test_login.java

@Test
public void login_successIfUserNameAndPasswordIsValid() {
    // Arrange
    String username = "username";
    String password = "password";

    when(userRepository.isUsernameValid(username)).thenReturn(true);
    when(userRepository.isPasswordValid(password)).thenReturn(true);

    // Act
    boolean result = authenticator.login(username, password);

    // Assert
    assertTrue(result);
}