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

In your specific case,

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

this.list is a collection (list, set, or array), and that code assigns c to each element of the collection.

So, if this.list were a collection {"2S", "3H", "4S"} then the cardString on the end would be this string:

2S
3H
4S

Answer 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.

Answer 3

Just to add, when used in a for-each loop, the ":" can basically be read as "in".

So

for (String name : names) {
    // remainder omitted
}

should be read "For each name IN names do ..."

Answer 4

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

Assuming that list is a Collection instance ...

public String toString() {
   String cardString = "";
   for (Iterator<PlayingCard> it = this.list.iterator(); it.hasNext(); /**/) {
      PlayingCard c = it.next();
      cardString = cardString + c + "\n";
   }
}

---

I should add the pedantic point that : is not an operator in this context. An operator performs an operation in an expression, and the stuff inside the ( ... ) in a for statement is not an expression ... according to the JLS.

Answer 5

It's used in for loops to iterate over a list of objects.

for (Object o: list)
{
    // o is an element of list here
}

Think of it as a for <item> in <list> in Python.

Answer 6

You usually see it in the ternary assignment operator;

Syntax

variable =  `condition ? result 1 : result 2;`

example:

boolean isNegative = number > 0 ? false : true;

which is "equivalent" in nature to the if else

if(number > 0){
    isNegative = false;
}
else{
    isNegative = true;
}

Other than examples given by different posters,

you can also use : to signify a label for a block which you can use in conjunction with continue and break..

for example:

public void someFunction(){
     //an infinite loop
     goBackHere: { //label
          for(int i = 0; i < 10 ;i++){
               if(i == 9 ) continue goBackHere;
          }
     }
}