Detailed explanation of if else branch structure of 200 cases 41 for Java beginners

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Author name: Ming Shiyin in programming world
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if structure

if statement is the most used conditional branch structure. It belongs to selection statement and can also be called conditional statement.

If selection structure is a syntax structure that is processed after judgment according to conditions. By default, the if statement controls the execution of the following statement. However, through statement blocks, if statements can control multiple statements.

The simplest syntax format of the if statement is as follows, which means "if certain conditions are met, some processing will be carried out".

if (conditional expression){
Statement block;
}

Example 1

Instance whose judgment condition is true

package demo.demo41;

public class Demo1 {

	public static void main(String[] args) {
		System.out.println("The program starts running");
		//The judgment condition is true, so it will enter the if branch statement
		if(true){
			System.out.println("if The statement was executed");
		}
		System.out.println("The program ends running");
	}
}

Operation results:

The program starts running
The if statement was executed
The program ends running

Example 2

Instance whose judgment condition is false

package demo.demo41;

public class Demo2 {

	public static void main(String[] args) {
		System.out.println("The program starts running");
		//The judgment condition is false, so it will not enter the if branch statement
		if(false){
			System.out.println("if The statement was executed");
		}
		System.out.println("The program ends running");
	}
}

Operation results:

The program starts running
The program ends running

Example 3

User input judgment instance

package demo.demo41;

import java.util.Scanner;

public class Demo3 {

	public static void main(String[] args) {
		System.out.println("The program starts running");
		System.out.println("Please enter a number:");
		Scanner input = new Scanner(System.in);
		int num = input.nextInt(); // Receive keyboard input data
		// Judge whether the data entered by the user is greater than 10
		if (num > 10) {
			System.out.println("The number entered is greater than 10");
		}
		// Judge whether the data entered by the user is equal to 10
		if (num == 10) {
			System.out.println("The number entered is equal to 10");
		}
		// Judge whether the data entered by the user is less than 10
		if (num < 10) {
			System.out.println("The number entered is less than 10");
		}
		System.out.println("The program ends running");
	}
}

Operation results:

The program starts running
Please enter a number:
5
The number entered is less than 10
The program ends running

Operation result 2:

The program starts running
Please enter a number:
11
The number entered is greater than 10
The program ends running

If else structure

A single if statement can only be used when the condition is met, and cannot perform any other operation (stop). If combined with else statement can define two operations. At this time, if... Else statement means "if the condition is correct, perform one operation, otherwise perform another operation".

The syntax format of the if... else statement is as follows:

if (expression){
Statement block 1;
} else {
Statement block 2;
}

Example 1

When the judgment condition is true

package demo.demo41;

public class Demo4 {

	public static void main(String[] args) {
		System.out.println("The program starts running");
		//The judgment condition is true, so it will enter the if branch statement
		if(true){
			System.out.println("if The statement was executed");
		}else {
			System.out.println("else The statement was executed");
		}
		System.out.println("The program ends running");
	}
}

Operation results:

The program starts running
The if statement was executed
The program ends running

Example 2

When the judgment condition is false

package demo.demo41;

public class Demo5 {

	public static void main(String[] args) {
		System.out.println("The program starts running");
		//The judgment condition is false, so it will enter the else branch statement
		if(false){
			System.out.println("if The statement was executed");
		}else {
			System.out.println("else The statement was executed");
		}
		System.out.println("The program ends running");
	}
}

Operation results:

The program starts running
else statement was executed
The program ends running

Example 3

Size judgment of two numbers

package demo.demo41;

public class Demo6 {

	public static void main(String[] args) {
		System.out.println("The program starts running");
	    int num1 = 10;
	    int num2 = 20;
	    // If num1 equals num2
	    if (num1 == num2) {
	        System.out.println("num1 be equal to num2");
	    }
	    // If num1 is greater than num2
	    if (num1 > num2) {
	        System.out.println("num1 greater than num2");
	    } else {
	        // Otherwise, num1 is less than num2
	        System.out.println("num1 less than num2");
	    }
		System.out.println("The program ends running");
	}
}

Operation results:

The program starts running
num1 is less than num2
The program ends running

Multi conditional if else if statement

The main function of if statement is to provide a branch for the program. However, sometimes only one more branch in the program is not enough, and sometimes the branch of the program will be very complex, which requires the use of multi branch if... else if statements.

It is usually expressed as "if certain conditions are met, some processing will be carried out, otherwise if another condition is met, another processing will be carried out... If these conditions are not met, the last condition will be executed".

The syntax format of if... else if multi branch statements is as follows:

If (expression 1){
Statement block 1;
}Else if (expression 2){
Statement block 2;
}Else if (expression n){
Statement block n;
} else {
Statement block n+1;
}

Example 1

Judge the size of two numbers

package demo.demo41;

public class Demo7 {

	public static void main(String[] args) {
		System.out.println("The program starts running");
	    int num1 = 10;
	    int num2 = 20;
	    if (num1 == num2) { // If num1 equals num2
	        System.out.println("num1 be equal to num2");
	    } else if (num1 > num2) { // If num1 is greater than num2
	        System.out.println("num1 greater than num2");
	    } else { // Otherwise, it is less than
	        System.out.println("num1 less than num2");
	    }
		System.out.println("The program ends running");
	}
}

Operation results:

The program starts running
num1 is less than num2
The program ends running

Example 2

Judge students' grades according to the input conditions

package demo.demo41;

import java.util.Scanner;

public class Demo8 {

	public static void main(String[] args) {
		System.out.println("The program starts running");
		System.out.println("Please enter the test result:");
		Scanner input = new Scanner(System.in);
		int score = input.nextInt(); // Receive keyboard input data
		if (score >= 90) { // Test score > = 90
			System.out.println("excellent");
		} else if (score >= 80) { // 90 > test score > = 80
			System.out.println("good");
		} else if (score >= 60) { // 80 > test score > = 60
			System.out.println("secondary");
		} else { // Test score < 60
			System.out.println("difference");
		}
		System.out.println("The program ends running");
	}
}

Operation conclusion: 1:

The program starts running
Please enter the test result:
100
excellent
The program ends running

Operation result 2:

The program starts running
Please enter the test result:
59
difference
The program ends running

Use of nested if

The usage of if statement is very flexible. It can not only be used alone, but also nest another if statement in the if statement. Similarly, if... else statement and if... else if statement can also nest another if structure statement to complete deeper judgment.

The syntax format of nested if is as follows:

If (expression 1){
If (expression 2){
Statement block 1;
} else {
Statement block 2;
}
} else {
If (expression 3){
Statement block 3;
}Else if (expression 4){
Statement block 4;
} else {
If (expression n){
Statement block n;
} else {
Statement block n+1;
}
}
}

example

package demo.demo41;

public class Demo9 {

	public static void main(String[] args) {
		String today = "weekend";
		boolean nvpengyou = true;//girl friend
		System.out.println("The program starts running");
		if("weekend".equals(today)){//It's the weekend
			if(nvpengyou){//Have a girlfriend
				System.out.println("Take your girlfriend shopping");
			}else{//No girlfriend
				System.out.println("Beat the king at home");
			}
		}else {//Not a weekend
			System.out.println("Go to work");
		}
		System.out.println("The program ends running");
	}
}

Operation results:

The program starts running
Take your girlfriend shopping
The program ends running

Summary

This section summarizes the "detailed explanation of if else branch structure". I hope it can be helpful to you. Please help [like] + [collection] + [punch in the comment area]. If you are interested in learning Java with brother Xiaoming, [pay attention to a wave] won't get lost.

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Navigation

✪ introduction to Java white 200 case series directory index
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► next    42.switch case statement

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Posted on Sat, 11 Sep 2021 13:54:04 -0400 by suzuki