Java/ JUnit - AssertTrue vs AssertFalse

java junit assert junit4 assertions
Thomas picture Thomas · Jul 13, 2010 · Viewed 306.8k times · Source

I'm pretty new to Java and am following the Eclipse Total Beginner's Tutorials. They are all very helpful, but in Lesson 12, he uses assertTrue for one test case and assertFalse for another. Here's the code:

// Check the book out to p1 (Thomas)
// Check to see that the book was successfully checked out to p1 (Thomas)
assertTrue("Book did not check out correctly", ml.checkOut(b1, p1));    // If checkOut fails, display message
assertEquals("Thomas", b1.getPerson().getName());

assertFalse("Book was already checked out", ml.checkOut(b1,p2));        // If checkOut fails, display message
assertEquals("Book was already checked out", m1.checkOut(b1,p2));

I have searched for good documentation on these methods, but haven't found anything. If my understanding is correct, assertTrue as well as assertFalse display the string when the second parameter evaluates to false. If so, what is the point of having both of them?

Edit: I think I see what was confusing me. The author may have put both of them in just to show their functionality (it IS a tutorial after all). And he set up one which would fail, so that the message would print out and tell me WHY it failed. Starting to make more sense...I think that's the explanation, but I'm not sure.

Answer

Matt Solnit picture Matt Solnit · Jul 13, 2010

assertTrue will fail if the second parameter evaluates to false (in other words, it ensures that the value is true). assertFalse does the opposite.

assertTrue("This will succeed.", true);
assertTrue("This will fail!", false);

assertFalse("This will succeed.", false);
assertFalse("This will fail!", true);

As with many other things, the best way to become familiar with these methods is to just experiment :-).

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