What does the Java assert keyword do, and when should it be used?

The assert keyword in Java is used to define an assertion, which is a statement that you expect to be true at a specific point in your program. If the assertion is not true, then the program will throw an AssertionError.

Assertions are typically used to test for logical errors in your code. For example, you might use an assertion to test that an object is not null before calling a method on it, or to test that the result of a computation is what you expect it to be.

Here is an example of how to use the assert keyword:

int num = 5;
assert num > 0 : "num must be positive";

In this example, the assertion is that num is greater than 0. If this assertion is true, then the program will continue to run. If the assertion is false, then the program will throw an AssertionError with the message "num must be positive".

You should use the assert keyword sparingly, as it is intended for use during development and testing, and not for production code. In particular, you should not use assertions to perform input validation or to handle expected error conditions, as they are not intended for these purposes and can be disabled at runtime.