How can I make my generics code compatible with this method signature?

generics, java

Solution

No, and you shouldn't.

Your `BlockingQueue<MyRunnable>` should, of course, only contain `MyRunnable`s. But the `ThreadPoolExecutor` can submit arbitrary `Runnable` tasks to the queue you give it: see `execute(Runnable command)`.

If that happens, you could have a non-`MyRunnable` instance in your queue. You then try to poll from your reference of that queue (typed as a `BlockingQueue<MyRunnable>`), and get a `ClassCastException`.

Simple example:

PriorityBlockingQueue<MyRunnable> queue = new PriorityBlockingQueue<>();
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(200, 200, 0L,
        TimeUnit.MILLISECONDS, queue);
threadPool.execute(new WhateverRunnable());
MyRunnable myRunnable = queue.poll(); // this could throw ClassCastException

The above code will throw an exception if the `queue.poll()` happens before the thread pool has had a chance to dequeue the `WhateverRunnable` instance.

Problem

I have a variation of the following code: ``` package com.test.package; import java.util.concurrent.BlockingQueue; import java.util.concurrent.PriorityBlockingQueue; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; public class TestClass { public static class MyRunnable implements Runnable { @Override public void run() { System.out.println("Called"); } } public void method() { PriorityBlockingQueue<MyRunnable> queue = new PriorityBlockingQueue<MyRunnable>(); method2(queue); } public void method2(BlockingQueue<? extends Runnable> queue) { System.out.println(queue); // Getting error here because BlockingQueue<? extends Runnable> is not a // subtype of BlockingQueue<Runnable>. ThreadPoolExecutor threadPool = new ThreadPoolExecutor(200, 200, 0L, TimeUnit.MILLISECONDS, queue); } } ``` And as you can see, my queue is not compatible with the `ThreadPoolExecutor` constructor. Is there any way to work around this than cast my queue to `(BlockingQueue<Runnable>)`? I obviously can't patch Java Standard Library.

Original source