冒号 (:) 运算符的作用是什么？

显然，冒号在 Java 中有多种使用方式。有人介意解释一下它的作用吗？

例如这里：

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

你会怎么写这个for-each以不同的方式循环，以便不合并:?

Java 代码中有几个地方使用了冒号：

1) 跳出标签 (Tutorial http://docs.oracle.com/javase/tutorial/java/nutsandbolts/branch.html):

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）三元条件（Tutorial http://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html):

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

3）for-each循环（Tutorial http://docs.oracle.com/javase/tutorial/java/nutsandbolts/for.html):

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

4）断言（Guide http://docs.oracle.com/javase/8/docs/technotes/guides/language/assert.html):

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) switch 语句中的 case (Tutorial http://docs.oracle.com/javase/tutorial/java/nutsandbolts/switch.html):

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

6）方法参考（Tutorial http://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html)

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

Arrays.sort(persons, Person::compareByAge);