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Additional assertions

Predicate-based

When using assertTrue or assertFalse, you'll end up with failure messages expected: <true> but was: <false> or expected: <false> but was: <true>. While these tell you there is a failure, they don't tell you why.

Class PredicateAssertions provides alternatives that take a predicate and the value to apply the predicate to, and provide failure messages like expected: matching predicate but was: <foo> and expected: not matching predicate but was: <foo>. For instance:

// assertTrue(StringUtils.isNotBlank(value));
assertMatches(StringUtils::isNotBlank, value);
// assertFalse(StringUtils.isBlank(value));
assertDoesNotMatch(StringUtils::isBlank, value);

// assertTrue(i > 0);
assertMatches(n -> n > 0, i);
// assertFalse(i < 0);
assertDoesNotMatch(n -> n < 0, i);

In case you want even more information in your failure messages, you should consider using Hamcrest.

IO related

Class IOAssertions provides some additional I/O related assertions, for instance for checking the content of a Reader or InputStream or to check the serializability of objects.

Optional related

Class OptionalAssertions provides some additional related assertions to check if Optional, OptionalInt, OptionalLong and OptionalDouble instances are present or empty.

Throwable related

Class Assertions has methods assertThrows, assertThrowsExactly and assertDoesNotThrow to assert that code throws or does not throw any exceptions. Class ThrowableAssertions provides some additional assertions, for instance:

  • assertions similar to assertDoesNotThrow that lets unchecked exceptions pass through (including AssertionFailedError)
  • assertions similar to assertThrows and assertThrowsExactly that check for more than one exception type
  • assertions similar to assertThrows and assertThrowsExactly that check for optional exceptions
  • assertions for checking exception causes
  • assertions for checking exception chains

ThrowableAsserter

In some cases, the “one-of” assertions of ThrowableAssertions is not sufficient, and you need different assertions for each of the possible exceptions. If that's the case, class ThrowableAsserter can be used. Besides specifying each error type that can be thrown, for each error type the type's specific assertions must be specified. For instance, from one of the pre-defined tests:

whenThrows(UnsupportedOperationException.class, () -> map.computeIfAbsent(key, function)).thenAssertNothing()
        .whenThrows(IllegalArgumentException.class).thenAssert(thrown -> assertSame(exception, thrown))
        .execute();