What is the purpose of the assert keyword in Java?

The assert keyword in Java is used as a debugging aid to check assumptions made by the program. It is typically used to check for conditions that should be true at a certain point in the program's execution. If the condition is not true, an AssertionError is thrown, which can help identify the cause of the problem.

The assert keyword takes a boolean expression as its argument, and can also optionally take a message to be displayed when the assertion fails. By default, assertions are disabled at runtime. To enable them, the -ea or -enableassertions option must be passed to the JVM when running the program.

One common use case for the assert keyword is to verify that preconditions and postconditions of methods are being met. For example, a method that takes an array as an argument might use an assert statement to check that the length of the array is greater than 0, to avoid an ArrayIndexOutOfBoundsException.