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📜  Java switch

📅  最后修改于: 2020-09-24 04:39:36             🧑  作者: Mango

Java Switch语句

Javaswitch语句从多个条件中执行一个语句。就像if-else-if阶梯语句。switch语句适用于字节，短型，整型，长型，枚举类型，字符串和某些包装类型，如字节，短型，整型和长型。从Java7开始，您可以在switch语句中使用字符串。

换句话说，switch语句针对多个值测试变量的相等性。

要记住的要点

  • 一个switch表达式可以有一个或N个case值 。
  • case值只能是switch表达式类型。大小写值必须为文字或常量 。它不允许变量 。
  • 大小写值必须唯一 。如果值重复，则会产生编译时错误。
  • Java switch表达式必须为byte，short，int，long(具有Wrapper类型)， 枚举和字符串 。
  • 每个case语句可以有一个break语句 ，该语句是可选的。当控件到达break语句时，它将在switch表达式后跳转控件。如果未找到break语句，则执行下一种情况。
  • case值可以具有默认标签 ，该标签是可选的。

句法：

"switch(expression){    
case value1:    
 //code to be executed;    
 break;  //optional  
case value2:    
 //code to be executed;    
 break;  //optional  
......    
    default:     
 code to be executed if all cases are not matched;    
}

例：

"public class SwitchExample {  
public static void main(String[] args) {  
    //Declaring a variable for switch expression  
    int number=20;  
    //Switch expression  
    switch(number){  
    //Case statements  
    case 10: System.out.println("10");  
    break;  
    case 20: System.out.println("20");  
    break;  
    case 30: System.out.println("30");  
    break;  
    //Default case statement  
    default:System.out.println("Not in 10, 20 or 30");  
    }  
}  
}

查找月份示例：

"//Java Program to demonstrate the example of Switch statement  
//where we are printing month name for the given number  
public class SwitchMonthExample {    
public static void main(String[] args) {    
    //Specifying month number  
    int month=7;    
    String monthString="";  
    //Switch statement  
    switch(month){    
    //case statements within the switch block  
    case 1: monthString="1 - January";  
    break;    
    case 2: monthString="2 - February";  
    break;    
    case 3: monthString="3 - March";  
    break;    
    case 4: monthString="4 - April";  
    break;    
    case 5: monthString="5 - May";  
    break;    
    case 6: monthString="6 - June";  
    break;    
    case 7: monthString="7 - July";  
    break;    
    case 8: monthString="8 - August";  
    break;    
    case 9: monthString="9 - September";  
    break;    
    case 10: monthString="10 - October";  
    break;    
    case 11: monthString="11 - November";  
    break;    
    case 12: monthString="12 - December";  
    break;    
    default:System.out.println("Invalid Month!");    
    }    
    //Printing month of the given number  
    System.out.println(monthString);  
}    
}

检查元音或辅音的程序：

如果字符是A，E，I，O或U，则它是元音，否则是辅音。它不区分大小写。

"public class SwitchVowelExample {    
public static void main(String[] args) {    
    char ch='O';    
    switch(ch)  
    {  
        case 'a':   
            System.out.println("Vowel");  
            break;  
        case 'e':   
            System.out.println("Vowel");  
            break;  
        case 'i':   
            System.out.println("Vowel");  
            break;  
        case 'o':   
            System.out.println("Vowel");  
            break;  
        case 'u':   
            System.out.println("Vowel");  
            break;  
        case 'A':   
            System.out.println("Vowel");  
            break;  
        case 'E':   
            System.out.println("Vowel");  
            break;  
        case 'I':   
            System.out.println("Vowel");  
            break;  
        case 'O':   
            System.out.println("Vowel");  
            break;  
        case 'U':   
            System.out.println("Vowel");  
            break;  
        default:   
            System.out.println("Consonant");  
    }  
}    
}

Java Switch语句失败

Javaswitch语句是失败的。这意味着如果没有break语句，它将在第一个匹配项之后执行所有语句。

例：

"//Java Switch Example where we are omitting the  
//break statement  
public class SwitchExample2 {  
public static void main(String[] args) {  
    int number=20;  
    //switch expression with int value  
    switch(number){  
    //switch cases without break statements  
    case 10: System.out.println("10");  
    case 20: System.out.println("20");  
    case 30: System.out.println("30");  
    default:System.out.println("Not in 10, 20 or 30");  
    }  
}  
}

带字符串的Java Switch语句

从JavaSE7开始，Java允许我们在开关表达式中使用字符串。case语句应为字符串文字。

例：

"//Java Program to demonstrate the use of Java Switch  
//statement with String  
public class SwitchStringExample {    
public static void main(String[] args) {    
    //Declaring String variable  
    String levelString="Expert";  
    int level=0;  
    //Using String in Switch expression  
    switch(levelString){    
    //Using String Literal in Switch case  
    case "Beginner": level=1;  
    break;    
    case "Intermediate": level=2;  
    break;    
    case "Expert": level=3;  
    break;    
    default: level=0;  
    break;  
    }    
    System.out.println("Your Level is: "+level);  
}    
}

Java嵌套开关语句

我们可以在Java中的其他switch语句中使用switch语句。它被称为嵌套switch语句。

例：

"//Java Program to demonstrate the use of Java Nested Switch  
public class NestedSwitchExample {    
    public static void main(String args[])  
      {  
      //C - CSE, E - ECE, M - Mechanical  
        char branch = 'C';                 
        int collegeYear = 4;  
        switch( collegeYear )  
        {  
            case 1:  
                System.out.println("English, Maths, Science");  
                break;  
            case 2:  
                switch( branch )   
                {  
                    case 'C':  
                        System.out.println("Operating System, Java, Data Structure");  
                        break;  
                    case 'E':  
                        System.out.println("Micro processors, Logic switching theory");  
                        break;  
                    case 'M':  
                        System.out.println("Drawing, Manufacturing Machines");  
                        break;  
                }  
                break;  
            case 3:  
                switch( branch )   
                {  
                    case 'C':  
                        System.out.println("Computer Organization, MultiMedia");  
                        break;  
                    case 'E':  
                        System.out.println("Fundamentals of Logic Design, Microelectronics");  
                        break;  
                    case 'M':  
                        System.out.println("Internal Combustion Engines, Mechanical Vibration");  
                        break;  
                }  
                break;  
            case 4:  
                switch( branch )   
                {  
                    case 'C':  
                        System.out.println("Data Communication and Networks, MultiMedia");  
                        break;  
                    case 'E':  
                        System.out.println("Embedded System, Image Processing");  
                        break;  
                    case 'M':  
                        System.out.println("Production Technology, Thermal Engineering");  
                        break;  
                }  
                break;  
        }  
    }  
}

Switch语句中的Java枚举

Java允许我们在switch语句中使用枚举。

例：

"//Java Program to demonstrate the use of Enum  
//in switch statement  
public class JavaSwitchEnumExample {      
       public enum Day {  Sun, Mon, Tue, Wed, Thu, Fri, Sat  }    
       public static void main(String args[])    
       {    
         Day[] DayNow = Day.values();    
           for (Day Now : DayNow)    
           {    
                switch (Now)    
                {    
                    case Sun:    
                        System.out.println("Sunday");    
                        break;    
                    case Mon:    
                        System.out.println("Monday");    
                        break;    
                    case Tue:    
                        System.out.println("Tuesday");    
                        break;         
                    case Wed:    
                        System.out.println("Wednesday");    
                        break;    
                    case Thu:    
                        System.out.println("Thursday");    
                        break;    
                    case Fri:    
                        System.out.println("Friday");    
                        break;    
                    case Sat:    
                        System.out.println("Saturday");    
                        break;    
                }    
            }    
        }    
}

Switch语句中的Java包装器

Java允许我们在switch语句中使用四个包装器类：Byte，Short，Integer和Long。

例：

"//Java Program to demonstrate the use of Wrapper class  
//in switch statement  
public class WrapperInSwitchCaseExample {       
       public static void main(String args[])  
       {         
            Integer age = 18;        
            switch (age)  
            {  
                case (16):            
                    System.out.println("You are under 18.");  
                    break;  
                case (18):                
                    System.out.println("You are eligible for vote.");  
                    break;  
                case (65):                
                    System.out.println("You are senior citizen.");  
                    break;  
                default:  
                    System.out.println("Please give the valid age.");  
                    break;  
            }             
        }  
}