Getting confused with == and = in "if" statement

Question

I know that we cant use assignment operator in if statements in java as we use in any other few languages.

that is

            int a;

            if(a = 1) {  }

will give a compilation error.

but the following code works fine, how?

           boolean b;

           if(b = true) {   }

EDIT : Is this the exception to rule that assignment cant be used in if statement.

It's not a bad idea to get in the habit of putting the constant first in your if statements.... That is: if (true == b) {} and if (1 == a) {} This habit guarantees that the compiler will smack you if you write = instead of ==. — O. Jones, Mar 06 '10 at 21:12
@Ollie: opinions widely differ on that one. As far as readability is concerned, this *is* a bad idea. Furthermore, it offers no benefit in languages such as Java since no confusion can arise here. I mean, nobody ever writes a test as `if (a == true)`, right? — Konrad Rudolph, Mar 06 '10 at 21:56
@Konrad. You're right in Java, but for this C retread who works in PERL and some other languages, it's saved me lots of times. — O. Jones, Mar 11 '10 at 02:54
Doesn't work in a huge variety of cases, like `if ( $a == $b )`. Horrible to read, nobody says '5 is a'. It's easier to just be more careful when writing `if` conditions, IMO. — rjh, Apr 06 '10 at 17:18
@Ollie most compilers these days have an option to warn you if you write `if(foo = x)` or something like that. — Spudd86, Jun 17 '10 at 20:38

Jon Skeet · Accepted Answer · 2010-03-06T21:32:13.597

40

Because the "result" of an assignment is the value assigned... so it's still a boolean expression in the second case. if expressions require the condition to be a boolean expression, which is satisfied by the second but not the first. Effectively, your two snippets are:

int a;

a = 1;
if (a) { }

and

boolean b;

b = true;
if (b) { }

Is it clear from that expansion that the second version will compile but not the first?

This is one reason not to do comparisons with true and false directly. So I would always just write if (b) instead of if (b == true) and if (!b) instead of if (b == false). You still get into problems with if (b == c) when b and c are boolean variables, admittedly - a typo there can cause an issue. I can't say it's ever happened to me though.

EDIT: Responding to your edit - assignments of all kinds can be used in if statements - and while loops etc, so long as the overall condition expression is boolean. For example, you might have:

String line;
while ((line = reader.readLine()) != null)
{
    // Do something with a line
}

While I usually avoid side-effects in conditions, this particular idiom is often useful for the example shown above, or using InputStream.read. Basically it's "while the value I read is useful, use it."

edited Mar 06 '10 at 21:32

answered Mar 06 '10 at 20:20

Jon Skeet

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Are you sure there are an extra pair of parentheses in the last example? I thought assignment has the last precedence in Java... – user85421 Mar 06 '10 at 21:05
2

@Carlos: Good catch. Maybe I'm so used to deliberately putting it in that I've never tried without :) – Jon Skeet Mar 06 '10 at 21:32
I usually think of that idiom (and use it when the analogy fits) in terms of the iterator's hasNext and next methods - it's basically implementing a combination of those for something that isn't explicitly an iterator (and I sure wish readers had implementation of that interface added in later versions). – Carl Mar 06 '10 at 23:28
Additionally, in some languages (C, C++) an integer can be substituted for a boolean value, with 0 being false and non-0 being true. Which widely increases the chances of the =/== bug. – mtruesdell Mar 07 '10 at 01:42
@mtruesdell: Indeed, although I believe most C and C++ compilers will warn you unless you have an extra pair of parentheses to reinforce that it's deliberate... at least if you have warnings turned up high enough. – Jon Skeet Mar 07 '10 at 07:15
You can avoid the danger of if(b == c) by instead using something like if(!(b ^ c)). – ILMTitan Mar 08 '10 at 19:25
@ILMTitan: Yes, but if you're thinking enough about it to write that, chances are you'll get it right anyway :) – Jon Skeet Mar 08 '10 at 19:47

score 10 · Answer 2 · answered Mar 06 '10 at 20:20

10

For if you need an expression that evaluates to boolean. b = true evalueates to boolean but a = 1 evaluates to int as assignments always evaluate to the assigned values.

answered Mar 06 '10 at 20:20

Gumbo

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score 1 · Answer 3 · answered Mar 06 '10 at 20:21

The reason the second code works okay is because it is assigning 'b' the value of true, and then comparing to see if b is true or false. The reason you can do this is because you can do assignment operators inside an if statement, AND you can compare against a boolean by itself. It would be the same as doing if(true).

score 1 · Answer 4 · answered Mar 06 '10 at 20:23

In java, you don't have implicit casting. So non-boolean values or not automatically transformed to booleans.

In the first case, the result of the statements is an int, which is non-boolean, which will not work. The last case, the result is boolean, which can be evaluated in an if-statement.

score 1 · Answer 5 · answered Mar 06 '10 at 20:23

The rule is not that "assignment can't be used in an if statement", but that "the condition in an if statement must be of type boolean". An assignment expression produces a value of the type being assigned, so Java only permits assignment in an if statement if you're assigning a boolean value.

This is a good reason why the style if (foo == true) should be avoided, and instead simply write if (foo).

Getting confused with == and = in "if" statement

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