java study notes (introduction)_Multiple selection structure switch statement

Multi-selection structure switch statement
In Java, a switch statement is specially provided for the multi-way branch selection process. The switch statement selects to run one of multiple operations based on the value of an expression. Its grammatical form is as follows:

witch(表达式){ 
case 表达式常量1：语句1； 
break； 
case 表达式常量2：语句2； 
break； 
...... 
case 表达式常量n：语句n； 
break； 
[default：语句n+1；] 
}

Among them, a case expression constant becomes a label, representing the entrance of a case branch. When the switch statement is running, it first calculates the value of the "expression" in the switch parentheses. This value must be an integer or character type. At the same time, the value type of each subsequent case expression constant should be the same as the "expression" in the switch parentheses. The value types are consistent. A case statement represents a formulation operation and then moves to the exit of the structure. The default clause is optional. When the value of the expression does not match the value of the case expression constant, the default clause is run and the structure exits.
Finally, let me talk about a few very important points to note about switch.
　
First, switch (integer or character variable ), the type of the variable is as marked in the text, and can only be integer and character types. They contain int,char. Of course unsigned types or integers of different lengths (unsigned int,short,unsigned char), etc. are fine. In addition, enumeration types (enum) are also internally implemented by integer or character types. So that's ok too. Real type (floating point) numbers will not work, such as:

float a = 0.123; 
switch(a) //错误！a不是整型或字符类型变量。 
{ 
.... 
}

Second, case can be followed by a direct constant value, such as 1, 2, 3, 4 in the example, or it can be a constant calculation formula, such as 2+2, etc., but it cannot be a variable or an expression with variables, such as a * 2, etc. Of course, it cannot be a real number, such as 4.1, or 2.0 / 2, etc.

switch(formWay) 
{ 
case 2-1 : //正确 
... 
case a-2 : //错误 
... 
case 2.0 : //错误 
... 
}

In addition, after case and constant value, a colon is required, please be careful not to neglect it.
　
Third, the role of break. break allows the program to jump out of the entire switch statement (that is, jump to the pair of {} connected to the switch) after executing the selected branch, and complete the switch. Without this break, the program will continue to advance to the next branch until it encounters a subsequent break or the switch is completed.
For example, suppose the program now enters the branch in case 1:, but the branch in case 1 does not have break:
　

case 1 : 
System.out.println("您是通过搜索引擎来到本网站的。"); 
case 2 : 
System.out.println("您是通过朋友介绍来到本网站的。"); 
　

Then, after the program outputs "You came to this website through a search engine.", it will Continue to output "You came to this website through a friend's introduction." in case 2.
　
Fourth, default is optional. We have already mentioned its use before, and if there is no default, after the program cannot find the matching case branch, it will do nothing within the scope of the switch statement and directly complete the switch. You can also comment out the default code in the example, then test run it, and enter custom code when selecting.
　
Fifth, when necessary, {} can be used in each case to explicitly generate independent compound statements. When we talked about if... statements and other flow control statements earlier, we used {} to generate compound statements:

if (条件) 
{ 
分支一; 
}

Unless there is exactly one statement in the branch, the curly braces {} are not needed here. But in each case statement of switch, we do not indicate the use of {} in the grammatical format. Please see:

switch ( 整型或字符型变量 ) 
{ 
　case 变量可能值1 ： 
　 分支一; 
　break; 
case 变量可能值2 ： 
.... 
} 
　

General textbooks only say that case branches do not need to use {}, but here I want to remind everyone and It is not necessary to add {} to the case branch under any circumstances. For example, if you want to define a variable in a certain case:

switch (formWay) 
{ 
case 1 : 
int a=2; //错误。由于case不明确的范围，编译器无法在此处定义一个变量。 
... 
case 2 : 
... 
} 
　

In this case, adding {} can solve the problem.

switch (formWay) 
{ 
case 1 : 
　｛　 
int a=2; //正确，变量a被明确限定在当前｛｝范围内。 
... 
　｝　 
case 2 : 
... 
}

Finally take a look at the example program:

public class TestSwitch //基于字符型 
{ 
public static void main(String[] args) 
{ 
//声明变量score，并为其赋值为'C' 
char score = 'C'; 
//执行swicth分支语句 
switch (score) 
{ 
case 'A': 
System.out.println("优秀."); 
break; 
case 'B': 
System.out.println("良好."); 
break; 
case 'C': 
System.out.println("中"); 
break; 
case 'D': 
System.out.println("及格"); 
break; 
case 'F': 
System.out.println("不及格"); 
break; 
default: 
System.out.println("成绩输入错误"); 
} 
} 
}

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