111 lines
3.2 KiB
Markdown
111 lines
3.2 KiB
Markdown
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---
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title: Control Flow
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localeTitle: 控制流
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---
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# 控制流
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控制流语句正是该术语的含义。它们是基于决策，循环和分支改变执行流程的语句，以便程序可以有条件地执行代码块。
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首先，Java具有以下用于流控制的构造：
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* `if`
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```java
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if( <expression that results in a boolean> ){
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//code enters this block if the above expression is 'true'
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}
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```
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* `if...else`
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```java
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if( <expression that results in a boolean> ){
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//execute this block if the expression is 'true'
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} else{
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//execute this block if the expression is 'false'
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}
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```
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* `switch`
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当有多个值和要检查的案例时，Switch是`if...else`构造的替代方案。
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```java
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switch( <integer / String / Enum > ){
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case <int/String/Enum>:
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<statements>
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break;
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case <int/String/Enum>:
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<statements>
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break;
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default:
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<statements>
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}
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```
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注：程序流程`falls through`下一`case` ，如果`break`语句丢失。例如，假设你对办公室里的每个人都说标准的“你好”，但对于坐在你旁边的女孩来说，你对你的老板说得好脾气。表示方式如下：
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```java
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switch(person){
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case 'boss':
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soundGrumpy();
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break;
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case 'neighbour':
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soundExtraNice();
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break;
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case 'colleague':
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soundNormal();
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break;
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default:
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soundNormal();
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}
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```
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```
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Note: The `default` case runs when none of the `case` matches. Remember that when a case has no `break` statement, it `falls through` to the next case and will continue to the subsequent `cases` till a `break` is encountered. Because of this, make sure that each case has a `break` statement. The `default` case does not require a `break` statement.
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```
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* `nested statements`
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可以嵌套任何先前的控制流。这意味着您可以嵌套`if` ， `if..else`和`switch..case`语句。也就是说，您可以将这些语句的任意组合放在另一个语句中，并且`nesting`的深度没有限制。
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例如，让我们考虑以下场景：
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* 如果你的钱少于25美元，你可以给自己一杯咖啡。
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* 如果你有超过25美元但不到60美元，你会得到一个体面的饭菜。
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* 如果你有超过60美元但不到100美元，你会得到一顿不错的美食和一杯葡萄酒。
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* 然而，当你有超过100美元，取决于你与谁在一起，你要么去一个烛光晚餐（与你的妻子）或你去体育酒吧（与你的朋友）。
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表示这一点的方法之一是：
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```java
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int cash = 150;
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String company = "friends";
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if( cash < 25 ){
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getCoffee();
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} else if( cash < 60 ){
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getDecentMeal();
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} else if( cash < 100 ){
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getDecentMeal();
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getGlassOfWine();
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} else {
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switch(company){
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case "wife":
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candleLitDinner();
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break;
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case "friends":
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meetFriendsAtSportsBar();
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break;
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default:
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getDecentMeal();
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}
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}
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```
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在此示例中，将执行`meetFriendsAtSportsBar()` 。
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 [运行代码](https://repl.it/CJZi/1)
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