Java多态性/重载惊喜

Question

Given this code:

class Task{};
class TaskA extends Task{};

class TopWorker{
  //... omitting constructor, instance variables, etc.
  public void work(Task task){
    this.subWorkerA.work(task)
    //followed by more subworkers...
  }
}

class SubWorkerA{
  public void work(TaskA task){
    // Subworker A does something interesting here
  }
  public void work(Task task){
    // Subworker A doesn't care, do nothing
  }
}

TaskA task = new TaskA();
(new TopWorker).work(task);

I would expect that the TaskA signature for work() would be called- instead, the Task version is being called. I assume the issue is that TopWorker only accepts a Task , and so java must dispatch SubWorker.work() accordingly.

Is there any way around this problem? Assume that I have many Tasks and many workers, and the TopWorker is there to coordinate their results.

Answer 1

The reason why the TaskA signature is not being called is because TopWorker.work guarantees only a Task to be passed in and not a TaskA instance. SubWorkerA.work(Task task) ends up getting called because it's the only work method that takes type Task in the arguments. If you instead call this.subWorkerA.work((TaskA)task) , the desired method will get called, albeit in an inherently unsafe manner.

I think you're probably confusing method overloading with late binding. Overloading simply allows you to name methods the same way so long as they have different numbers of arguments or argument types. When you call overloaded methods, the correct method is chosen based on the makeup of the arguments. In late binding, you override a method by duplicating its signature in a sub-class, which forces the overriding method to be called even if you refer to its parent object as an instance of the super class.

Answer 2

Here is a workaround...

public void work(Task task){
    if (task instanceof TaskA) {
        work((TaskA) task);
    }
    // Subworker A doesn't care, do nothing
}

Answer 3

I think that Task would better be an interface, at runtime you can figure out

public void work(Task task){

    if (task instanceof TaskA)
        this.subWorkerA.work((TaskA) task);
  }