java17学习笔记-switch总结

本文会介绍switch的历史。新版本更新内容及代码示例

发展史

Java 1.0（1996）：首次引入switch语句，支持byte、short、char、int及枚举类型。
Java 7（2011）：支持字符串类型（String）作为表达式。
Java 12（2018）：引入模式匹配(第一次预览)JEP 325: Switch Expressions (Preview)
Java 13（2019）：引入模式匹配(第二次预览)JEP 354: Switch Expressions (Second Preview)
Java 14（2019）：引入模式匹配(上线)JEP 361: Switch Expressions
Java 17（2021）：匹配增强(第一次预览)JEP 406: Pattern Matching for switch (Preview)引入保护模式和括号模式
Java 18（2021）：匹配增强(第二次预览)JEP 420: Pattern Matching for switch (Second Preview)优化保护模式，表达式扩展密封类sealed
Java 19（2022）：匹配增强(第三次预览)JEP 427: Pattern Matching for switch (Third Preview)引入when替换保护和括号模式，表达式拓展null
Java 20（2022）：匹配增强(第四次预览)JEP 433: Pattern Matching for switch (Fourth Preview)编译检查case的详尽，表达式拓展泛型类型参数推断，记录类record 

Java 21（2023）：匹配增强(上线)JEP 441：交换机的模式匹配 删除括号模式，表达式拓展枚举

java12之前的switch

示例

        String a = "2";switch (a) {case "1":System.out.println("it is Monday");break;case "2":System.out.println("it is Tuesday");break;case "3":System.out.println("it is Wednesday");break;case "4":System.out.println("it is Thursday");break;case "5":System.out.println("it is Friday");break;case "6":System.out.println("it is Saturday");break;case "7":System.out.println("it is Sunday");break;default:System.out.println("this is not one day of a week");break;}

也是比较常见的用法，每一个case必须要用break，当case子句未使用break时，程序会继续执行后续case的代码（称为“case穿透”）

        String a = "2";switch (a) {case "1":case "2":case "3":case "4":case "5" : System.out.println("it is workday");break;case "6", "7":System.out.println("it is weekday");break;default:System.out.println("this is not one day of a week");break;}

java14第一次switch语法更新上线

引入全新模式"->"                     依旧支持":"+"break"模式

        String dayName = "Monday";switch (dayName) {case "Monday", "Tuesday", "Wednesday", "Thursday", "Friday"->System.out.println("Workday");case "Saturday", "Sunday"->System.out.println("Weekend");default->System.out.println(dayName + " is not a day");};

以上代码可以改写成如下，->默认带有返回值 switch 新增了返回值。

        String dayName = "Monday";System.out.println(switch (dayName) {case "Monday", "Tuesday", "Wednesday", "Thursday", "Friday"->"Workday";case "Saturday", "Sunday"->"Weekend";default->dayName + " is not a day";});

还支持":"+:yield" 用于case中处理复杂逻辑之后返回

    public String getDayType(String dayName) {return switch (dayName) {case "Monday", "Tuesday", "Wednesday", "Thursday", "Friday":yield "Workday";case "Saturday", "Sunday":yield "Weekday";default://模拟处理数据String resString = dayName + " is not a day";yield resString;};}

java21第二次switch语法更新上线

在JDK 14中，Switch表达式已经准备好成为最终和永久的，不再进行任何更改。

但是随着java16增强instanceof的上线switch也进行了第二次增强

java21之前如果一个对象可能属于很多种类，那么只能写很多种if else如下

    static String formatter(Object o) {String formatted = "unknown";if (o instanceof Integer i) {formatted = String.format("int %d", i);} else if (o instanceof Long l) {formatted = String.format("long %d", l);} else if (o instanceof Double d) {formatted = String.format("double %f", d);} else if (o instanceof String s) {formatted = String.format("String %s", s);}return formatted;}

表达式拓展instanceof

static String formatterPatternSwitch(Object o) {return switch (o) {case Integer i -> String.format("int %d", i);case Long l    -> String.format("long %d", l);case Double d  -> String.format("double %f", d);case String s  -> String.format("String %s", s);default        -> o.toString();};
}

表达式拓展null

static String formatterPatternSwitch(Object o) {return switch (o) {case null -> "it is null";case Integer i -> String.format("int %d", i);case Long l    -> String.format("long %d", l);case Double d  -> String.format("double %f", d);case String s  -> String.format("String %s", s);default        -> o.toString();};
}

保护模式

范围越小越靠前(包括when条件和实现继承)

    static void error(Object o) {switch(o) {case CharSequence cs ->System.out.println("A sequence of length " + cs.length());case String s ->    // Error - pattern is dominated by previous patternSystem.out.println("A string: " + s);default -> {break;}}}

在switch加入条件函数when

（第三次预览加入代替之前使用的括号模式，且在java21发布版中括号模式删除）

    static void test(Object o) {switch (o) {case String s when (s.length() == 1) -> System.out.println("传入的是String且长度为1");case String s -> System.out.println("传入的是String且长度测试");case CharSequence s  -> System.out.println("mmm");default -> {System.out.println("喵喵叫");}}}

表达式拓展枚举，记录类record 

    record Point(int i, int j) {}enum Color { RED, GREEN, BLUE; }static void typeTester(Object obj) {switch (obj) {case null     -> System.out.println("null");case String s -> System.out.println("String");case Color c  -> System.out.println("Color: " + c.toString());case Point p  -> System.out.println("Record class: " + p.toString());case int[] ia -> System.out.println("Array of ints of length" + ia.length);default       -> System.out.println("Something else");}}

表达式拓展泛型类型参数推断

    record MyPair<S,T>(S fst, T snd){};static void recordInference(MyPair<String, Integer> pair){switch (pair) {case MyPair(var f, var s) -> {System.out.println(f + " " + s);}// Inferred record Pattern MyPair<String,Integer>(var f, var s)default -> System.out.println("Something else");}}

表达式扩展密封类sealed

        static void testTriangle(Shape s) {switch (s) {case Square square  -> square.side();case Rectangle rectangle-> rectangle.length();case Circle circle-> circle.center();}}
public  sealed interface Shape permits Circle, Rectangle, Square {}
public non-sealed class Circle implements Shape {void center() {System.out.println("圆心");}
}
public non-sealed class Rectangle implements Shape {void length() {System.out.println("长度");}
}
public final class Square implements Shape {int side;void side() {System.out.println("边长");}
}