reason of IllegalMonitorStateException if I use wait within sync block

Question

I try to understand java core synchronization. I wrote code sample: Program should write left right 10 times ``` package concurrency; public class LeftRightWaitNotifyExample { final static String str = "1"; public static void main(String[] args) { new LeftLegThread(str).start(); new RightLegThread(str).start(); } } class LeftLegThread extends Thread { String monitor; public LeftLegThread(String str) { monitor = str; } @Override public void run() { try { makeStep(); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } private void makeStep() throws InterruptedException { synchronized (monitor) { for (int i = 0; i < 10; i++) { System.out.println("Left "); wait(); } } } } class RightLegThread extends Thread { String monitor; public RightLegThread(String str) { monitor = str; } @Override public void run() { try { makeStep(); } catch (InterruptedException e) { } } private void makeStep() throws InterruptedException { synchronized (monitor) { while…

Accepted Answer

You are synchronizing on `monitor`, so you should `wait()` on monitor, too:

```
monitor.wait();
```

Right now you are waiting on `this`, which is not the owner of the monitor because synchronization is on `monitor`.

Note that of course the `notify` should also be done on the `monitor` object, and that you might want to consider using `notify`/`notifyAll` in both threads. Otherwise it may happen that one thread starves waiting for a missing notification. Using a timeout (the overloaded version of `wait`) might also be a good idea to catch corner cases.

reason of IllegalMonitorStateException if I use wait within sync block

Solution

Problem