Java threads: wait and notify methods

Question

I have a thread that calls the wait method and can only be awoken when the notify method called from some other class:

 class Threa         


        

Answer 1

1) You need to add some flag that is used to communicate between the threads, so that B can signal to A when it is finished. A simple boolean variable is fine, as long as it is only read and written within the synchronized blocks.

synchronized(this) {
    for(int i=0;i<100;i++) {
        total += i;
    }
    isDone = true;
    notify();
}

2) A needs to loop while waiting. So if your boolean variable was called isDone, and was set to true by threadB, then threadA should have some code like this:

synchronized(b) {
    System.out.println("Waiting for b to complete...");
    while( ! isDone ) b.wait();
}

In this particular case, there's actually no reason to have the synchronized block in A - since threadB doesn't do anything after it finishes running, and A doesn't do anything except wait for B, threadA could simply call b.join() to block until it finishes. I assume that your actual use case is more complex than this.