Chapter V: process control

The fifth chapter ： Process control

5.1、 Branch statement

brief introduction ：Java The branch statement of is executed only when part of the program code meets certain conditions .

5.1.1、if else sentence

Basic grammar ：

if( Boolean expression ){
    
 Program code block 
}else{
    
 Program code block 
}

matters needing attention ：

• if The following expression must be a Boolean expression , It cannot be of numeric type .
• if hinder else Statement is not necessary .
• Join in if After statement or else The program code block after the statement contains multiple statements , Must be stored in {} Inside . If there is only one sentence in the code block, it can not be used {}
• if else Statements have a special kind of concatenation programming lattice .

public void method(int x){
    
    if(x>0){
    
        System.out.println(" Greater than 0");
    }else if(x==0){
    
        System.out.println(" be equal to 0");
    }else{
    
        System.out.println(" Less than 0");
    }
}

Classic case ：

// Judge whether a certain year is a leap year 
// Method 1 
public boolean idleapYear(int year){
    
    if((year%4==0&&year%100!==0)||(year%400==0))
        return true;
    else
        return false;
}

5.1.2、switch sentence

brief introduction ：switch A statement is a multi branch statement , Basic grammar

switch (expr){
    
    case value1:
        ...
            break;
    case value2:
        ...
            break;
        ...
            default:
        ...
            break;
}

matters needing attention ：

• The parameter value types in the expression include the following
• And int Type compatible types
• String type
• Enumeration type
• switch There can be at most one... In the statement default Clause .
• switch Sentence with that case Matching starts from that place . encounter break Just jump out of the whole switch
• switch Statement can be used if else Statement to implement

Classic case ：

public void sortScore(char grade){
    
    switch(grade){
    
            case'A':
            System.out.println(grade+"is 85~100");
            break;
            case'B':
            System.out.println(grade+"is 70~84");
            break;
            case'C':
            System.out.println(grade+"is 60~69");
            break;
            case'D':
            System.out.println(grade+"is<60");
            break;
            default:
            System.out.println(" There is no hierarchy ");
            break;
    }
}

String type parameters

String color="red";
switch(color){
    
    case "blue":
        System.out.println(" Blue ");
        break;
    case "red":
        System.out.println(" Red ");
        break;
    default:
        System.out.println(" Other colors ");
        break;
}

Enumeration type parameters ：

public class SwitchTest{
    
    enum Color{
    red,blue,yellow}// Define an enumeration class 
    public static void main(String [] args){
    
        Color c=Color.blue;
        switch(c){
    
            case red:
                System.out.println(" Red ");
                break;
            case blue:
                System.out.println(" Blue ");
                break;   
            case yellow:
                System.out.println(" yellow ");
                break; 
            default:
                System.out.println(" Other colors ");               
        }
    }
}

5.2、 Loop statement

brief introduction ： The function of a loop statement is to execute a piece of code repeatedly , Until the conditions are not met .

Four major parts ：

• Initialization part ： Used to set some initial conditions of the cycle , Set the initial value of the loop control variable
• The loop condition ： This is a Boolean expression
• The loop body ： This is the main content of the cycle
• Iteration part ： To change the value of a loop variable .

5.2.1、while sentence

brief introduction ：while Loop statements are the most basic loop statements in loop statements . The basic grammar is as follows ：

[ Initialization part ]
while( The loop condition ){
    
 The loop body （ Include iteration part ）
}

Precautions for use ：

• If the loop body contains multiple statements , Must be placed in braces , If the loop body has only one statement , You don't need braces .
• while The expression is judged at the beginning of the loop , If the return value is false Then the loop body will not be executed once .
• The loop body can be empty
• Ensure that there are conditions for terminating the cycle , Avoid the dead cycle .

Classic case ：

public static int max(int [] array){
    
    if(array==null||array.length==0){
    
        throw new IllegalArgumentException(" Invalid array ");
        int index=1,location=0;// Initialization part 
        while(index<array.length){
    // The loop condition ,index Is the loop control variable 
            // The loop body 
            if(array[location]<array[index]){
    
                location=index;
                index++;
            }
        }
        return array[location];
    }
}

5.2.2、do while sentence

brief introduction ： And while Statements are all circular statements , however do while The loop will first execute the loop body , Then judge the conditions , In any case, the loop body will be executed once .

Grammar format ：

[ Initialization part ]
do{
    
     The loop body  , Include iteration part 
}while( The loop condition );

5.2.3、for sentence

brief introduction ：for Circulation and while Like the cycle, we judge the cycle condition first , Then execute the loop body . The grammar is as follows ：

for( Initialization part , The loop condition , Iteration part ){
    
 The loop body 
}

matters needing attention ：

• If for The body of the loop has only one line , May not be used {}
• control for Loop variables are often used only for this loop , It cannot be used elsewhere in the program . Its scope is current for Cycle cannot be in for Use it outside the loop .
• You can use comma statements in the initialization and iteration sections , Comma statements are used to separate statement sequences .
• for The initialization part and iteration part of the loop can be empty
• for Loop statements are generally used when the number of loops can be determined in advance .while Circulation and do while Cycles are used when the number of cycles is not clear .

5.2.4、foreach sentence

brief introduction ：foreach The sentence is for A special simplified version of the statement , It can simplify the code of traversing arrays and collections .

Grammar format ：

for( Element type   Element variables X ： Array to be traversed ){
    
     References to variables x Of Java sentence 
}

5.2.5、 Multiple cycles

brief introduction ： Various loop statements can be nested with each other , Form multiple loops ;

Classic case ： multiplication table

public static void main(String[] args){
    			
		for (int i = 1; i < 10; i++) {
    
			for (int j = 1; j <= i; j++) {
    
				System.out.print(j+"*"+i+"="+j*i+"\t");
			}
			System.out.println();
		}
	}

5.3、 Process jump statement

brief introduction ：break、continue、return Statements are all process jump statements , But they have different use conditions and functions

• break： To jump out of switch Or the current cycle .
• continue： Jump out of this cycle , Execute next cycle .
• return： Means to exit this method , Return the method called by the previous layer , If the method is not void, You need to provide the corresponding return value .