冒号（:)运算符有什么作用？ [英] What does the colon (:) operator do?

问题描述

显然在Java中以多种方式使用冒号。有人会介意解释它的作用吗？

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

例如这里：

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

你怎么写这个 for-each 以不同的方式循环，以便不包含：？

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

推荐答案

在Java代码中使用冒号的几个地方：

There are several places colon is used in Java code:

1）跳出标签（教程）：

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（教程 ）：

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循环（教程）：

3) For-each loop (Tutorial):

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

4 ）断言（指南）：

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 教程）：

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）方法参考（教程）

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

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

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