Case_KeyWord

雨点打透心脏的1/2处 2021-11-05 06:38 219阅读 0赞

## Java Switch Case Statement with Examples ##

In this quick article, we will look into Switch case statement, which is used when we have a number of options (or choices) and we may need to perform a different task for each choice.  
Table of contents  
Switch Case Simple Example  
Switch Case Statement with Break  
Java Switch Statement with String  
Java Switch Statement with Enum  
Nested switch Statements  
The switch statement is Java’s multiway branch statement. It provides an easy way to dispatch execution to different parts of your code based on the value of an expression. As such, it often provides a better alternative than a large series of if-else-if statements.  
Syntex  
Here is the general form of a switch statement:

switch (expression) {
        case value1:
            // statement sequence
            break;
        case value2:
            // statement sequence
            break;
            .
            .
            .
            case valueN:
                // statement sequence
                break;
        default:
            // default statement sequence
    }

For versions of Java prior to JDK 7, an expression must be of type byte, short, int, char, or an enumeration. Beginning with JDK 7, expression can also be of type String. Each value specified in the case statements must be a unique constant expression (such as a literal value). Duplicate case values are not allowed. The type of each value must be compatible with the type of expression.  
Switch Case Flow Diagram

The switch statement works like this: The value of the expression is compared with each of the values in the case statements. If a match is found, the code sequence following that case statement is executed. If none of the constants matches the value of the expression, then the default statement is executed. However, the default statement is optional. If no case matches and no default is present, then no further action is taken.

### 1. Java switch case Simple Example ###

The following code example, SwitchDemo, declares an int named month whose value represents a month. The code displays the name of the month, based on the value of the month, using the switch statement.

public class SwitchDemo {
        public static void main(String[] args) {
    
            int month = 8;
            String monthString;
            switch (month) {
                case 1:
                    monthString = "January";
                    break;
                case 2:
                    monthString = "February";
                    break;
                case 3:
                    monthString = "March";
                    break;
                case 4:
                    monthString = "April";
                    break;
                case 5:
                    monthString = "May";
                    break;
                case 6:
                    monthString = "June";
                    break;
                case 7:
                    monthString = "July";
                    break;
                case 8:
                    monthString = "August";
                    break;
                case 9:
                    monthString = "September";
                    break;
                case 10:
                    monthString = "October";
                    break;
                case 11:
                    monthString = "November";
                    break;
                case 12:
                    monthString = "December";
                    break;
                default:
                    monthString = "Invalid month";
                    break;
            }
            System.out.println(monthString);
        }
    }

In this case, August is printed to standard output.

### 2. Java switch Statement with Break (break statements are optional) ###

The break statement is optional. If you omit the break, execution will continue on into the next case. It is sometimes desirable to have multiple cases without break statements between them.

package net.javaguides.corejava.controlstatements.switchcluase;
    
    public class MissingBreak {
        public static void main(String args[]) {
            for (int i = 0; i < 12; i++)
                switch (i) {
                    case 0:
                    case 1:
                    case 2:
                    case 3:
                    case 4:
                        System.out.println("i is less than 5");
                        break;
                    case 5:
                    case 6:
                    case 7:
                    case 8:
                    case 9:
                        System.out.println("i is less than 10");
                        break;
                    default:
                        System.out.println("i is 10 or more");
                }
        }
    }

Output:

i is less than 5
    i is less than 5
    i is less than 5
    i is less than 5
    i is less than 5
    i is less than 10
    i is less than 10
    i is less than 10
    i is less than 10
    i is less than 10
    i is 10 or more
    i is 10 or more

As you can see, execution falls through each case until a break statement (or the end of the switch) is reached.

### 3.Java switch Statement with String ###

Beginning with JDK 7, we can use a string to control a switch statement.  
Below example demonstrates use a string to control a switch statement.

package net.javaguides.corejava.controlstatements.switchcluase;
    
    public class StringsSwitch {
        public static void main(String args[]) {
            String str = "two";
            switch (str) {
                case "one":
                    System.out.println("one");
                    break;
                case "two":
                    System.out.println("two");
                    break;
                case "three":
                    System.out.println("three");
                    break;
                default:
                    System.out.println("no match");
                    break;
            }
        }
    }

Output:

two

### 4. Java switch Statement with Enum ###

In real-time projects, enums are frequently used with switch case statement.  
For example:

package net.javaguides.corejava.controlstatements.switchcluase;
    
    public class EnumInSwitchStatement {
    
        public static void main(String[] args) {
            System.out.println(enumInSwitch(Days.SUNDAY));
        }
    
        enum Days {
            SUNDAY,
            MONDAY,
            TUESDAY,
            WEDNESDAY,
            THURSDAY,
            FRIDAY,
            SATURDAY;
        }
    
        public static String enumInSwitch(Days day) {
            switch (day) {
                case SUNDAY:
                    return "Its Sunday!!";
                case MONDAY:
                    return "Its Monday";
                case TUESDAY:
                    return "Its Tuesday";
                case WEDNESDAY:
                    return "Its Wednesday";
                default:
                    return "Rest of the week....";
            }
        }
    }

Output:

Its Sunday!!

### 5. Java Nested switch Statements ###

We can use a switch as part of the statement sequence of an outer switch. This is called a nested switch.  
For example, the following fragment is perfectly valid:

switch (count) {
        case 1:s
            switch (target) { // nested switch
                case 0:
                    System.out.println("target is zero");
                    break;
                case 1: // no conflicts with outer switch
                    System.out.println("target is one");
                    break;
            }
            break;
        case 2: // ...

Here, the case 1: statement in the inner switch does not conflict with the case 1: statement in the outer switch. The count variable is compared only with the list of cases at the outer level. If a count is 1, then the target is compared with the inner list cases.

Case_KeyWord

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