（可能）相当简单的Java多线程问题

Question

I'm making a platformer in which I make the player jump by, every 30ms, adding a small amount of force upwards. I thought I'd use multi-threading, as I've done a little bit of it before and it seemed simple enough. Anyway, I tried this:

public void jump() {
    new Runnable() {
        public void run() {
            for (int i = 0; i <= jumpForce; i++) {
                velocity.y++;
                System.out.println("Adding force");
                try {
                    wait(30);
                } catch (InterruptedException e) {}
            }
        }
    }.run();
}

Now, what I thought this would do is every loop from 0 to jumpForce (in this case 50), it adds 1 onto the y velocity variable then waits 30ms, but what actually happens is I get an error called:

java.lang.IllegalMonitorStateException

and I have no idea what this means so can someone tell me what I'm doing wrong?

Answer 1

The javadoc says for wait :

This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.

you own a monitor with a synchronized block, change your code to :

synchronized (this) {
  wait(30);
}

Answer 2

If you want to execute this code in a different thread, than you should NOT invoke the run() method of the Runnable. What you should do is create an instance of a thread and put your Runnable implementation in its constructor. Then invoke .start() .

 public void jump() {
        new Thread(new Runnable() {
            public void run() {
                for (int i = 0; i <= jumpForce; i++) {
                    velocity.y++;
                    System.out.println("Adding force");
                    try {
                        Thread.sleep(30);
                    } catch (InterruptedException e) {}
                }
            }
        }).start();
    }

Also you should sleep() the thread in this case rather than wait() , because you have not taken the object lock. In order to implement what you need using wait(), the method jump() must be synchronized and another thread must invoke it.

Answer 3

You are not multithreading, because you are not creating any threads.
The Runnable is running in the same thread that is going to run it. If you want to create a new Thread you have to explicitly do so with new Thread(new Runnable()) and use Thread#start() .

Anyway, you probably just want to schedule a task so better to use a ScheduledExecutorService:

ScheduledExecutorService ex = Executors.newSingleThreadScheduledExecutor();
ex.scheduleAtFixedRate(new Runnable() {
    @Override
    public void run() {
        // do something
    }
}, 0, 30, TimeUnit.SECONDS);