Java process control

Java process control

User interaction Scanner

The java.util.Scanner package provides the Scanner to obtain user input

Scanner s=new Scanner(System.in)

scanner.close()

package Scanner;


import java.util.Scanner;

public class Demo01 {
    public static void main(String[] args) {
        //Create a scanner object to accept keyboard data
        Scanner scanner=new Scanner(System.in);

       // System.out.println("receive in next mode:");
        System.out.println("use nextLine Reception mode:");
        //Judge whether the user has entered a string
        if(scanner.hasNextLine()){
            //Receive in next mode
           // String str=scanner.next();
            String str=scanner.nextLine();
            System.out.println("The output contents are:"+str);
        }
        //All classes belonging to IO streams will always occupy resources and form good habits if they are not closed
        scanner.close();
    }
}

next():

Be sure to read valid characters before you can end the input

The next() method will automatically remove the blanks encountered before the valid characters entered

Only after entering a valid character will the space entered after it be used as a separator or terminator

next() cannot get a string with spaces

nextLine():

With Enter as the terminator, the nextLine() method returns all characters before the input carriage return

You can get spaces

example:

package Scanner;

import java.util.Scanner;

public class Demo02 {
        public static void main(String[] args) {

            Scanner scanner=new Scanner(System.in);
            int i=0;
            float f=0.0f;

            System.out.println("Please enter an integer:");
            //Judge whether the user has entered an integer
            if(scanner.hasNextInt()){

                i=scanner.nextInt();
                System.out.println("The output contents are:"+i);
            }
            else{
                System.out.println("The input is not integer data!");
            }

            if(scanner.hasNextFloat()){
                f= scanner.nextFloat();
                System.out.println("Decimal data:"+f);
            }
            else{
                System.out.println("The input is not decimal data");
            }
            //All classes belonging to IO streams will always occupy resources and form good habits if they are not closed
            scanner.close();
        }

}

Sequential structure

Step by step, from top to bottom

Select structure

if radio structure

if(Boolean expression){

//If the Boolean expression has a value of true

}

if double selection structure

if(Boolean expression){

//If the Boolean expression has a value of true

}else{

//If the value of the Boolean expression is false

}

if multiple selection structure

if(Boolean expression 1){

//If the value of Boolean expression 1 is true

}else if(Boolean expression 2){

//If the value of Boolean expression 2 is true

}else if(Boolean expression 3){

//If the value of Boolean expression 3 is true

}else{

//If none of the above Boolean expressions is true

}

switch selection structure

  • The switch case statement determines whether a variable is equal to a value in a series of values

  • The variable type can be byte, short, int or char, and the String type (starting from Java SE 7) is supported

  • The case tag must be a string constant or literal

switch(expression){
    case value:
	//sentence
		breakļ¼›//Optional
    case value:
        //sentence
        break;//No break will output all of the following
    //You can use any number of case statements
    default://Optional
        //sentence

}

example

public class Switch01 {
    public static void main(String[] args) {
        char grade='C';
        //case penetration, matching a specific value
        switch (grade){
            case 'A':
                System.out.println("excellent");
                break;
            case 'B':
                System.out.println("good");
                break;
            case 'C':
                System.out.println("pass");
                break;
            case 'D':
                System.out.println("unqualified");
                break;
            default:
                System.out.println("Unknown level");
        }
    }
}

Cyclic structure

while Loop

Judgment before execution

while(Boolean expression){

//Cyclic content

}

do while loop

Execute before Judge

Execute at least once even if the conditions are not met

do{

//Code statement

}while(Boolean expression)


for loop

for(initialization;Boolean expression;to update){

//Code statement

}
//Dead cycle
for(;;){
}

Quick input 100.for

 for (int i = 0; i < 100; i++) {
           
        }

example

public class ForDemo01 {
    public static void main(String[] args) {
        //Calculate 1 to 100
        int a=1;//Initialization condition
        int sum=0;
        while(a<=100){//Conditional judgment
            sum=sum+a;
            a++;
        }
        System.out.println(sum);
        System.out.println("while End of cycle!");
        sum=0;
        //Initialize. / / judge Boolean values. / / update iteratively
        for(int i=1;i<=100;i++)
        {
            sum=sum+i;

        }
        System.out.println(sum);
        System.out.println("for End of cycle!");
    }
}

Print 99 multiplication table

1*1=1
1*2=2 2*2=4
1*3=3 2*3=6 3*3=9
1*4=4 2*4=8 3*4=12 4*4=16
1*5=5 2*5=10 3*5=15 4*5=20 5*5=25
1*6=6 2*6=12 3*6=18 4*6=24 5*6=30 6*6=36
1*7=7 2*7=14 3*7=21 4*7=28 5*7=35 6*7=42 7*7=49
1*8=8 2*8=16 3*8=24 4*8=32 5*8=40 6*8=48 7*8=56 8*8=64
1*9=9 2*9=18 3*9=27 4*9=36 5*9=45 6*9=54 7*9=63 8*9=72 9*9=81

public class ForDemo02 {
    //Output 99 multiplication table
    public static void main(String[] args) {
        for(int j=1;j<=9;j++){
            for(int i=1; i<=j;i++){

              System.out.print(i+"*"+j+"="+(j*i));
              System.out.print('\t');

            }
            System.out.println();
        }
    }
}

Enhanced for loop

It is mainly used to traverse arrays and collection objects

for(Declaration statements: expressions)

{

//Code statement

}

example

public class ForDemo03 {
    public static void main(String[] args) {
        //Define an array
        int[] numbers={10,20,30,40,50};

        //The following two outputs are equal
        for (int i=0;i<5;i++){
            System.out.println(numbers[i]);
        }
        System.out.println("==============");
        //Traverse array elements
        for(int x:numbers){
            System.out.println(x);
        }

    }
}

break and continue

break: it can be used in the main part of any loop statement. break is used to forcibly exit the loop, do not execute the remaining statements in the loop, and do not terminate the program (terminate the loop)

continue: used in the body of a loop statement. It is used to terminate a loop process, skip the unexecuted statements in the loop body, and then determine whether to execute the loop next time (without terminating the loop)

No goto keyword

The difference between print and println

print will not wrap lines after output, and it will be output all the time

println will wrap after output

System.out.print('\n');
System.out.println();//The two statements are equivalent

Tags: Java

Posted on Tue, 02 Nov 2021 00:55:21 -0400 by gnu2php