Java 12 –开关表达式

刺骨的言语ヽ痛彻心扉 2023-02-15 05:07 15阅读 0赞

![切换表达][format_png]![切换表达][format_png]

Java 12， [JEP 325：开关表达式][JEP 325]增强了传统的`switch`语句，以支持以下新功能：

*  多个案例标签
 *  开关表达式通过break返回值（在[Java 13开关表达式中][Java 13]替换为`yield` ）
 *  通过标签规则切换表达式返回值（箭头）

*PS Switch表达式是预览功能，默认情况下处于禁用状态。*

一个传统的`switch`示例。

JEP325.java

package com.mkyong.java12;
    
    public class JEP325 {
    
        public static void main(String[] args) {
    
            System.out.println(getTextBefore12(1));
            System.out.println(getTextBefore12(3));
            System.out.println(getTextBefore12(6));
            System.out.println(getTextBefore12(10));
    
        }
    
        // Traditional switch statement
        private static String getTextBefore12(int number) {
            String result = "";
            switch (number) {
                case 1:
                case 2:
                    result = "one or two";
                    break;
                case 3:
                    result = "three";
                    break;
                case 4:
                case 5:
                case 6:
                    result = "four or five or six";
                    break;
                default:
                    result = "unknown";
            };
            return result;
        }
    
    }

输出量

one or two
    three
    four or five or six
    unknown

## 1.多个案例标签 ##

现在，我们可以用逗号分隔的标签声明多个情况。

private static String getTextMultipleLabels(int number) {
            String result = "";
            switch (number) {
                case 1, 2:
                    result = "one or two";
                    break;
                case 3:
                    result = "three";
                    break;
                case 4, 5, 6:
                    result = "four or five or six";
                    break;
                default:
                    result = "unknown";
            };
            return result;
        }

## 2.通过中断返回值 ##

现在，此`switch`是一个表达式（返回值），也称为值中断。

private static String getTextViaBreak(int number) {
            String result = switch (number) {
                case 1:
                case 2:
                    break "one or two"; 
                case 3:
                    break "three";
                case 4:
                case 5:
                case 6:
                    break "four or five or six";
                default:
                    break "unknown";
            };
            return result;
        }

> **注意**  
> 该值中断被Java 13 `yield`关键字取代，请阅读此[Java 13开关表达式][Java 13] 。

## 4.通过箭头返回值 ##

我们还可以通过箭头或标签规则返回值。

private static String getTextViaArrow(int number) {
            String result = switch (number) {
                case 1, 2 -> "one or two";
                case 3 -> "three";
                case 4, 5, 6 -> "four or five or six";
                default -> "unknown";
            };
            return result;
        }

## 5.启用预览功能 ##

5.1正常的`javac`编译将提示以下错误：

$ javac JEP325.java
    JEP325.java:39: error: multiple case labels are a preview feature and are disabled by default.
                case 1, 2:
                        ^
      (use --enable-preview to enable multiple case labels)
    
    JEP325.java:56: error: switch expressions are a preview feature and are disabled by default.
            String result = switch (number) {
                            ^
      (use --enable-preview to enable switch expressions)
    JEP325.java:75: error: switch rules are a preview feature and are disabled by default.
                case 1, 2 -> "one or two";
                          ^
      (use --enable-preview to enable switch rules)
    3 errors

5.2我们需要手动启用预览功能：

javac --enable-preview --release 12 Example.java
    java --enable-preview Example

## 参考文献 ##

*  [Java 13 –开关表达式][Java 13]
 *  [OpenJDK 12项目][OpenJDK 12]
 *  [JEP 325：开关表达式][JEP 325]
 *  [JEP 305：用于instanceof的模式匹配][JEP 305_instanceof]
 *  [JEP-354：开关表达式（预览）][JEP-354]
 *  [Java版本历史][Java]

标签： [Java][Java 1] [Java 12的][Java 12] [新功能][Link 1] [开关][Link 2]

> 翻译自: [https://mkyong.com/java/java-12-switch-expressions/][https_mkyong.com_java_java-12-switch-expressions]

[format_png]: http://imgconvert.csdnimg.cn/aHR0cHM6Ly9ta3lvbmcuY29tL3dwLWNvbnRlbnQvdXBsb2Fkcy8yMDE5LzA5L2phdmExMi1zd2l0Y2gtZXhhbXBsZS5wbmc?x-oss-process=image/format,png
[JEP 325]: https://openjdk.java.net/jeps/325
[Java 13]: https://www.mkyong.com/java/java-13-switch-expressions/
[OpenJDK 12]: https://openjdk.java.net/projects/jdk/12/
[JEP 305_instanceof]: https://openjdk.java.net/jeps/305
[JEP-354]: https://openjdk.java.net/jeps/354
[Java]: https://en.wikipedia.org/wiki/Java_version_history
[Java 1]: https://mkyong.com/tag/java/
[Java 12]: https://mkyong.com/tag/java-12/
[Link 1]: https://mkyong.com/tag/new-features/
[Link 2]: https://mkyong.com/tag/switch/
[https_mkyong.com_java_java-12-switch-expressions]: https://mkyong.com/java/java-12-switch-expressions/