Boolean expressions in Java

Question

I have a question about the meaning (evaluation) of Boolean variables in return statements in Java.

I know that:

if (var) { ... }

is the same as:

if (var==true) { ... }

In the second case we explicitly say var==true, but we don't need to do this, because Java evaluates var as true anyway. I hope I have understood this right.

My question is: is it the same when Boolean variables are returned? When we have a return statement?

For example, a task specifies: the method looksBetter() will return true only if b < a. My solution was:

public boolean looksBetter() {
     if (b < a) {
         return true;
     } else {
         return false;
     }
}

The simple answer was:

public boolean lookBetter() {
      return b < a;
}

So, it seems that here we have again this implicit assumption that in case b < a == true, the return of the method is true. I am sorry ... it seems very trivial, but I am somehow not comfortable with this, and I don't know why. Thank you.

Solution 1

It's not an "implicit assumption," it's what the compiler's doing. The b < a is just an expression, the same as if it were used for an if statement. The expression evaluates to a boolean, which is then returned.

Also noteworthy, you seem to interchange boolean and Boolean as though they're the same, but they're actually not. boolean is the primitive form while Boolean is an Object that wraps a boolean.

Solution 2

Yes, this is true for all booleans. You can think of if(expression) evaluating 'expression' to see if it's 'true' or 'false'. When you do

if(b < a == true)

it first tests to see if b < a and if it is, it now tests:

if(true == true)

It now tests whether true == true (which it obviously does). Java isn't doing anything tricky when you leave out the extra '== true', it just needs to perform one fewer test. There's no reason you couldn't say:

if(((b < a == true) == true) == true)

but it would cause Java to perform an extra test each time it sees an equals sign.

Solution 3

Don't needlessly complicate your code. If you feel the need to say "a < b == true", then you can follow that to its logical conflusion (conclusion + confusion) and say "((((((((...(a<b) == true) == true).... == true)"

"a < b" is a boolean expression. If you already have a boolean, why compare it to another boolean? You're not making it more boolean that way.

Solution 4

A Java conditional requires a boolean value. If you can put it into an if statement, it's already a boolean, and requires no further fiddling if what you want is a boolean.

Indeed, constructs like value == true can be tricky. I don't offhand remember the promotion rules in Java, but in C++ a bool can be promoted to an int, with false becoming 0 and true becoming 1. Therefore, int a = 2; if (a) and int a = 2; if (a == true) will do different things.

Solution 5

Your confusion might be eased if you try thinking about operators as methods. Using your example, you had the < "less than" operator. For our purposes, the < operator can really be considered a "method" (it only doesn't look like one), which takes two parameters and returns a boolean result. If the "method" were named lessThan, your example would be equivalent to this:

public boolean lookBetter() {     
  return lessThan(b, a);     
} 

Perhaps seeing it like that makes it a tad easier to understand? Incidentally, when I was leading exercise groups in the 'Programming 101' course back in Uni, this proved to be by far the hardest thing to teach, and a lot of people had trouble grasping the concepts involved. It almost seemed to be akin to learning to ride a bike or to swim, once you get how it works it becomes self-evident in a way.

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