Empty if-statements [duplicate]

Answer 1

In the statement

if  (eval) { //pseudo-code
}

Sometimes data is actually changed in evaluation of (eval). For example, in

while (someIterator.next()) {
}

Calling next() actually changes the state of the someIterator object.

And of course there is the classic example that usually happens from a typo (and is not recommended)

int x;  
if (x = getNumberOfWidgets() > 5) {
}

Conventional wisdom advises against coding this way, as it is harder to tell what is going on. However, the statements are legal and so that is one reason why such an 'if' statement is allowed.

Answer 2

Most Java compilers make no optimization of code at all. They delegate this work during runtime to the Java Runtime Environment. The JVM verifies the code before it's executed, but this verification consists (mainly) of three checks:

1. Branches in valid positions(not the same as correct) .
2. Data is always initialized and references are always type-safe
3. Access to private data is controlled.

So, your if statement may be wrong, but it's still valid. As valid as

 if (a==a)

Answer 3

The condition could be a function call with side effects. It wouldn't be correct to treat it as an error or warning.

Answer 4

I believe that they left it in because it can increase code readability. Even if nothing should be done for a case you may still want to let people know that the case is important.

Answer 5

I don't think this is truly relevant to the intent of the question but I think it should be stated as it is relevant to the essence of the question.

There is an effect of an:

if(variable);

if the variable is volatile. It''s effect is to cause a memory barrier to be honoured between the current thread and any other threads accessing the variable.

public volatile variable;
....
if(variable);

See here for a more detailed discussion.

I cannot imagine any real value to putting this kind of statement in your code but I felt it important to note that there is a real effect to this statement in this very specific situation.

Answer 6

I'm mostly a C# developer, although I have a little Java background. But I think my answer applies to both. I suspect it's not an intentional feature, but more of an emergent feature. The grammar of the language goes (roughly)

if (*condition*)
    *statement*

Unfortunately the below are both valid statements (I checked, you can drop as many into C# as you like and the compiler doesn't complain):

;

{
}

Therefore the construct that you highlighted is allowed.