Java 冒号 (:) 运算符有什么作用？
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What does the colon (:) operator do?
提问by lemon
Apparently a colon is used in multiple ways in Java. Would anyone mind explaining what it does?
显然，冒号在 Java 中以多种方式使用。有人介意解释一下它的作用吗？
For instance here:
例如这里：
String cardString = "";
for (PlayingCard c : this.list) // <--
{
cardString += c + "\n";
}
How would you write this for-eachloop a different way so as to not incorporate the :?
您将如何for-each以不同的方式编写此循环以不包含:?
采纳答案by Randy Sugianto 'Yuku'
There are several places colon is used in Java code:
Java 代码中有几个地方使用了冒号：
1) Jump-out label (Tutorial):
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) Ternary condition (Tutorial):
2）三元条件（教程）：
int a = (b < 4)? 7: 8; // if b < 4, set a to 7, else set a to 8
3) For-each loop (Tutorial):
3）for-each循环（教程）：
String[] ss = {"hi", "there"}
for (String s: ss) {
print(s); // output "hi" , and "there" on the next iteration
}
4) Assertion (Guide):
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) Case in switch statement (Tutorial):
5）switch语句中的案例（教程）：
switch (type) {
case WHITESPACE:
case RETURN:
break;
case NUMBER:
print("got number: " + value);
break;
default:
print("syntax error");
}
6) Method references (Tutorial)
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);
回答by lemon
It is used in the new short hand for/loop
它用于新的简写 for/loop
final List<String> list = new ArrayList<String>();
for (final String s : list)
{
System.out.println(s);
}
and the ternary operator
和三元运算符
list.isEmpty() ? true : false;
回答by Claudiu
There is no "colon" operator, but the colon appears in two places:
没有“冒号”运算符，但冒号出现在两个地方：
1: In the ternary operator, e.g.:
1：在三元运算符中，例如：
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".
在这种情况下，三元运算符充当表达式的“if”。如果 bigInt 为真，则 x 将获得 10000 分配给它。如果不是，则为 50。这里的冒号表示“其他”。
2: In a for-each loop:
2：在 for-each 循环中：
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.
这依次将 x 设置为 'vals' 中的每个值。因此，如果 vals 包含 [10, 20.3, 30, ...]，则 x 在第一次迭代时为 10，在第二次迭代时为 20.3，以此类推。
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.
注意：我说它不是运算符，因为它只是语法。它不能单独出现在任何给定的表达式中，只是 for-each 和三元运算符都使用冒号的机会。
回答by Mike Cialowicz
It's used in for loops to iterate over a list of objects.
它在 for 循环中用于迭代对象列表。
for (Object o: list)
{
// o is an element of list here
}
Think of it as a for <item> in <list>in Python.
把它想象成for <item> in <list>Python 中的 a。
回答by Ritwik Bose
The colon actually exists in conjunction with ?
冒号实际上与 ?
int minVal = (a < b) ? a : b;
is equivalent to:
相当于：
int minval;
if(a < b){ minval = a;}
else{ minval = b; }
Also in the for each loop:
同样在 for each 循环中：
for(Node n : List l){ ... }
literally:
字面上地：
for(Node n = l.head; n.next != null; n = n.next)
回答by ultrajohn
You usually see it in the ternary assignment operator;
您通常会在三元赋值运算符中看到它；
Syntax
句法
variable = `condition ? result 1 : result 2;`
example:
例子：
boolean isNegative = number > 0 ? false : true;
which is "equivalent" in nature to the if else
这在本质上与 if else 是“等价的”
if(number > 0){
isNegative = false;
}
else{
isNegative = true;
}
Other than examples given by different posters,
除了不同海报给出的例子，
you can also use : to signify a label for a block which you can use in conjunction with continue and break..
您还可以使用 : 来表示块的标签，您可以将其与 continue 和 break 结合使用。
for example:
例如：
public void someFunction(){
//an infinite loop
goBackHere: { //label
for(int i = 0; i < 10 ;i++){
if(i == 9 ) continue goBackHere;
}
}
}
回答by Randy Sugianto 'Yuku'
In your specific case,
在您的具体情况下，
String cardString = "";
for (PlayingCard c : this.list) // <--
{
cardString = cardString + c + "\n";
}
this.listis a collection (list, set, or array), and that code assigns cto each element of the collection.
this.list是一个集合（列表、集合或数组），并且该代码分配c给集合的每个元素。
So, if this.listwere a collection {"2S", "3H", "4S"} then the cardStringon the end would be this string:
所以，如果this.list是一个集合 {"2S", "3H", "4S"} 那么cardString最后就是这个字符串：
2S
3H
4S
回答by Stephen C
How would you write this for-each loop a different way so as to not incorporate the ":"?
你将如何以不同的方式编写这个 for-each 循环，以便不包含“:”？
Assuming that listis a Collectioninstance ...
假设这list是一个Collection实例......
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 forstatement is not an expression ... according to the JLS.
:在这种情况下，我应该添加不是运算符的迂腐点。根据 JLS ，运算符在表达式中执行操作，而( ... )infor语句中的内容不是表达式……。
回答by helpermethod
Just to add, when used in a for-each loop, the ":" can basically be read as "in".
补充一点，当在 for-each 循环中使用时，“:”基本上可以读作“in”。
So
所以
for (String name : names) {
// remainder omitted
}
should be read "For each name IN names do ..."
应该读作“对于每个名字 IN 名字做......”
回答by helpermethod
It will prints the string"something" three times.
它将打印字符串“something”三遍。
JLabel[] labels = {new JLabel(), new JLabel(), new JLabel()};
for ( JLabel label : labels )
{
label.setText("something");
panel.add(label);
}
