What makes a good unit test?

A good unit test is fast, deterministic, isolated and focused on behaviour.

Characteristics:

• Tests one behaviour, with a clear name such as rejects_negative_amount, so a failure tells you what broke.
• Follows Arrange-Act-Assert, keeping setup, action and verification distinct.
• Has no dependence on execution order, shared mutable state, the clock, the network or random values; inject these as dependencies.
• Asserts on observable outcomes rather than internal implementation, so refactoring does not break it.
• Has a single logical assertion concept; multiple asserts are fine if they describe one behaviour.
• Fails with a message that shows expected versus actual.
• Runs in milliseconds, so the suite can run on every save.

@Test void rejectsNegativeAmount() {
    assertThrows(IllegalArgumentException.class,
        () -> account.deposit(-1));
}

Avoid over-mocking, which couples tests to implementation. A test that never fails, or one that fails for unrelated reasons, costs more than it protects.