本节内容教学视频链接：Java流程控制04：if选择结构_哔哩哔哩_bilibilihttps://www.bilibili.com/video/BV12J41137hu?p=36&vd_source=b5775c3a4ea16a5306db9c7c1c1486b5

   在Java中，if 选择结构用于根据特定条件执行不同的代码块。

        if语句有四种结构：if单选择结构、if双选择结构、if多选择结构、if嵌套结构；

1.if单选择结构（if）

代码示例1：单选择结构

import java.util.Scanner;

public class IfDemo01 {
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入内容");
        String s = scanner.nextLine();
        if (s.equals("Hello")){
            System.out.println(s);
        }
        System.out.println("End");
        scanner.close();
    }
}

代码示例2：单选择结构

int number = 10;
if (number > 0) {
    System.out.println("Number is positive");
}

2.if双选择结构（if...else）

代码示例1：

import java.util.Scanner;

public class IfDemo02 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入成绩");
        int score = scanner.nextInt();
        if (score>60){
            System.out.println("成绩及格");
        }else{
            System.out.println("成绩不及格");
        }
        scanner.close();
    }
}

代码示例2：

int number = -10;
if (number > 0) {
    System.out.println("Number is positive");
} else {
    System.out.println("Number is non-positive");
}

3.if多选择结构（if...else if...else）

代码示例1：

import java.util.Scanner;

public class IfDemo03 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入考生成绩");
        int score = scanner.nextInt();
        if (score==100){
            System.out.println("恭喜您获得满分！！！");
        } else if (score<100 && score>=90) {
            System.out.println("您的成绩为A");
        } else if (score<90 && score>=80) {
            System.out.println("您的成绩为B");
        } else if (score<80 && score>=70) {
            System.out.println("您的成绩为C");
        } else if (score<70 && score>=60) {
            System.out.println("您的成绩为D");
        } else if (score<60 && score>=0) {
            System.out.println("您的成绩不及格，请再接再厉");
        }else {
            System.out.println("您的输入的成绩不合法");
        }
    }
}

代码示例2：

int number = 25;
if (number <= 0) {
    System.out.println("Number is non-positive");
} else if (number < 10) {
    System.out.println("Number is between 1 and 10");
} else {
    System.out.println("Number is greater than 10");
}

3.if嵌套结构

int x = 10;
int y = 20;
if (x > 0) {
    if (y > 0) {
        System.out.println("Both x and y are positive");
    } else {
        System.out.println("x is positive and y is non-positive");
    }
} else {
    if (y > 0) {
        System.out.println("x is non-positive and y is positive");
    } else {
        System.out.println("Both x and y are non-positive");
    }
}