Why does this code result in illegalMonitorState exception?

Question

The code below is trying to insert a random value into a circular queue and remove it. However, there are some synchronization issues. I know I can use higher level routines

Answer 1

another problem in this code is that even if isEmpty/isFull return true - until you call the adjacent wait the state of the queue might change.
For example:
- queue is empty
- thread 1 calls isEmpty()
- context switch
- thread 2 calls enqueue (now the queue is not empty)
- context switch
- thread 1 not calls lock.wait() event though the queue is not empty

this problem of course will be solved when calls to wait()/notify() will be placed in a synchronized block.

Answer 2

Because java.lang.Object#wait, java.lang.Object#notify, java.lang.Object#notifyAll must be called from synchronized block.

As a solution(need to check) you should put your conditions inside synchronized blocks:

void enqueue(int number) throws InterruptedException
{

    synchronized(lock)
    {
        if(isFull())
           lock.wait();

        array[rear] = number;
        System.out.println("Rear is:"+ rear+ "value is:"+number+"Size is:"+size);

        rear = (rear+1)%size;
        count++;
        lock.notify();
    }
}

void dequeue() throws InterruptedException
{
    synchronized(lock)
    {
        if(isEmpty())
           lock.wait();

        int retVal = 0;
        retVal = array[front];
        System.out.println("Front is:"+ front+ "value is:"+retVal);

        front = (front+1)%size;
        count--;
        lock.notify();
    }

}