A trustworthy unit test is a test you can trust. It should fail when the code is broken, and succeed when – and only when – the code behaves like expected. If these two conditions are not met then the unit test is not trustworthy.

Example​

The following test is not trustworthy. According the test name, we want to validate that the balance updates accordingly after a deposit using the deposit() method. One thing we notice right away is that the assertion we used is not enough: If for some reason the deposit() we will never know since the unit test will be still successful, which is not good.

@Test
void deposit_Should_Update_Balance_when_Non_Null_Amount_Provided(){
    // arrange
    double AMOUNT = 100.00;
    BankAccount bankAccount = new BankAccount();

    // act
    bankAccount.deposit(AMOUNT);

    // assert
    assertTrue( bankAccount.getBalance() > 0  ) ;
}

Improved example​

To make the unit test trustworthy, we need to define an expected balance and use a more accurate assertion:

@Test
void deposit_Should_Update_Balance_when_Non_Null_Amount_Provided(){
    // arrange
    double AMOUNT = 100.00;
    double EXPECTED_BALANCE = 100.00;
    BankAccount bankAccount = new BankAccount();

    // act
    bankAccount.deposit(AMOUNT);

    // assert
    assertEquals(EXPECTED_BALANCE, bankAccount.getBalance() );
}

note

In java, it’s a bad practice to use the primitive type double for real world monetary values. In production code, use more appropriate types like BigDecimal.