Java 13 –开关表达式

电玩女神 2023-02-15 05:07 12阅读 0赞

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

在Java 13中， [JEP 354：交换表达式][JEP 354]通过添加新的`yield`关键字从交换表达式返回值来扩展了先前的Java 12交换表达式。

*PS Switch表达式是预览功能，默认情况下处于禁用状态。*

> **注意**  
> 这是Java 14中的标准功能。

## 1.没有更多的价值突破！ ##

1.1 Java 12 `value breaks`语法不再在Java 13中编译，而是使用`yield` 。

// value breaks are superseded by Java 13 'yield' statements.
    	private static int getValueViaBreak(String mode) {
            int result = switch (mode) {
                case "a":
                case "b":
                    break 1;
                case "c":
                    break 2;
                case "d":
                case "e":
                case "f":
                    break 3;
                default:
                    break -1;
            };
            return result;
        }

1.2在Java 13中，我们可以使用`yield`返回一个值。

private static int getValueViaYield(String mode) {
            int result = switch (mode) {
                case "a", "b":
                    yield 1;
                case "c":
                    yield 2;
                case "d", "e", "f":
                    // do something here...
                    System.out.println("Supports multi line block!");
                    yield 3;
                default:
                    yield -1;
            };
            return result;
        }

1.3 Java 13仍支持`switch`表达式（Java 12）的标签规则或箭头。

private static int getValueViaArrow(String mode) {
            int result = switch (mode) {
                case "a", "b" -> 1;
                case "c" -> 2;
                case "d", "e", "f" -> {
                    // do something here...
                    System.out.println("Supports multi line block!");
                    yield 3;
                }
                default -> -1;
            };
            return result;
        }

## 2. Java 13开关表达式 ##

Java 13中的完整`switch`表达式示例。

JEP354.java

package com.mkyong.java13;
    
    public class JEP354 {
    
        public static void main(String[] args) {
    
            System.out.println(getValueViaYield("a"));
            System.out.println(getValueViaYield("c"));
            System.out.println(getValueViaYield("e"));
            System.out.println(getValueViaYield("z"));
    
        }
    
        // Traditional switch
        private static int getValueBefore12(String mode) {
            int result;
            switch (mode) {
                case "a":
                case "b":
                    result = 1;
                    break;
                case "c":
                    result = 2;
                    break;
                case "d":
                case "e":
                case "f":
                    result = 3;
                    break;
                default:
                    result = -1;
            }
            ;
            return result;
        }
    
        // Java 12, multiple comma-separated labels
        private static int getValueMultipleLabels(String mode) {
            int result;
            switch (mode) {
                case "a", "b":
                    result = 1;
                    break;
                case "c":
                    result = 2;
                    break;
                case "d", "e", "f":
                    result = 3;
                    break;
                default:
                    result = -1;
            }
            ;
            return result;
        }
    
        // Java 13, value breaks are superseded by 'yield' statements
        // Java 12, switch expression returning value via break
        /*private static int getValueViaBreak(String mode) {
            int result = switch (mode) {
                case "a":
                case "b":
                    break 1;
                case "c":
                    break 2;
                case "d":
                case "e":
                case "f":
                    break 3;
                default:
                    break -1;
            };
            return result;
        }*/
    
        // Java 12, switch expression returns a value via label rules (arrow)
        private static int getValueViaArrow(String mode) {
            int result = switch (mode) {
                case "a", "b" -> 1;
                case "c" -> 2;
                case "d", "e", "f" -> {
                    // do something here...
                    System.out.println("Supports multi line block!");
                    yield 3;
                }
                default -> -1;
            };
            return result;
        }
    
        // Java 13, switch expression returns a value via yield
        private static int getValueViaYield(String mode) {
            int result = switch (mode) {
                case "a", "b":
                    yield 1;
                case "c":
                    yield 2;
                case "d", "e", "f":
                    // do something here...
                    System.out.println("Supports multi line block!");
                    yield 3;
                default:
                    yield -1;
            };
            return result;
        }
    
    }

输出量

1
    2
    Supports multi line block!
    3
    -1

## 3.启用预览功能 ##

3.1正常的`javac`编译将提示以下错误：

$ javac JEP354.java
    D:\test>javac JEP354.java
    JEP354.java:39: error: multiple case labels are a preview feature and are disabled by default.
                case "a", "b":
                          ^
      (use --enable-preview to enable multiple case labels)
    JEP354.java:76: error: switch expressions are a preview feature and are disabled by default.
            int result = switch (mode) {
                         ^
      (use --enable-preview to enable switch expressions)
    JEP354.java:77: error: switch rules are a preview feature and are disabled by default.
                case "a", "b" -> 1;
                              ^
      (use --enable-preview to enable switch rules)
    3 errors

3.2我们需要手动启用预览功能：

javac --enable-preview --release 13 Example.java
    java --enable-preview Example

## 参考文献 ##

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

标签： [Java][Java 1] [Java 13][] [新功能][Link 1] [开关][Link 2]

> 翻译自: [https://mkyong.com/java/java-13-switch-expressions/][https_mkyong.com_java_java-13-switch-expressions]

[format_png]: http://imgconvert.csdnimg.cn/aHR0cHM6Ly9ta3lvbmcuY29tL3dwLWNvbnRlbnQvdXBsb2Fkcy8yMDE5LzA5L2phdmExMy1zd2l0Y2gtZXhhbXBsZS5wbmc?x-oss-process=image/format,png
[JEP 354]: https://openjdk.java.net/jeps/354
[OpenJDK 13]: https://openjdk.java.net/projects/jdk/13/
[Java 12]: https://www.mkyong.com/java/java-12-switch-expressions/
[JEP 325]: https://openjdk.java.net/jeps/325
[JEP 305_instanceof]: https://openjdk.java.net/jeps/305
[Java]: https://en.wikipedia.org/wiki/Java_version_history
[Java 1]: https://mkyong.com/tag/java/
[Java 13]: https://mkyong.com/tag/java-13/
[Link 1]: https://mkyong.com/tag/new-features/
[Link 2]: https://mkyong.com/tag/switch/
[https_mkyong.com_java_java-13-switch-expressions]: https://mkyong.com/java/java-13-switch-expressions/