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)
.
来源:https://stackoverflow.com/questions/2393988/getting-confused-with-and-in-if-statement