What does the colon (:) operator do?

Question

Apparently a colon is used in multiple ways in Java. Would anyone mind explaining what it does?

For instance here:

String cardString = "";
for (PlayingCard c : this.list)  // <--
{
    cardString += c + "\n";
}

How would you write this for-each loop a different way so as to not incorporate the :?

Answer 1

There are several places colon is used in Java code:

1. Jump-out label (Tutorial):

   label: for (int i = 0; i < x; i++) {
       for (int j = 0; j < i; j++) {
           if (something(i, j)) break label; // jumps out of the i loop
       }
   } 
   // i.e. jumps to here
   

2. Ternary condition (Tutorial):

   int a = (b < 4)? 7: 8; // if b < 4, set a to 7, else set a to 8
   

3. For-each loop (Tutorial):

   String[] ss = {"hi", "there"}
   for (String s: ss) {
       print(s); // output "hi" , and "there" on the next iteration
   }
   

4. Assertion (Guide):

   int a = factorial(b);
   assert a >= 0: "factorial may not be less than 0"; // throws an AssertionError with the message if the condition evaluates to false
   

5. Case in switch statement (Tutorial):

   switch (type) {
       case WHITESPACE:
       case RETURN:
           break;
       case NUMBER:
           print("got number: " + value);
           break;
       default:
           print("syntax error");
   }
   

6. Method references (Tutorial)

   class Person {
      public static int compareByAge(Person a, Person b) {
          return a.birthday.compareTo(b.birthday);
      }}
   }
   
   Arrays.sort(persons, Person::compareByAge);