Java: switch lambda-like syntax

Posted 2023-04-09 ascertain

 

The switch expression has an additional lambda-like syntax and it can be used not only as a statement, but also as an expression that evaluates to a single value.

 

With the new lambda-like syntax, if a label is matched, then only the expression or statement to the right of the arrow is executed; there is no fall through

package com.example.prom;

import java.util.Scanner;

public class D 
    public static void main(String[] args) 
        Scanner scanner = new Scanner(System.in);
        String next = scanner.next();
        byte result = switch (next) 
            case "A" -> 1;
            case "B" -> 2;
            default -> throw new IllegalStateException("Unexpected value: " + next);
        ;
        System.out.println("result = " + result);
    

 

package com.example.prom;

import java.util.Scanner;

public class D 
    public static void main(String[] args) 
        Scanner scanner = new Scanner(System.in);
        String next = scanner.next();
        int result;

        switch (next) 
            case "A" -> result = 1;
            case "B" -> result = 2;
            case "C" -> 
                result = 3;
                System.out.println("3!");
            
            default -> 
                System.err.println("Unexpected value: " + next);
                result = -1;
            
        
        System.out.println("result = " + result);
    

 

yield In the situation when a block is needed in a case, yield can be used to return a value from it

package com.example.prom;

import java.util.Scanner;

public class D 
    public static void main(String[] args) 
        Scanner scanner = new Scanner(System.in);
        String next = scanner.next();

        var result = switch (next) 
            case "A" -> 1;
            case "B" -> 2;
            case "C", "D", "E" -> 
                System.out.println("3!");
                yield 3;  // return
            
            default -> throw new IllegalStateException("Unexpected value: " + next);
        ;
        System.out.println("result = " + result);
    

 

    protected double calculator(char operator, double x, double y) 
        return switch (operator) 
            case \'+\' -> x + y;
            case \'-\' -> x - y;
            case \'*\' -> x * y;
            case \'/\' -> 
                if (y == 0)
                    throw new IllegalArgumentException("Can\'t divide by 0");
                yield x / y;
            
            default -> throw new IllegalArgumentException("Unknown operator `%s`".formatted(operator));
        ;
    

 

java基础之switch

switch 语句由一个控制表达式和多个case标签组成。

switch 控制表达式支持的类型有byte、short、char、int、enum（Java 5）、String(Java 7)。

switch-case语句完全可以与if-else语句互转，但通常来说，switch-case语句执行效率要高。

default在当前switch找不到匹配的case时执行。default并不是必须的。

一旦case匹配，就会顺序执行后面的程序代码，而不管后面的case是否匹配，直到遇见break。

switch语法格式

switch (表达式) {
case 条件1:
    语句1;
    break;
case 条件2:
    语句2;
    break;
...
default:
    语句;
}

分支语句先对表达式进行求值，然后依次匹配条件1，条件2... ...当表达式的值于条件相匹配时，执行对应的语句，如果没有对应对条件，则执行default语句。

byte：

 1 byte b1=12;
 2         switch(b1){
 3         case 1:
 4             System.out.println("wrong");
 5             break;
 6         
 7         case 2:
 8             System.out.println("wrong2");
 9             break;
10         
11         case 12:
12             System.out.println("right");
13             break;
14             
15         default:
16             System.out.println("all wrong");
17         }

 int:

 1 int num1=12;
 2         switch(num1){
 3         case 1:
 4             System.out.println("wrong");
 5             break;
 6         
 7         case 2:
 8             System.out.println("wrong2");
 9             break;
10         
11         case 12:
12             System.out.println("right");
13             break;
14             
15         default:
16             System.out.println("all wrong");
17         }

char:

 1 char c1=‘a‘;
 2         switch (c1) {
 3         case ‘a‘:
 4             System.out.println("a is right");
 5             break;
 6             
 7         case ‘b‘:
 8             System.out.println("b is right");
 9             break;
10             
11         case ‘c‘:
12             System.out.println("c is right");
13             break;
14         default:
15             break;
16         }

string:

 1 String str1="123";
 2         switch (str1) {
 3         case "111":
 4             System.out.println("111 is right");
 5             break;
 6 
 7         case "112":
 8             System.out.println("112 is right");
 9             break;
10             
11         case "123":
12             System.out.println("123 is right");
13             break;
14         default:
15             System.out.println(" no right");
16             break;
17         }

 Enum:

 1 static enum E{
 2         A,B,C,D
 3     }
 4     public static void main(String[] args) {
 5         E e=E.C;
 6         
 7         switch (e) {
 8         case A:
 9             System.out.println("A is right");
10             break;
11 
12         case B:
13             System.out.println("B is right");
14             break;
15 
16         case C:
17             System.out.println("C is right");
18             break;
19             
20         case D:
21             System.out.println("D is right");
22             break;                
23         default:
24             System.out.println("no right");
25             break;
26         }