Empty if-statements [duplicate]

Question

By "empty if-statement", I mean something like this (note the semicolon):

if (condition);

I'm having trouble thinking of an application for this. With a while loop you can do this:

while (callUntilReturnsFalse());

But there's no such application for an if-statement. What's more, the Java compiler doesn't issue an error or a warning when confronted with such a statement. This can lead to large and silent problems, especially with a long and convoluted statement:

if ((functionA() && functionB(getFoo()) ||
    checkForComplexCondition(arg1, arg2, getBar(getFoo())));
{
    doStuff();
}

My question is: why is this allowed in Java? And, more importantly, can I enable an option to cause a warning when this happens?

(This question was asked before with regards to C#, which does issue a warning, but I was hoping to find a way to cause a warning with Java.)

Answer 1

why is this allowed in Java?

See Java Language Specification (14.6. The Empty Statement):

An empty statement does nothing.

It's simply allowed and it's equivalent to (and will be translated to):

if (condition) {  }

Which means, if the condition is true, do nothing.

If you're using eclipse, you can look here, you might find something useful (I'm not sure there exists such an option for semicolon terminator):

Window → Preferences → Java → Compiler → Error/Warnings

---

EDIT

As @nullptr pointed out in his answer, there exist an IDE warning for this, you need to set warning on Empty statement.

Answer 2

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 3

There's one construct that I use fairly frequently which the "null statement" makes clearer and easier to understand. Here's an example:

for (int i=0;  i < argc;  i++)
{
   if (argv[i]=="left")
      hpos++;
   else if (argv[i]=="right")
      hpos--;
   else if (argv[i]=="up")
      ;
   else if (arv[i]=="down")
      ;
   else fprintf(stderr, "Unknown option \"%s\\n".", argv[i]);
}

In this case, I still want to check for the existence of certain options, while only executing code for some of them. In this case, using the null statement, as above, makes the function and structure of the code more readable and comprehensible to the next guy who has to come along and maintain it.

There are certainly ways to restructure this code to not require the null statement. But I don't believe that its intention will be as clear as in the code snippet.