If switch branch structure

if

grammar ： if( Boolean expression ){ // Code block 1 }

• Execute the process ： Judge Boolean expressions . The result is true, Then execute the code block first 1, Then execute the following code . The result is false, Then skip the code block 1, Execute subsequent code directly .

​
import java.util.Scanner;
​
/**
 *   Reward according to achievements 
 *   If 
 *      java Results are greater than 90  also   Music scores   Greater than 85 
 *   perhaps 
 *      java Results are greater than 80  also   Dance performance   be equal to 100
 * 
 *    Reward apples 12 bag 
 * @author WHD
 *
 */
public class Test3 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        
        System.out.println(" Please enter java achievement ");
        
        double javaScore = input.nextDouble();
        
        System.out.println(" Please enter your music score ");
        
        double musicScore = input.nextDouble();
        
        
        System.out.println(" Please enter your dance score ");
        
        double danceScore = input.nextDouble();
        // java Results are greater than 90  also   Music scores   Greater than 85 
        // java Results are greater than 80  also   Dance performance   be equal to 100
        if((javaScore > 90 && musicScore > 85) || (javaScore > 80 && danceScore == 100)) {
            System.out.println(" Reward apples 12 bag ");
        }
        System.out.println(" Program end ");
        
    }
}

2.2 if-else

grammar ： if( Boolean expression ){ // Code block 1 }else{ // Code block 2 } Subsequent code ...

• Execute the process ： Judge Boolean expressions . The result is true, Then execute the code block first 1, Then exit the whole structure , Execute subsequent code . The result is false, Then execute the code block first 2, Then exit the whole structure , Execute subsequent code .

• if-else amount to 2 choose 1 A code block must be executed , Then exit the whole if-else structure

package com.qfedu.test3;
​
import java.util.Scanner;
​
/**
 *  if-else choice   structure 
 * @author WHD
 *
 */
public class Test1 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        
        System.out.println(" Please enter age ");
        
        int age = input.nextInt();
        
        
        if(age >= 18){
            System.out.println(" Grown up ");
        }else {
            System.out.println(" A minor ");
        }
        
        System.out.println(" Program end ");
    
    }
}
​

2.3 multiple if

grammar ： if( Boolean expression 1){ // Code block 1 }else if( Boolean expression 2){ // Code block 2 }else if( Boolean expression 3){ // Code block 3 }else{ // Code block 4 }

• Execute the process ： expression 1 by true, Then execute the code block 1, Then exit the whole structure .

  expression 2 by true, Then execute the code block 2, Then exit the whole structure .

  expression 3 by true, Then execute the code block 3, Then exit the whole structure .

  All of the above are false, Then execute the code block 4, Then exit the whole structure .

  Be careful ： Mutually exclusive , There is one for true, Others will not be executed , Applicable to interval judgment .

multiple if

• else if Not alone Must be combined with if

• else It can't be alone Can combine else if perhaps combination if

multiple if be used for Deal with the case where a value is in a continuous interval

When writing judgment conditions Use ascending or descending order Do not use out of sequence

package com.qfedu.test4;
​
import java.util.Scanner;
​
/**
 *   multiple if
 *  else if  Not alone   Must be combined with  if
 *  else  It can't be alone   Can combine  else if  perhaps   combination  if 
 * 
 *   demand ： Grade according to grades 
 *  100 branch   perfect 
 *  90 More than   good 
 *  80 More than   good 
 *  70 More than   secondary 
 *  60 Above and above   pass 
 *  60 Below   fail, 
 * 
 *   multiple if be used for   Deal with the case where a value is in a continuous interval 
 *   When writing judgment conditions   Use ascending or descending order   Do not use out of sequence 
 * @author WHD
 *
 */
public class Test1 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        
        System.out.println(" Please enter your score ");
        
        int score = input.nextInt();
        if(score == 100) {
            System.out.println(" perfect ");
        }else if(score > 90) {
            System.out.println(" good ");
        }else if(score > 80) {
            System.out.println(" good ");
        }else if(score > 70) {
            System.out.println(" secondary ");
        }else if(score >= 60) {
            System.out.println(" pass ");
        }else {
            System.out.println(" fail, , come on. ~");
        }   
        System.out.println(" Program end ");
    }
}
​

2.4 nesting if

if( Outer expression ){ if( Inner expression ){ // Inner code block 1 }else{ // Inner code block 2 } }else{ // Outer code block }

• Execute the process ： When the outer conditions are met , Then judge the inner conditions .

  Be careful ： In a selection structure , Another selection structure can be nested . When the nesting format is correct , Support any combination .

package com.qfedu.test5;
​
import java.util.Scanner;
​
/**
 *   nesting if   A complete if One or more... Are nested in the structure if structure 
 *   The grammatical format is correct   Can be nested arbitrarily   No layers   ceiling   The actual development shall not exceed three layers 
 * 
 * 
 *   demand ：
 *   The school held a 100 meter race , If the time is less than 15 second , Can enter the finals 
 *  
 *   Men are divided according to sex   perhaps   Women's group 
 * @author WHD
 *
 */
public class Test1 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        
        System.out.println(" Please enter your running time ");
        
        double time = input.nextDouble();
        
        
        
        if(time < 15) {
            //  Can enter the finals   grouping 
            System.out.println(" Congratulations on entering the finals , Start grouping ");
            //  adopt sex.eqauls(" Content ")   Said judgment  sex Whether the value of the variable is the same as that in the brackets 
            //  For the same true  Otherwise false
            
            
            System.out.println(" Please enter gender ");
            
            String sex = input.next();
            
            if(sex.equals(" male ")) {
                System.out.println(" Enter the men's group ");
            }else if(sex.equals(" Woman ")){
                System.out.println(" Into the women's group ");
            }else {
                System.out.println(" The third sex is not supported for the time being , Please wait for the next ");
            }
        }else {
            //  Come on next time    Make persistent efforts 
            System.out.println(" Come on next time ！");
        }
        
        System.out.println(" Program end ");
​
    }
}
​

3. Branching structure

3.1 switch

grammar ： switch( Variable | expression ){ case value 1: Logic code 1; case value 2: Logic code 2; case value n: Logic code n; default: Logic code when not satisfied ; }

• Execute the process ： If the value in the variable is equal to the value 1, Execute the logic code 1. If the value in the variable is equal to the value 2, Execute the logic code 2. If the value in the variable is equal to the value n, Execute the logic code n. If the value in the variable does not match case When the value of , perform default Logical code in .

package com.qfedu.test6;
​
import java.util.Scanner;
​
/**
 *  switch Branching structure 
 *   demand ： Reward according to the ranking 
 *       The first name ： Reward summer camp activities 
 *       proxime accessit ： Reward apples 12 bag 
 *       The third ： Reward a notebook 
 *       A fourth ： Oral praise once 
 *       other ： Continue to work hard 
 * @author WHD
 *
 */
public class Test1 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        
        System.out.println(" Please enter your rank ");
        
        int num = input.nextInt();
        
        switch(num) {
            
            case 1:
                System.out.println(" Summer camp ");
                break;
            case 2:
                System.out.println(" Apple 12 bag ");
                break;
            case 3:
                System.out.println(" A notebook ");
                break;
            case 4:
                System.out.println(" Oral praise once ");
                break;
            default:
                System.out.println(" Continue refueling ~");
                break;
        }
        System.out.println(" Program end ");
    }
}
​

3.2 Supported data types

byte short int char String(JDK7+) enumeration

package com.qfedu.test6;
​
import java.util.Scanner;
​
/**
 *  switch Branching structure 
 *   Supported data types 
 * @author WHD
 *
 */
public class Test3 {
    public static void main(String[] args) {

        
        
        char ch1 = 'B';
        
        String str = "D";
        
        switch(str) {
            
            case "A":
                System.out.println(" Summer camp ");
                break;
            case "B":
                System.out.println(" Apple 12 bag ");
                break;
            case "C":
                System.out.println(" A notebook ");
                break;
            case "D":
                System.out.println(" Oral praise once ");
                break;
            default:
                System.out.println(" Continue refueling ~");
                break;
        }
        
        System.out.println(" Program end ");
    }
}
​

4. local variable

characteristic	describe
Scope of action	Within the nearest brace to the current variable
Define the location	In the method
The problem of duplicate names	The overlapping scope cannot have the same name
Storage location ( understand )	There is a stack of basic data types (stack) in , Reference data type , The name is on the stack (stack), The contents are piled up (heap)
Life cycle ( understand )	As the method is stacked ( Pressing stack ) And take effect , As the method goes out of the stack ( Bomb stack ) And death