Control statement if switch for while while break continue

     Control statement

    * Selection structure
    - if,if...else
    -switch

    * Loop structure
    -for
    -while
    -do while

    * Structure of control cycle
    -break
    -continue

if The four expressions of a sentence ：

     The first one is ：
        if( Boolean expression ){        （ The Boolean expression is true perhaps false）
            java sentence ;
            java sentence ;
            java sentence ;
            }
    
     The second kind ：
        if( Boolean expression ){
            java sentence ;
            java sentence ;
            java sentence ; 
            }else{
                java sentence ;
                java sentence ;
                java sentence ;
                }
     The third kind of ：
        if( Boolean expression ){
            java sentence ;
            java sentence ;
            java sentence ;
        } else if( Boolean expression ){
            java sentence ;
            java sentence ;
            java sentence ;
        }
     A fourth ：
    if( Boolean expression ){
            java sentence ;
            java sentence ;
            java sentence ;
        } else if( Boolean expression ){
            java sentence ;
            java sentence ;
            java sentence ;
        } else{
            java sentence ;
            java sentence ;
            java sentence ;
        }

/*
 demand ： If KFC is within the kilometer range of the location , Go to KFC Drink spicy Hu soup   If not, go home 6 Steamed bread 
*/

public class A
{
	public static void main(String[] args){
		double distance =6.0;
		if (distance<5)
		{
			System.out.println(" Let's go together KFC Drink spicy Hu soup ！");
		}
		else{
			System.out.println(" get home 6 Let's go ！");
		}
	}
}

How to make users input

First step Create a keyboard scanner object
java.util.Scanner s = new java.util.Scanner(System.in);
Accept text 【 Receive... As a string 】
String userInputContent = s.next();
Receive numbers 【 In integer form int To receive 】
int num =s.nextInt();
System.out.println(" The number you entered ："+num);

/*
	System.out.println();  Responsible for outputting to the console 【 From memory to console , The process of output 】

	 How to receive user keyboard input ？  【 from “ keyboard ” To “ Memory ”】

*/

public class A
{
	public static void main(String[] args){
		//  First step   Create a keyboard scanner object 
		java.util.Scanner s = new java.util.Scanner(System.in);
		//  The second step   call scanner Object's next（） Method to start receiving the user's keyboard input 
		//  The program will stop when it reaches here   Waiting for user input 
		//  When the user enters   And when you finally hit the Enter key   The input information is automatically assigned to userInputContent

		//  Accept text 【 Receive... As a string 】
		//String userInputContent = s.next();
		//  Output the data in memory to the console 
		//System.out.println(" You entered :"+userInputContent);

		// Receive numbers 【 In integer form int To receive 】
		int num =s.nextInt();
		System.out.println(" The number you entered ："+num);
}
}

    

demand ： Given a person's age
    【0~5】 Young children
    【6~10】 a juvenile
    【11~18】 teenagers
    【19~35】 youth
    【36~55】 middle-aged
    【56~150】 The elderly

public class A
{
	public static void main(String[] args){
		java.util.Scanner s = new java.util.Scanner(System.in);
		int userAge = s.nextInt();
		System.out.print(" The age you entered is ："+userAge);
		if (userAge>=0 & userAge<=5)
		{
			System.out.println(" Young children ");
		}
		else if (userAge>=6 & userAge<=10)
		{
			System.out.println(" a juvenile ");
		}
		else if (userAge>=11 & userAge<=18)
		{
			System.out.println(" teenagers ");
		}
		else if (userAge>=19 & userAge<=35)
		{
			System.out.println(" youth ");
		}
		else if (userAge>=36& userAge<=55)
		{
			System.out.println(" middle-aged ");
		}
		else if (userAge>=56 & userAge<=150)
		{
			System.out.println(" The elderly ");
		}
		}
	}

Improved version

/*
	 demand ： Given a person's age 
	【0~5】 Young children 
	【6~10】 a juvenile 
	【11~18】 teenagers 
	【19~35】 youth 
	【36~55】  middle-aged 
	【56~150】  The elderly 
*/

public class A
{
	public static void main(String[] args){
		java.util.Scanner s = new java.util.Scanner(System.in);
		int userAge = s.nextInt();
		System.out.print(" The age you entered is ："+userAge);
		//  Default a value 
		String str =" The elderly ";
		if (userAge<0 || userAge>150)
		{
			str =" The age you entered is not 【0~150】 Between   Please re-enter ！";
		}
		else if (userAge<=5)
		{
			str =" Young children ";
		}
		else if (userAge<=10)
		{
			str =" a juvenile ";
		}
		else if (userAge<=18)
		{
			str =" teenagers ";
		}
		else if (userAge<=35)
		{
			str =" youth ";
		}
		else if (userAge<=55)
		{
			str =" middle-aged ";
		}
		System.out.println(str);
	}
	}

Be careful ：if (" It's raining ".equals(wea))    

  demand ：
         Judge the current weather ：
             When it rains outside ：
                 Take an umbrella
                     Determine gender ：
                         When the gender is male ： Bring a big black umbrella
                         When the gender is female ： Bring a small flower umbrella
             When it's sunny outside ：
                 Judge the temperature of the weather ：
                     When the weather temperature is 30 Over degrees ：
                         When the gender is male ： Wear sunglasses
                         When the gender is female ： Wear sunscreen

Tips ： weather condition 、 temperature 、 Both sexes need to be entered from the keyboard . There is no need to judge the temperature when the weather is cloudy .

public class A
{
	public static void main(String[] args){
			java.util.Scanner s = new java.util.Scanner(System.in);
				
			//  Enter weather conditions 
			System.out.print(" Please enter the current weather conditions :");
			String wea = s.next();
			
			//  Enter gender 
			System.out.print(" Please enter gender :");
			String gender = s.next();
			//  When the weather is rainy 
			if (" It's raining ".equals(wea)){
				//  Take an umbrella  
				//  Determine gender 
				if (" male ".equals(gender))
				{
					System.out.println(" Take a small black umbrella ！");
				}
				else if (" Woman ".equals(gender))
				{
					System.out.println(" Bring a small flower umbrella ！");
				}
				else{
					System.out.println(" What about sex ！");
				}
			}
			
			//  When the weather is sunny 
			else if (" a sunny day ".equals(wea))
			{
				//  Judge the temperature 
				System.out.print(" Please enter the current weather temperature :");
				int temp = s.nextInt();
				if (temp>30)
				{
					if (" male ".equals(gender))
					{
						System.out.println(" Wear sunglasses ！");
					}
					else if (" Woman ".equals(gender))
					{
						System.out.println(" Wear sunscreen ！");
					}
					else{
						System.out.println(" What about sex ！");
					}
				}
				else {
					System.out.println(" The temperature is less than 30 Centigrade   Go out and have fun ~");
				}
				}
			else{
				System.out.println(" The weather input is not accurate ！");
			}
}
}

switch Control statement

1、switch Execution principle ：
    switch In the following parentheses “ data ” and case hinder “ data ” Match one by one , Match successful branch execution .

2、 Match successful branch execution , At the end of the branch is “break” In words , Whole switch Statement termination    

3、 Match successful branch execution , There's no... In the branch “break” In words , Go directly to the next branch for execution without matching case, This phenomenon is called case Penetration phenomenon .【 Provide break Can avoid penetrating 】

4、 If all branches fail to match , When there is default Sentence words , Will execute default The program in the branch

5、switch Back and case It can only be int perhaps String Data of type , It can't be detecting other types

        * Of course byte,short,char You can also write directly to switch and case Back , Because they can do automatic type conversion byte,short,char It can be automatically converted to int type  

6、case Can be combined

int i =10;

switch(i){

        case1: case2: case3: case10:

                System.out.println("...");

}

7、 usage ：
    switch(int or String A literal or variable of type ){
        case int or String A literal or variable of type :  // Match data If unsuccessful, match down
            java sentence ;
            ...
            break;    // Program end
        case int or String A literal or variable of type :
            java sentence ;
            ...
            break;
        case int or String A literal or variable of type :
            java sentence ;
            ...
            break;
        ...
        default;            // If they don't match, they will default
            java sentence ;
            ...    
    }
 

public class A
{
	public static void main(String[] args){
		java.util.Scanner s = new java.util.Scanner(System.in);
		System.out.print(" Please enter a number ：");
		int num=s.nextInt();
		switch(num){
			case 1:
				System.out.println(" Monday ");
			break;
			case 2:
				System.out.println(" Tuesday ");
			break;
			case 3:
				System.out.println(" Wednesday ");
			break;
			case 4:
				System.out.println(" Thursday ");
			break;
			case 5:
				System.out.println(" Friday ");
			break;
			case 6:
				System.out.println(" Saturday ");
			break;
			case 7:
				System.out.println(" Sunday ");
			break;
			default :
				System.out.println(" Please enter the correct number ");
		}
	}
}



 The second kind  String type ：

public class A
{
	public static void main(String[] args){
		java.util.Scanner s = new java.util.Scanner(System.in);
		System.out.print(" Please enter the day of the week ：");
		String num=s.next();
		switch(num){
			case " Monday ":
				System.out.println("1");
			break;
			case " Tuesday ":
				System.out.println("2");
			break;
			case " Wednesday ":
				System.out.println("3");
			break;
			default :
				System.out.println(" Please enter the correct week ");
		}
	}
}

demand ： A simple calculator computing system         Be careful ： The operator is a string String type

public class A
{
	public static void main(String[] args){
		java.util.Scanner s = new java.util.Scanner(System.in);
		System.out.println(" Welcome to the simple calculator system ");
		System.out.print(" Please enter the first number ：");
		int num_1 = s.nextInt();
		System.out.print(" Please enter operator :");
		String i =s.next();
		System.out.print(" Please enter the second number :");
		int num_2 =s.nextInt();
		switch(i){
			case "+":
			int a = num_1+num_2;
			System.out.println(num_1+"+"+num_2+" The result of the calculation is :"+a);
			break;
			case "-":
			int b = num_1-num_2;
			System.out.println(num_1+"-"+num_2+" The result of the calculation is :"+b);
			break;
			case "*":
			int c = num_1*num_2;
			System.out.println(num_1+"*"+num_2+" The result of the calculation is :"+c);
			break;
			case "/":
			int d = num_1/num_2;
			System.out.println(num_1+"/"+num_2+" The result of the calculation is :"+d);
			break;
			case "%":
			int e = num_1%num_2;
			System.out.println(num_1+"%"+num_2+" The result of the calculation is :"+e);
			break;
			default :
				System.out.println(" Incorrect input   Please re-enter ！");
				break;
		}
	}
}

Upgraded version ：

public class A
{
	public static void main(String[] args){
		java.util.Scanner s = new java.util.Scanner(System.in);
		System.out.println(" Welcome to the simple calculator system ");
		System.out.print(" Please enter the first number ：");
		int num_1 = s.nextInt();
		System.out.print(" Please enter operator :");
		String i =s.next();
		System.out.print(" Please enter the second number :");
		int num_2 =s.nextInt();
		int result = 0;
		switch(i){
			case"+":
				result = num_1+num_2;
				break;
			case"-":
				result = num_1-num_2;
				break;
			case"*":
				result = num_1*num_2;
				break;
			case"/":
				result = num_1*num_2;
				break;
			case"%":
				result = num_1*num_2;
				break;
				}
		System.out.println(num_1+i+num_2+" The result of the calculation is ："+result);

		
	}
}

     Assume a given student grade （ Grades may be decimal ）
     The effective range of grades 【0-100】
    【90-100】  A
    【80-90】    B
    【70-80】    C
    【60-70】    D
    【0-60】    E
     Tips :(int)( achievement /10)
    0
    1
    2
    3
    ...
    10

public class A
{
	public static void main(String[] args){
		java.util.Scanner s = new java.util.Scanner(System.in);
		System.out.println(" Welcome to the examinee score determination system ");
		System.out.print(" Please enter the test result ：");
		double score1 = s.nextDouble();
		int score = (int)(score1/10);
		switch(score){
			case 9: case 10:
				System.out.println("A");
				break;
			case 8:
				System.out.println("B");
				break;
			case 7:
				System.out.println("C");
				break;
			case 6:
				System.out.println("D");
				break;
			default:
				System.out.println("E");
				break;
			
		}

		

	}
}

for loop

for The grammatical structure of a loop ：
    
    for( Initialization expression ; Boolean expression ; Update expression ){
        // It's a piece of code that needs to be executed repeatedly 【 The loop body ： from java Sentence structure 】
    }

for The execution of the loop / Execution principle ？【***** a key 】
   * Initialization expression 、 Boolean expression 、 Updating expressions is not necessary ！【 But two semicolons are necessary （ Dead cycle ）】
    * The initialization expression is executed first , And in the whole for Only execute once in the loop
    * The Boolean expression must be true/false It can't be anything else
    * If the Boolean expression is true Then execute the loop body If it's false The loop body... Is not executed

 public class A
{
    public static void main(String[] args){
        for (int i=1;i<=10 ;i++ )
        {
            System.out.println(i);
        }

    }
}

  Be careful ：

public class A
{
    public static void main(String[] args){
        int i =0;
        for (;i<10 ;i++ )   // The default initial value is i=0
        {
            System.out.println("i ===>"+i);
        }
        System.out.println(i);   // Because when i++ by 10 When Judge 10<10 by false So the output is 10
    }
}

The second kind ：

public class A
{
    public static void main(String[] args){
        int i ;
        for (i=0;i<10 ;i++ )
        {
            System.out.println("i ===>"+i);
        }
        System.out.println(i);   // Because when i++ by 10 When Judge 10<10 by false So the output is 10
    }
}

for A nested loop

 public class A
{
    public static void main(String[] args){
        
            for (int i=1;i<=10 ;i++ )    // Cycle ten times j
            {
                for (int j=1;j<=3 ;j++ )    // The result of the cycle j：1 2 3
                {
                    System.out.println("j ===>"+j);
                }
            }
            
        }        
}

 

 

multiplication table  

public class A
{
	public static void main(String[] args){
		
		for (int i=1;i<=9 ;i++ )
		{
			for (int k=1;k<=i ;k++ )
			{
				System.out.print( k+ "×" +i+ "=" +i*k);
				
			}
			//  There are two ways to wrap lines 
			//System.out.println();
			System.out.println("\n");
			
			
		}
			
		}		
}

while loop

/*
while Loop structure ：

    while( Boolean expression ){
         The loop body ;
    }

*/

// Dead cycle
public class A
{
    public static void main(String[] args){
        while (true)
{
    System.out.println(" Ha ha ha ");
}
    }
}

Knowledge point ( Add ：--k Yes, first 1 To assign a value   k-- Is the assignment first Re reduction 1)

public class A
{
	public static void main(String[] args){
		while (10>3)	//  When while（） When Boolean expressions are numeric comparisons   The compiler can detect 10>3 by true
						//	 So it's also an endless cycle   Below hello world  cannot access 
                    {
	System.out.println(" Ha ha ha ");
        }
    System.out.println("hello world");

	}
}

public class A
{
	public static void main(String[] args){
		int i =10;
		int j =3;
		while (i>j)	{	//  It can be determined to be true artificially, but the compiler cannot judge whether it is true or not   Therefore, the output results are always in an endless loop " Ha ha ha "
	
	System.out.println(" Ha ha ha ");
					}
		System.out.println("Hello world");
	}
}

do   while Loop structure

do while loop ：

    1、 Grammatical structure ：
    do{ The loop body ;}
    while( Boolean expression );

    2、 Execution principle ： First execute the loop body Then proceed while Judge If true, continue with the loop body End for false

    3、do while cycles ： The lowest execution cycle body
 

 

 break     continue    sentence

break; Represents a break statement

By default ： break The statement terminates the nearest circular statement .

break Indicates that the loop is no longer executed
continue It means to enter the next cycle directly Carry on .

public class A
{
	public static void main(String[] args){
		for (int i=1;i<=10 ;i++ )
		{
			if (i==5)
			{
				break;
			}
			System.out.println(">>>"+i); // 1 2 3 4
		}
		System.out.println("hello world！");
		
	}
}

 

public class A
{
	public static void main(String[] args){
		for (int i=1;i<=10 ;i++ )
		{
			if (i==5)
			{
				continue;
			}
			System.out.println(">>>"+i); // 1 2 3 4
		}
		System.out.println("hello world！");
		
	}
}