can you change? — Scorpion, Jan 18 '13 at 18:40
@Scorpion: No, this is not a trick question. It comes from an actual use case where the middle class is setting up some data, the final class requires to work with. However, the execution of the base classes method must still be executed at last. I can change any of these classes. — Niklas R, Jan 18 '13 at 19:25
Well, If its an actual use case, then the final class should support it as suggested in my answer along with the javadocs mentioning what it does and the special use case that it caters to. — Scorpion, Jan 18 '13 at 19:29

score 2 · Answer 1 · answered Jan 18 '13 at 18:34

2

Write a public method in final class doStuffDifferently() and invoke these methods in that order. I am not sure it's possible to do it via any other tricks in the doStuff() method.

answered Jan 18 '13 at 18:34

Scorpion

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score 1 · Accepted Answer · answered Jan 18 '13 at 18:38

One possible way, if you can make the middle class abstract:

public abstract class MiddleClass extends BaseClass {

    // {BaseClass} Overrides
    public void doStuff() {
        performMiddleTasks();
        doProxyExec();
        super.doStuff();
    }

    public abstract void doProxyExec();
}

You override the proxy method in your subclass:

public class FinalClass extends MiddleClass {

    // {BaseClass} Overrides
    public void doStuff() {
        super.doStuff();
    }

    // {MiddleClass} Overrides
    public void doProxyExec(
        performFinalTasks();
    }
}

A not very polymorphic way of method call chaining, but then again the original design is kind of ... odd.

Overwriting methods: how to "inject" into the super-method?

2 Answers2