Can not put a thread on wait() - IllegalMonitorStateException

Background:

I have an application of miniature robots (these extend Thread class) that make deals with one another at nodes of a map. I am trying to program the logic that goes into the node. The node is responsible for carrying out the deals between two bots that meet at the node.

The logic I want to code into the node is as follows:

• Bot A arrives.
• IF there is another Bot present at the node (e.g. Bot B)
• then broker a deal between Bot A and Bot B.
• ELSE tell Bot A to wait until another Bot arrives at the Node.

My Attempt

Here is my attempt at coding the logic described above.

public void trade(StrippedBot trader)
{
    // check to see if there are any other bots waiting
    if(bots.size() > 0)
    {
        for (StrippedBot b : bots.keySet()) {
            if(!b.equals(trader) && !b.getFamily().getName().equals(trader.getFamily().getName()))
            {
                b.notify();
                trader.getResource().adjust(COOPERATION_REWARD);
                b.getResource().adjust(COOPERATION_REWARD);
                trace(trader);
            }
        }
    } else {
        // this is the first bot here, so shall wait for others to come.
        try
        {
            bots.put(trader, true); // keeping track - true to mean this bot is on wait().
            trader.wait(); // Tell Bot to wait till another bot comes along.
        } catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

But when I run it, I get the IllegalMonitorStateException on the trader.wait() line. I have researched, and apparently it is a commonly encountered problem. So I tried the synchronized(trader) but that just froze everything, I couldn't even debug through the code, as everything was frozen.

To deal with the IllegalMonitorStateException: You cannot call wait or notify on an unsynchronized object. You must synchronize on the object you wish to call wait or notify on.

But I think your code has other issues, for example, if you wait on trader, you'll want to notify on trader. And, I suspect there are even more issues, but I'll restrict my answer to addressing the OPs direct question.