Pragmatic Unit Testing in Java with JUnit, 3rd Edition

Assertion Messages: Redundant Messages for Assertions

Most verifications are self-explanatory, at least in terms of the code bits they’re trying to verify. Sometimes, it’s helpful to have a bit of “why” or additional context to explain an assertion. “Just why does this test expect the total to be 42?”

Most JUnit assert forms support an optional final argument named message. The message argument allows you to supply a nice verbose explanation of the rationale behind the assertion:

utj3-junit/01/src/test/java/scratch/AnAccount.java

@Test
void balanceRepresentsTotalOfDeposits() {
   account.deposit(50);
   account.deposit(51);

   var balance = account.getBalance();

   assertEquals(101, balance, "account balance must be total of deposits" ...