Getting confused with == and = in “if” statement

问题:

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.

回答1:

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."

回答2:

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.

回答3:

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).

回答4:

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.

回答5:

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).