CODEWITHSUNDEEP
java
Tim
Tim

Reputation: 4365

Why does code with successive semi-colons compile?

According to my scientific Java experimentation, int x = 0; is equivalent to int x = 0;; which is equivalent to int x = 0;;;;;;;;;;;;;;;

  1. Why does Java allow for this? Does it have any practical application?
  2. Are each of these just empty statements? Do they actually take up any extra processing time during runtime? (I would assume they're just optimized out?)
  3. Do other languages do this? I'm guessing it's something inherited from C, like a lot of things in Java. Is this true?

Upvotes: 9

Views: 1703

Answers (4)

Jon Lin
Jon Lin

Reputation: 143906

One thing to note is that if you do something like this:

public int foo()
{
    return(0);;
}

The compiler will/may complain about an unreachable statement because there's an empty statement after a return. At least with Oracle's 1.6 sdk it does.

Upvotes: 0

user unknown
user unknown

Reputation: 36250

As Jake King writes, you can produce empty statements to do nothing in a loop:

while (condition);

but to make it obvious, you would write

while (condition)
    ;

or even better:

while (condition)
/** intentionally empty */
    ;

or even better, as Michael Kjörling pointed out in the comment, 

while (condition)
{
    /** intentionally empty */
}

More often, you see it in for-statements for endless loops:

for (;;)

or only one empty statement

for (start;;) 
for (;cond;) 
for (;;end)

Another thing you can do, is, to write a program, once with one, and once with 2 semicolons: 

public class Empty
{
    public static void main (String args[])
    {
        System.out.println ("Just semicolons");;
    }
}

Compile it, and run list the size of byte code (identic) and do an md5sum on the bytecode (identic).

So in cases, where the semantics aren't changed, it is clearly optimized away, at least for the 1.6-Oracle compiler I can say so.

Upvotes: 18

Alexis King
Alexis King

Reputation: 43872

Yes, they are empty statements, which don't do anything at all. They are optimized out by the compiler, but in some cases this empty statement actually does something, doing the same thing as a set of empty braces ({}). They are inherited from C syntax, but they have few uses in Java. In C, sometimes doing something like this was useful:

while (condition);

This will loop until condition is false, preventing code from progressing. However, this is discouraged in modern code, and shouldn't ever really be used in Java. However, the empty statement does count as a statement, just a useless one, so constructs like this:

if (condition);
    doSomething();

....may cause somewhat baffling results. The if statement will call the empty statement, not the method, so the method will always be called. Just a caveat to keep in mind.

Upvotes: 7

Jordonias
Jordonias

Reputation: 5848

The extra semicolons are treated as empty statements. Empty statements do nothing, so that's why Java doesn't complain about it.

Upvotes: 21

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