JAVA flow control

First, the process control structure


Scanner class

In the basic grammar, only simple operation results can be printed, and the interaction between programs and people cannot be realized.

Among the new features of Java 5, a tool class Scanner is provided to facilitate the acquisition of user input.


basic grammar

Scanner s = new Scanner(;

Create a scanner that accepts user input


Judgment statement expression

if (Scanner.hasNextLine());

First use the hasNext() and hasNextLine() methods to determine whether the character is input

Similar judgment methods include:

hasNextByte() //Determine whether the byte type is input

hasNextInt() //Determine whether the integer type is input

hasNextShort() //Determine whether the Short type is input

hasNextLong() ////Determine whether the Long type is input

hasNextFloat() //Determine whether the Float type is input

hasNextDouble() //Determine whether the Double type is input

hasNextBoolean() //Determine whether the Boolean type is input

The difference between next() and nextLine()

next can only get characters without spaces, nextLine can get a line of characters


method get expression

Get the input characters through the next() and nextLine() methods of the Scanner class

String str = next()/nextLine()//Get characters and put them in str

Similar methods are also available:

nextByte() //Get byte type

nextShort() //Get Short type

nextInt() //Get Int type

nextLong() //Get Long type

nextFloat() //Get Float type

nextDouble() //Get Double type

nextBoolean() //Get Boolean type


close IO stream


If it is not closed, it will take up resources and form a habit



if conditional statement

The if selection structure is generally used to make certain judgments and meet certain conditions to execute the statement.


if single-choice structure

If the if boolean expression is satisfied, the statement in the if bracket is executed, otherwise, skip to the next step directly


        //if single-choice structure
        boolean select = true;
        if (select == false){
            System.out.println("boolean value false");//Execute if the condition is met, jump out if it is not met


if double select structure

If the value of the Boolean expression is true, execute the statement within the if brackets, otherwise execute the statement within the else.


        //if double select structure
        int age = 17;
        if (age >= 18){
            System.out.println("age of adulthood");//Execute the statement under if if the condition is satisfied, and execute the statement under the else if not satisfied
            System.out.println("Underage age!");


if multiple choice structure

On the basis of the if double selection structure, several else if keywords can be added in the middle. When the Boolean expression is satisfied, only one of the branches is executed and the program is executed downward.


        //if multiple choice structure
        int score = 44;
        if (score == 60){
            System.out.println("Just passing grade");//Which branch of the judgment is satisfied, execute the statement of which branch, if not satisfied, jump out of the entire judgment
        }else if(score >= 90){
            System.out.println("Great score");
        }else if(score >= 80){
            System.out.println("good grades");
        }else if (score >= 60){
            System.out.println("Failed grades");//When the above branch judgments are not satisfied, the else statement is executed at the end



switch switch logic statement

The Boolean expression of the if selection structure is generally used to determine a range, while the Boolean expression of the switch selection structure is used to determine a specific value.


Scanner in = new Scanner(;
System.out.println("enter int type constant");
        int a = in.nextInt();
            case 1:
                System.out.println("result is equal to 1");//Constant expression satisfying case
            case 5:
                System.out.println("the result is equal to 5");
                System.out.println("Inputs are not 1 and 5!");//Executed only if all case s do not match, not affected by the order



while loop structure

single while loop structure

The sequence structure and the selection structure can only execute the program once while can realize the repeated execution of a certain section of the statement, which is widely used in practical programming.

  • The entire while structure can be called a loop body

  • If there is no condition set in the while, the statement in the parentheses will always be looped, which is called an infinite loop

  • A condition is added to the while The program will stop when the while loop does not satisfy the condition


while(boolean expression){
    //loop statement

Case using a single while loop

		int num = 0;
        while (num < 10) {
            System.out.print(num + " ");//The num variable loops up until the Boolean expression is not satisfied and jumps out


do while loop structure

The do while structure executes the loop statement once and then judges whether the loop is executed or not. Do is written before while while is written after.


    //loop statement
}while(conditional expression);

The case of using do while loop

		int a = 1;
        do {
            System.out.println("code output" + a);//If the condition is not met, it will also be executed once
        }while(a > 10);



for loop structure

The for structure contains three options: variable declaration, judgment condition and variable value change, separated by semicolons. The code is concise and concise when it is consistent with the execution effect of while

When a large number of loops are required, it is recommended to use the for loop for loop, and declare that the condition update value can be empty, and there is no fixed specification.

working process:

After definition, judge the condition, output first, then auto-increment, and finally update

//declare; condition; iterate;
for (int a = 1;a > 1;a++;){
    //loop statement

Use a for loop to output numbers from 1 to 10 sequentially

for (int i = 0;i < 10;i++){
            System.out.print(i+" ");


Enhanced for loop structure

A new feature referenced by Java 5

Enhanced for loops are mainly used for arrays and collections


for(//Variable declaration: expression){//statement block};

Mainly used for looping operations on arrays and collections


Posted by dandaman2007 on Sun, 08 May 2022 17:22:36 +0300