Queue with Threads

[Abu Nene]

Hi guys, any idea is there anything wrong with my Queue implementation using ArrayList? I'm suspecting something is wrong as sometimes dequeue method will return null.

import java.util.ArrayList;

import org.apache.log4j.Logger;

@SuppressWarnings("serial")
public class Queue extends ArrayList<Object> {

	private static Logger logger = Logger.getLogger(Queue.class.getName());

	public synchronized void enqueue(Object o) {
		this.add(o);
		notify();
	}

	public synchronized Object dequeue() {

		try {
			if (this.isEmpty()) {
				wait();
			}
		} catch (InterruptedException e) {
			logger.fatal(e);
		}
		
		return !this.isEmpty() ? this.remove(0) : null;
	}
}

[Ireneusz Kordal]

Hi,

Change lines 18-20 to:

  
while (this.isEmpty()) {
				wait();
			}

the reason is explained in the java specification:
http://java.sun.com/docs/books/jls/third_edition/html/memory.html#17.8

2. Thread t does not execute any further instructions until it has been removed from m's wait set. The thread may be removed from the wait set due to any one of the following actions, and will resume sometime afterward.
...........
* An internal action by the implementation. Implementations are permitted, although not encouraged, to perform "spurious wake-ups" -- to remove threads from wait sets and thus enable resumption without explicit instructions to do so. Notice that this provision necessitates the Java coding practice of using wait only within loops that terminate only when some logical condition that the thread is waiting for holds.

[Abu Nene]

Got it, thanks for the info Ireneusz.

Ireneusz Kordal wrote:Hi,

Change lines 18-20 to:

  
while (this.isEmpty()) {
				wait();
			}

the reason is explained in the java specification:
http://java.sun.com/docs/books/jls/third_edition/html/memory.html#17.8

2. Thread t does not execute any further instructions until it has been removed from m's wait set. The thread may be removed from the wait set due to any one of the following actions, and will resume sometime afterward.
...........
* An internal action by the implementation. Implementations are permitted, although not encouraged, to perform "spurious wake-ups" -- to remove threads from wait sets and thus enable resumption without explicit instructions to do so. Notice that this provision necessitates the Java coding practice of using wait only within loops that terminate only when some logical condition that the thread is waiting for holds.