Assertions in depth

Assertions are methods that check for a condition that can be evaluated. If the condition is not met, the assertion method will throw an exception, thereby aborting the execution of the test.

The JUnit API includes the class Assert. This is the base class of all the TestCase classes that hold several assertion methods useful for writing tests. These inherited methods test for a variety of conditions and are overloaded to support different parameter types. They can be grouped together in the following different sets, depending on the condition checked, for example:

The condition tested is pretty obvious and is easily identifiable by the method name. Perhaps the ones that deserve some attention are assertEquals() and assertSame(). The former, when used on objects, asserts that both objects passed as parameters are equally calling the objects' equals() method. The latter asserts that both objects refer to the same object. If, in some case, equals() is not implemented by the class, then assertEquals() and assertSame() will do the same thing.

When one of these assertions fails inside a test, an AssertionFailedException is thrown, and this indicates that the test has failed.

Occasionally, during the development process, you might need to create a test that you are not implementing at that precise time. However, you want to flag that the creation of the test was postponed (we did this in Chapter 1, Getting Started with Testing, when we added just the test method stubs). In such cases, you can use the fail() method, which always fails and uses a custom message that indicates the condition:

  public void testNotImplementedYet() {
    fail("Not implemented yet");
  }

Still, there is another common use for fail() that is worth mentioning. If we need to test whether a method throws an exception, we can surround the code with a try-catch block and force a fail if the exception was not thrown. For example:

public void testShouldThrowException() {
    try {
      MyFirstProjectActivity.methodThatShouldThrowException();
      fail("Exception was not thrown");
    } catch ( Exception ex ) {
      // do nothing
    }
  }

public void testMax() {
int a = 10;
int b = 20;

int actual = Math.max(a, b);

String failMsg = "Expected: " + b + " but was: " + actual;
assertEquals(failMsg, b, actual);
}

  public void testAlignment() {
 int margin = 0;
   ...
 android.test.ViewAsserts.assertRightAligned(errorMsg, editText, margin);
  }

import static android.test.ViewAsserts.assertRightAligned;

public void testAlignment() {
   int margin = 0;
 assertRightAligned(errorMsg, editText, margin);
}