class  IfStruct1{
    public static void main(String[] args) {
        if(布尔表达式){
            //语句体
        }
    }
}

class  IfStruct2{
    public static void main(String[] args) {
        if(布尔表达式){
            //语句体1
        }else{
            //语句体2
        }
    }
}

class  IfStruct3{
    public static void main(String[] args) {
        if(布尔表达式1){
            //语句体1
        }else if(布尔表打死2){
            //语句体2
        }else{
            //语句体3
        }
    }
}

package com.xuweiwei;

public class IfDemo {
    public static void main(String[] args) {
        int x = 0;
        if(x ==0){
            System.out.println("你好啊");
        }
        System.out.println("世界");
    }
}

package com.xuweiwei;

public class IfDemo {
    public static void main(String[] args) {
        int x = 2;
        if(x % 2 == 0){
            System.out.println("是偶数");
        }else{
            System.out.println("是奇数");
        }

    }
}

package com.xuweiwei;

public class IfDemo {
    public static void main(String[] args) {
        char c = ‘男‘;
        if (c == ‘男‘){
            System.out.println("你是男生");
        }else if(c ==‘女‘){
            System.out.println("你是女生");
        }else{
            System.out.println("你的性别暂时不能识别");
        }

    }
}

package com.xuweiwei;

public class IfTest {
    public static void main(String[] args) {
        int a = 10;
        int b = 20;
        int c = 30;

        if(a > b){
            if(a > c){
                System.out.println("a="+a+"最大");
            }else{
                System.out.println("c="+c+"最大");
            }
        }else{
            if(b > c){
                System.out.println("b="+b+"最大");
            }else{
                System.out.println("c="+c+"最大");
            }
        }
    }
}

switch(表达式){//表达式可以是byte short int char 枚举，JDK7新增了String
            case 值1:
                语句体1;
            break;
            case 值2:
                语句体2;
            break;
            ……
            default:
                语句体n;
            break;
}

package com.xuweiwei;

public class IfTest {
    public static void main(String[] args) {
        String str = "a";
        switch (str) {
            case "a":
                System.out.println("a");
                break;
            case "b":
                System.out.println("b");
                break;
            default:
                System.out.println("你好");
                break;
        }
    }
}

package com.xuweiwei;

import java.util.Scanner;

public class IfTest {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.println("请输入1-7：");
        int num = input.nextInt();
        if (num < 0 || num > 7) {
            System.out.println("抱歉，输入数字不合法");
        } else {
            switch (num) {
                case 1:
                    System.out.println("星期一");
                    break;
                case 2:
                    System.out.println("星期二");
                    break;
                case 3:
                    System.out.println("星期三");
                    break;
                case 4:
                    System.out.println("星期四");
                    break;
                case 5:
                    System.out.println("星期五");
                    break;
                case 6:
                    System.out.println("星期六");
                    break;
               default:
                    System.out.println("星期天");
                    break;

            }
        }
    }
}