How does "return" work for a method with a "boolean" return type?

[Jordan D. Williams]

Hello there!

I am systematically going through the Java Tutorials and I came up to something I don't quote understand. I am looking at the answers for question #2. Here is a direct link to the question. It's a little frustrating that they have not even mentioned assertion when they ask us readers to do this, but that's OK.

Basically I am looking at the Card.java class and I see this:

// Code preceding this method

public static boolean isValidRank(int rank) {
    return ACE <= rank && rank <= KING;

// More code after this method

ACE and KING are of type int.

This is what I *think* is happening here. Please correct me if I am wrong. Basically the code that follows the return statement is like an if expression to be evaluated - except it does not have parentheses around it. If the conditions are true (ACE is less than or equal to rank AND rank is less than or equal to KING) then the method will return true. If the one of the conditions is NOT true, then the method will return false. Is that correct so far?

If someone could shed some might for me and others who might be confused about this, it would be greatly appreciated!

Thanks so much for all your time and effort!

[Bear Bibeault]

asserts have nothing at all to so with return values. Why are you mashing them together?

[Jordan D. Williams]

Bear Bibeault wrote:asserts have nothing at all to so with return values. Why are you mashing them together?

Well... the Java Tutorials example used boolean return type fuctions whose return statement I did not understand and those same methods were used with assert. I thought that putting them together would give people a better handle of the context in which the question occurred. I can separate them if you think it would be better.

Thanks.

[Paul Clapham]

Unfortunately the kind of question which looks like "How does feature A work when you use it with feature B" is almost always answered by "The same way it works when you use it in any other context". And so it is here. Methods which return boolean data as their value do so whether or not that value is going to be inspected by an assert statement.

So I don't think that kind of question is really useful. It just leads people to jumble up different concepts into a big heap, whereas what should happen is that beginners learn early on that you don't need to look at everything all at once. Looking at small pieces (e.g. what is a boolean value, which is one of your questions, and a perfectly good question at that) is much easier when you don't have to worry about how those small pieces are going to be put together into a larger structure.

[Jordan D. Williams]

Paul Clapham wrote:... Looking at small pieces (e.g. what is a boolean value, which is one of your questions, and a perfectly good question at that) is much easier when you don't have to worry about how those small pieces are going to be put together into a larger structure.

What I asked previously and the way I formulated it was really not a very well thought out approach. I edited the question and the topic title. Is that better?

Thanks.

[Paul Clapham]

Jordan D. Williams wrote:Basically the code that follows the return statement is like an if expression to be evaluated - except it does not have parentheses around it. If the conditions are true (ACE is less than or equal to rank AND rank is less than or equal to KING) then the method will return true. If the one of the conditions is NOT true, then the method will return false. Is that correct so far?

Yes, that's right. It's called a "boolean value", and its value can only be true or false. The if-statement also requires a boolean value in the parentheses after the "if" keyword, so you've made a valid connection there.

Here's a link to the tutorial about primitive data types. It mentions the boolean data type, but that's about all it does. It kind of assumes you already know what it means and why you would need a boolean value, but from what I've seen that isn't necessarily a safe assumption with regard to beginning programmers. Basically you need a boolean value if you want to keep track of whether some condition is (or was) true or not.

[dennis deems]

Jordan D. Williams wrote:This is what I *think* is happening here. Please correct me if I am wrong. Basically the code that follows the return statement is like an if expression to be evaluated - except it does not have parentheses around it. If the conditions are true (ACE is less than or equal to rank AND rank is less than or equal to KING) then the method will return true. If the one of the conditions is NOT true, then the method will return false. Is that correct so far?

Your understanding is basically correct. This kind of expression is called a boolean expression: some logic is performed, and the expression is evaluated to a boolean. We could write a method that looks, one by one, at each condition we are interested in:

Pseudocode:
if rank is lower than ACE, return false.
if rank is higher than KING, return false.
otherwise, return true.

Instead, this method compacts all that into a single statement.

Something else to point out is the use of the short-circuiting AND operator, '&&'. In order for an AND expression to return true, the expressions on both sides of the operator must themselves evaluate to true. If the value to the left of the operator is false, then there is no need to evaluate the rest of the statement. So if we see that rank is less than ACE, we can return false immediately. That's exactly what the short-circuiting operators do.

Edit: too, too slow....

[Junilu Lacar]

Dennis Deems wrote:Edit: too, too slow....

It happens...

As a consolation, you correctly pointed out that it's a boolean expression that evaluates to a boolean value of either true or false.

One of my pet peeves when doing code reviews is the kind of code in your "long" example.

// Blah blah blah - way too much code 
if (n > max) {
    return true;
} else {
    return false;
}

// And this kind of code really gets me mad
boolean result = (n > max);
if (result == true) {
    return true;
} else {
    return false;
}

// just cut to the chase already!
return (n > max);

In the short form given above, the parentheses are optional but I add them anyway because I feel it makes it clearer that it's the result of a boolean expression that's returned; at least that's how it feels in my head.

[Jordan D. Williams]

Wow! Every day I post/read on these forums someone blows my mind!

Junilu Lacar wrote:

...
// just cut to the chase already!  
return (n > max);  

That is just awesome! I really have no other comments about that!

Thanks so much for the help all!