How does continue work?

Question

I am trying to understand how "continue" works. I understood the concept of the keyword but when I run different programs, it works differently :-/ Let me show you few examples:

If I run this program :

int j = 0; int i = 0; LABEL1: for (; i < 3; i++) {   if (true)     continue; } 

The value of i is gonna be 3. so far so good. Let's add an outer loop :

int j = 0; int i = 0; LABEL2: for (; j < 3; j++) {   LABEL1: for (; i < 3; i++) {     if (true)       continue LABEL2;   } } 

The value of i is gonna be ... 0 !! I don't understand why i is not incremented if continue is used with a label that goes to the outer loop. Can somebody explain it why ? Do you have some tricky things like that with break ? or with a do {} while ?

I really appreciate any help you can provide.

Answer 1

The basic for statement structure is as follows:

 BasicForStatement: for ( [ForInit] ; [Expression] ; [ForUpdate] ) Statement 

Now, from JLS §14.14.1.3. Abrupt Completion of for Statement:

If execution of the Statement completes abruptly because of a continue with no label, then the following two steps are performed in sequence:

1. First, if the ForUpdate part is present, the expressions are evaluated in sequence from left to right; their values, if any, are discarded. If the ForUpdate part is not present, no action is taken.

2. Second, another for iteration step is performed.

If execution of the Statement completes abruptly because of a continue with label L, then there is a choice:

• If the for statement has label L, then the following two steps are performed in sequence:

  1. First, if the ForUpdate part is present, the expressions are evaluated in sequence from left to right; their values, if any, are discarded. If the ForUpdate is not present, no action is taken.

  2. Second, another for iteration step is performed.

• If the for statement does not have label L, the for statement completes abruptly because of a continue with label L.

(emphasis mine)

ForUpdate, in your case, is i++. Based on what's above:

• Your first snippet falls under the first case, so i is incremented.

• Your second snippet falls under the second case, so i is not incremented because the for statement completes abruptly.

Note that if you had continue LABEL1 in your second snippet, i would have been incremented as in your first snippet (in accordance with the JLS).

As a tip for the future, for definitive answers regarding language rules/semantics, you should always consult the language specification. For Java, that's the JLS.