Calling Object.wait outside of a loop may cause problems because the thread does not go back to sleep after a spurious wake-up call. This results in the program continuing before the expected condition is met.

Ensure that wait is called within a loop that tests for the condition that the thread is waiting for. This ensures that the program only proceeds to execute when the relevant condition is true. Note that the thread that calls wait on an object must be the owner of that object's monitor.

In the following example, obj.wait is called within a while loop until the condition is true, at which point the program continues with the next statement after the loop:

J. Bloch, Effective Java (second edition), p. 276. Addison-Wesley, 2008.

Java API Documentation: Object.wait().

The Java Tutorials: Guarded Blocks.