Java 13 –开关表达式
在Java 13中， JEP 354：交换表达式通过添加新的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// value breaks are superseded by Java 13 'yield' statements.private static int getValueViaBreak(String mode) {int result = switch (mode) {case "a":case "b":break 1;case "c":break 2;case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 switchprivate 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 labelsprivate 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= 2;break;case "d", "e", "f":result = 3;break;default:result = -1;};return result;}// Java 13, value breaks are superseded by 'yield' statements// Java 12, switch expression returning value via break/*private static int getValueViaBreak(String mode) {int result = switch (mode)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 yieldprivate 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
12Supports 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.javaD:\test>javac JEP354.javaJEP354.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.javajava --enable-preview Example
参考文献
- OpenJDK 13项目
- JEP-354：开关表达式（预览）
- Java 12 –开关表达式
- JEP 325：开关表达式
- JEP 305：用于instanceof的模式匹配
- Java版本历史
标签： Java Java 13 新功能 开关
翻译自: https://mkyong.com/java/java-13-switch-expressions/
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