What does the colon (:) operator do?

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 :?

Solution 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);

Solution 2:

There is no "colon" operator, but the colon appears in two places:

1: In the ternary operator, e.g.:

int x = bigInt ? 10000 : 50;

In this case, the ternary operator acts as an 'if' for expressions. If bigInt is true, then x will get 10000 assigned to it. If not, 50. The colon here means "else".

2: In a for-each loop:

double[] vals = new double[100];
//fill x with values
for (double x : vals) {
    //do something with x
}

This sets x to each of the values in 'vals' in turn. So if vals contains [10, 20.3, 30, ...], then x will be 10 on the first iteration, 20.3 on the second, etc.

Note: I say it's not an operator because it's just syntax. It can't appear in any given expression by itself, and it's just chance that both the for-each and the ternary operator use a colon.