Java Switch Statement

Java Switch Java Switch Statement

switch语句的执行规则如下

    1、从第一个case开始判断，不匹配则跳到下一个case继续判断；

    2、遇到break则跳出switch语句；

    3、default一般是没有匹配项才执行的，一般是放在switch语句末尾。在如下情况下，它会被执行：一、没有匹配项的时候；二、匹配项最后没有break语句，default跟在这个匹配项后面。

switch表达式支持的数据类型

=>从JDK7开始switch表达式开始支持枚举enum、String以及包装类

* byte and Byte
* short and Short
* char and Character
* int and Integer
* enum
* String

switch语句注意事项

• switch(表达式)中表达式的返回值必须是下述几种类型之一：byte，short，char，int，枚举enum，String以及对应的包装类(Byte、Short、Character、Integer)；

• case子句中的值必须是常量不允许为变量，而且必须和switch(表达式)中表达式的数据类型一致，且所有case子句中的值应是不同的；

• break语句用来在执行完一个case分支后使程序跳出switch语句块；如果没有break，程序会顺序执行到switch结尾；

• default子句是可选的，当没有匹配的case时，执行default；

• default子句在最后一行时，下面的break可以省略不写，但如果没有在最后一行，default子句下面需要加上break，否则执行完default后会继续执行下面的代码直到遇到break跳出循环；

实践案例

public class SwitchDemo {
	public static void main(String[] args) {
		int type = 4;
		switch (type) {
		default:
			System.out.println(4);
		case 1:
			System.out.println(1);
		case 2:
			System.out.println(2);
			break;
		case 3:
			System.out.println(3);
		}
	}
}

参考答案

```
4
1
2
```

public class SwitchDemo2 {
	public static void main(String[] args) {
		int x = 5;
		switch (x) {
		case 1:
			System.out.println("A");
			break;
		case 2:
			System.out.println("B");
			break;
		default:
			System.out.println("结束");
		case 3:
			System.out.println("C");
			break;
		case 4:
			System.out.println("D");
			break;
		}
	}
}

参考答案

```
结束
C
```

public class SwitchDemo3 {
	public static void main(String[] args) {
		System.out.println(getValue(2));
	}

	public static int getValue(int i) {
		int result = 0;
		switch (i) {
		default:
			System.out.println("default");
		case 1:
			result = result + i;
		case 2:
			result = result + i * 2;
		case 3:
			result = result + i * 3;
		}
		return result;
	}
}

参考答案

```
10
```

public class SwitchDemo4 {
	public static void main(String[] args) {
		byte a = 4;
		switch (a) {
		default:
			System.out.println("default");
		case 1:
			System.out.println("A");
		case 2:
			System.out.println("B");
		case 3:
			System.out.println("C");
			break;
		case 4:
			System.out.println("D");
		}
	}
}

参考答案

```
D
```

public class SwitchDemo5 {
	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);
	}
}

参考答案

```
Number of Days = 29
```

public class SwitchDemo6 {
	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");
		}
	}
}

参考答案

```
20
30
Not in 10, 20 or 30
```

public class SwitchDemo7 {
	public static void main(String args[]) {
		// Byte age = 18;
		// Short age = 18;
		// Character age = 18;
		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;
		}
	}
}

参考答案

```
You are eligible for vote.
```

参考资料

• https://www.javatpoint.com/java-switch
• https://docs.oracle.com/javase/tutorial/java/nutsandbolts/switch.html
• https://www.geeksforgeeks.org/switch-statement-in-java/
• https://www.baeldung.com/java-switch