The magic of assertions

assert.java

if (3 != 3)
{
    fail("equality check failed on 3");
}
/*
 * That's fine, except that we're usually comparing dynamic data retrieved from the app.
 * Sometimes we aren't even comparing against static data
 */
for (String result : results)
{
    if ( page.verifyData(result) == false)
    {
        fail("result did not match expected value");
    }
}
/*
 * In this example, we have no knowledge of what was actually being compared.
 * We only know whether it was successful or not. This might lead you to think about printing out the data
 * before comparing it, but that leads to more complex code (less readable, more boilerplate), and more
 * noise in the logs, with little benefit
 */

/*
 * The proper way is to use assertions. Javadoc: http://testng.org/javadocs/org/testng/Assert.html
 */
assertEquals(3, 4, "equality check failed on 3");
/*
 * This is functionally equivalent to the first validation statement in this example file. It will even print the
 * same message. But, if the condition fails, it will also print the data that it failed on.
 *
 * java.lang.AssertionError: equality check failed on 3 expected [4] but found [3]
 * Expected :4
 * Actual   :3
 */

/*
 * All assertion methods take at least two parameters: The actual value, and the expected value (in that order)
 *
 * They also all are overloaded to take a 3rd argument, a string with a message for when the assertion fails.
 */

/*
 * There are assertions for just about anything you can imagine, and assertEquals is overloaded for most types
 */
String[] foo = {"foo", "bar"};
String[] bar = {"bar", "foo"};
//fails
assertEquals(foo, bar);
/* java.lang.AssertionError: Lists differ at element [0]: bar != foo expected [bar] but found [foo]
 * Expected :bar
 * Actual   :foo
 */
//passes
assertEqualsNoOrder(foo, bar);

/*
 * Using this assetion, we can rewrite the second example to be:
 */
assertEquals(page.getData(), results);

Object nothing = null;
assertNotNull(nothing);
/* java.lang.AssertionError: expected object to not be null
 */

Integer three = new Integer(3);
Integer tres = three;
Integer anotherThree = new Integer(3);
//passes
assertSame(tres, three);
//fails
assertSame(anotherThree, three);
/* java.lang.AssertionError: expected [3] but found [3]
 * Expected :3
 * Actual   :3
 */

/*
 * ok, so that error message sucks, but you can provide your own.
 */
assertSame(anotherThree, three, "Objects were not the same! Values were:");

/*
 * There are plenty more assertions too. Go read the javadocs!
 */