Java programming question. Boolean

[Drenriza]

Hi guys.

So i was told if i wanted to check a boolean, if it is true or false. I could do it as follows

boolean end = false;

while(!end)

And that this would check if the boolean is false

while(end)

And this if it is true.

But i was wondering, how is it that you can skip the != and == signs in each loop. Is this just something that Java have implemented? Or is their a higher meaning. And can this only be done with boolean's or can it also be used in other situations? For loops, if then statements and so on.

Kind regards.

[muteXe]

both cases evaluate to a boolean so it's the same thing really.

i tend not to do:

while(!end)

but do

while(end == false)

It's easier for a pleb like me to understand it when i re-read it again six months down the line.

[davetv]

It is the same in c/c++. I don't know about java, but in c you can use the same construct to check an integer value for zero or non-zero where zero=false, any other value =true.

Sometimes you might use the same for a pointer, declare a pointer and set it to NULL and you can test it with the same boolean construct and it will be false, if it has a value it =true.

And yes ... it should work for if for and repeat statements. Any sort of boolean test.


lol @ your next comment muteXe

[muteXe]

It is the same in c/c++. I don't know about java, but in c you can use the same construct to check an integer value for zero or non-zero where zero=false, any other value =true.

thankfully this type of thing is not allowed in java.

[davetv]

Hi muteXe ... why is that bad? Your "thankfully" comment gives me the vibe. Don't you think that is a versatile use of that construct?

I don't see it as confusing as long as you know the typedef and history of the tested variable.

[schauerlich]

The while loop condition only requires and expression that returns a boolean. There are many ways to have that happen, and not all of them involve an equals/not equals sign. This is a simple case, where just the variable's name gives you a boolean value, which the while loop checks for true/false. You can also do things like negate it, use a boolean and/or on it with other expressions, or return a boolean from a function (eg, while(iter.hasNext()).

The situation in C is slightly different. C has no innate boolean class, so the while loop condition looks for integers. If the integer is 0, the condition evaluates to false. Otherwise, it is true. C implicitly "casts" (reinterprets) a lot of things to int, which lets you put a few more things in the condition expression. Any numeric type checks whether it equals 0, and any other type casts to int and checks if that value is 0. In most cases this means checking if a pointer is null or not. C++ has a similar situation for primitive types because it is (mostly) a superset of C.

[trent.josephsen]

C implicitly "casts" (reinterprets) a lot of things to int

I know I should probably let this go, but casts are always explicit. The word you're looking for is "converts".

int i = (int) 1.0; /* this line contains a cast (explicit conversion) */
int j = 1.0; /* this line relies on implicit conversion and doesn't have a cast */

[muteXe]

Just to add an example to extend to what schauerlich quite correctly is saying:

If you have 2 ints, variable1 and variable2, in C or C++ this is perfectly allowable:

if(variable1 - variable2)
{
    // do some stuff --- A
}
else
{
    // do some otherstuff   --- B
}

so you are 'if'ing on an integer (not a true or false), and if variable1 and 2 were the same then you'd get a if(0) i.e. it would evaluate to false and path B would execute.

This kinda thing wouldn't even compile in java. It expects, in an 'if', a built-in boolean type.

[Bachstelze]

C implicitly "casts" (reinterprets) a lot of things to int, which lets you put a few more things in the condition expression. Any numeric type checks whether it equals 0, and any other type casts to int and checks if that value is 0.

No! In C, when you write

if (foo)

it is interpreted as

if (foo != 0)

and then the usual rules for comparison apply. In some cases, there will be a conversion (not a cast!), but not always. In particular, if foo is a pointer, the expression foo != 0 will have value 0 is foo is a null pointer, and 1 otherwise. There is no conversion performed!

And it is not just pedantry, either. The conversion from pointer to int is implementation-defined, so it is possible that a null pointer foo, when cast to an int, will give a value of 1. Even then, though, the expression foo != 0 will still have value 0, because foo is a null pointer.

[dagoth_pie]

while(end == false)

It's easier for a pleb like me to understand it when i re-read it again six months down the line.

But 6 months later... "What the hell does end mean!?"