为什么我们需要事后声明？

#1 楼

case 'A':
case 'B':
case 'C':
    doSomething();
    break;

case 'D':
case 'E':
    doSomethingElse();
    break;

#2 楼

#3 楼

class SwitchDemo2 {
    public static void main(String[] args) {

        int month = 2;
        int year = 2000;
        int numDays = 0;

        switch (month) {
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                numDays = 31;
                break;
            case 4:
            case 6:
            case 9:
            case 11:
                numDays = 30;
                break;
            case 2:
                if ( ((year % 4 == 0) && !(year % 100 == 0))
                     || (year % 400 == 0) )
                    numDays = 29;
                else
                    numDays = 28;
                break;
            default:
                System.out.println("Invalid month.");
                break;
        }
        System.out.println("Number of Days = " + numDays);
    }
}

#4 楼

#5 楼

switch (someValue)
{
    case extendedActionValue:
        // do extended action here, falls through to normal action
    case normalActionValue:
    case otherNormalActionValue:
        // do normal action here
        break;
}

#6 楼

#7 楼

public class SwitchExpressionsNoFallThrough {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int value = scanner.nextInt();
        /*
         * Before JEP-325
         */
        switch (value) {
            case 1:
                System.out.println("one");
            case 2:
                System.out.println("two");
            default:
                System.out.println("many");
        }

        /*
         * After JEP-325
         */
        switch (value) {
            case 1 ->System.out.println("one");
            case 2 ->System.out.println("two");
            default ->System.out.println("many");
        }
    }
}

//input
1
// output from the implementation before JEP-325
one
two
many
// output from the implementation after JEP-325
one

//input
2
// output from the implementation before JEP-325
two
many
// output from the implementation after JEP-325
two

// input
3
many // default case match
many // branches to 'default' as well

#8 楼

case THIS:
case THAT:
{
    code;
    break;
}

case THIS:
{
   do this;
}
case THAT:
{
   do that;
}

#9 楼

#10 楼

#11 楼

public class SwitchTester{
    private static final Log log = LogFactory.getLog(SwitchTester.class);
    public static void main(String[] args){
        log.info(monthsOfTheSeason(Season.WINTER));
        log.info(monthsOfTheSeason(Season.SPRING));
        log.info(monthsOfTheSeason(Season.SUMMER));
        log.info(monthsOfTheSeason(Season.AUTUMN));
    }

    enum Season{WINTER, SPRING, SUMMER, AUTUMN};

    static String monthsOfTheSeason(Season season){
        switch(season){
            case WINTER:
                return "Dec, Jan, Feb";
            case SPRING:
                return "Mar, Apr, May";
            case SUMMER:
                return "Jun, Jul, Aug";
            case AUTUMN:
                return "Sep, Oct, Nov";
            default:
                //actually a NullPointerException will be thrown before reaching this
                throw new IllegalArgumentException("Season must not be null");
        }        
    }
}   

12:37:25.760 [main] INFO lang.SwitchTester - Dec, Jan, Feb
12:37:25.762 [main] INFO lang.SwitchTester - Mar, Apr, May
12:37:25.762 [main] INFO lang.SwitchTester - Jun, Jul, Aug
12:37:25.762 [main] INFO lang.SwitchTester - Sep, Oct, Nov

#12 楼

switch(a) {
  case 1: //I'm between 1 and 3
  case 2: //I'm between 1 and 3
  case 3: //I'm between 1 and 3
          break;
}

#13 楼

#14 楼

case 404:
case 500:
        {
            Log.v("Server responses","Unable to respond due to server error");
            break;
        }
        case 401:
        {
             //regenerate token
        }
        case 200:
        {
            // log in user
            break;
        }

#15 楼

public static void spanishNumbers(String span){

    span = span.toLowerCase().replace(" ", "");
    switch (span){
     case "1":    
     case "jan":  System.out.println("uno"); break;    
     case "2":      
     case "feb":  System.out.println("dos"); break;    
     case "3":     
     case "mar":  System.out.println("tres"); break;   
     case "4":   
     case "apr":  System.out.println("cuatro"); break;
     case "5":    
     case "may":  System.out.println("cinco"); break;
     case "6":     
     case "jun":  System.out.println("seis"); break;
     case "7":    
     case "jul":  System.out.println("seite"); break;
     case "8":    
     case "aug":  System.out.println("ocho"); break;
     case "9":   
     case "sep":  System.out.println("nueve"); break;
     case "10":    
     case "oct": System.out.println("diez"); break;
     }
 }

#16 楼

while(1){
    switch(state){
      case state1:
        // Wait for user input code
        state = state3; // Jump to state3
        break;
      case state2:
        //Print the sum code
        state = state3; // Jump to state3;
      case state3:
        // Calculate the sum code
        state = wait; // Jump to state1
        break;
    }
}

#17 楼