This applet runs a simple loop that would be quite robust in a single-threaded world. But by running the same loop in multiple threads, you render the loop unsafe. Adding synchronization gives you back safety, but with a significant performance penalty.

Experiment:

• Run the loop unsynchronized a few times. Note how many null pointer exceptions you see on average and how long the two million loops (2 threads times 1 million loops per thread) take.
• Now try the same loops synchronized, but be prepared to wait a while (hint: up to several minutes).