How can I get a specific thread to be the next one to enter a synchronized block?

Question

I was asked this in an interview.

There are four threads t1,t2,t3 and t4. t1 is executing a synchronized block and the other threads are waiting for t1 to complete. What operation would you do, so that t3 executes after t1.

I answered that join method should do the trick, but it looks like it isn't the right answer.The reason he gave was, the join method and the setPriority method would not work on threads that are wait state.

Can we achieve this? If yes, how?

Answer 1

Every thread should simply wait() on a separate object. So t3 should wait on t3Mutex. You could then simply notify that specific thread.

final Object t1Mutex = new Object();
final Object t3Mutex = new Object();
...
synchronized(t3Mutex) {
    //let thread3 sleep
    while(condition) t3Mutex.wait();
}
...
synchronized(t1Mutex) {
   //do work, thread1
   synchronized(t3Mutex) {t3Mutex.notify();}
}