What's the intended use of IllegalStateException?

Question

This came up in a discussion with a colleague today.

The Javadocs for Java\'s IllegalStateException state that it:

Signals that a method has b

Answer 1

Given a library, it should throw an IllegalStateException or IllegalArgumentException whenever it detects a bug due to the user code, whereas the library should throw an AssertionError whenever it detects a bug due to the library's own implementation.

For example, in the library's tests, you may expect the library throws an IllegalStateException when the order of method calls are wrong. But you will never expect the library throws an AssertionError.