You can't wait() on an object unless the current thread owns that object's monitor. To do that, you must synchronize on it:
class Runner implements Runnable
{
public void run()
{
try
{
synchronized(Main.main) {
Main.main.wait();
}
} catch (InterruptedException e) {}
System.out.println("Runner away!");
}
}
The same rule applies to notify()/notifyAll() as well.
The Javadocs for wait() mention this:
This method should only be called by a thread that is the owner of this object's monitor. See the
Throws:notifymethod for a description of the ways in which a thread can become the owner of a monitor.
IllegalMonitorStateException– if the current thread is not the owner of this object's monitor.
And from notify():
A thread becomes the owner of the object's monitor in one of three ways:
- By executing a synchronized instance method of that object.
- By executing the body of a
synchronizedstatement that synchronizes on the object.- For objects of type
Class, by executing a synchronized static method of that class.