【JAVA】我和我的第一个“对象”相遇

• 表达式1必须是一个布尔表达式
• 如果表达式1为真，那么执行表达式2，否则执行表达式3

自我检验

根据以下代码思考打印的结果是什么？

public class TestDemo2 {
    public static void main(String[] args) {
        boolean flg = true == true ? true : true == false ? false : false;
        System.out.println(flg);
        boolean flg2 = true == false ? true : true == false ? false : false;
        System.out.println(flg2);
    }
}

提示：全局观——broaden your perspective

正确答案：true，false

JAVA中的逻辑控制

其实程序和人生是一样:顺序中夹杂着循环伴随一次次选择不断成长

---

顺序语句

public class SequentialStatementsExample {
    public static void main(String[] args) {
        // 声明并初始化两个整型变量
        int num1 = 10;
        int num2 = 20;
        
        // 计算两个数的和
        int sum = num1 + num2;
        
        // 打印计算结果
        System.out.println("Sum: " + sum);
        
        // 修改num1的值
        num1 = 5;
        
        // 计算两个数的差
        int difference = num1 - num2;
        
        // 打印计算结果
        System.out.println("Difference: " + difference);
    }
}

---

选择语句

单分支

if (布尔表达式){
            //语句1
        }

双分支

        if (布尔表达式){
            //语句1
        }else{
        	//语句2
        	}

多分支

        if (布尔表达式1){
            //语句1
        }else if(布尔表达式2){
        	//语句2
        }else{
        	//语句3
        }

---

switch语句

    public static void main(String[] args) {
// switch语句
        int a = 10;
        switch (a){
            case 1:
                System.out.println(1);
                break;
            case 2:
                System.out.println(2);
            default:
                System.out.println("Aileen");
                break;
        }

面试题：不能作为Switch参数的数据类型是什么？
float double boolean long
switch和if语句最本质的区别就是：switch语句后面括号跟的必须是只能是以下类型的表达式：

• 基本类型：byte、char、short、int （不能是long类型）
• 引用类型：String常量串、枚举类型

而if后面可以是复杂的条件语句。

---

JAVA中读入一个整数的写法——Scanner

import java.util.Scanner;

public class TestDemo2 {


    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);
        //判断奇偶数
        int num = scan.nextInt();//输入一个整数
        if (num % 2 == 0){
            System.out.println(num +" 是偶数");
        }else{
            System.out.println(num +" 是奇数");
        }
    }

小试牛刀：判断一个年份是否是闰年

import java.util.Scanner;

public class TestDemo2 {


    public static void main(String[] args) {
        //四年一闰；百年不闰,四百年再闰
        Scanner scan = new Scanner(System.in);
        int year = scan.nextInt();
        if(year % 4 == 0 && year % 100 != 0 || year % 400 ==0) {
            System.out.println(year + " 是闰年");
        } else {
            System.out.println(year + " 是平年");
        }

    }

---

循环语句

while循环

public class TestDemo2 {
    public static void main(String[] args) {
        while(循环条件){
            循环语句;
        }
    }

1. 打印1-10的数字

public class TestDemo2 {
    public static void main(String[] args) {
        int i = 1;
        while(i<=10){
            System.out.println(i);
            i++;
        }
    }

---

2. 打印1-10的和

public class TestDemo2 {
    public static void main(String[] args) {
        int i = 1;
        int sum = 0;
        while(i<=10){
            sum = sum + i;
            i++;
        }
        System.out.println(sum);
    }

---

3. 打印5的阶乘

public class TestDemo2 {

    public static void main(String[] args) {
        int i = 1;
        int j = 1;
        while(i<=5){
            j \*= i;
            i++;
        }
        System.out.println(j);
    }

---

4. 打印1-5的阶乘之和

public class TestDemo2 {

    public static void main(String[] args) {
        int i = 1;
        int j = 1;
        int sum = 0;
        while(i<=5){
            j \*= i;
            sum += j;
            i++;
        }
        System.out.println(sum);
    }

---

5. 判断素数

public class TestDemo2 {
    public static void main(String[] args) {
        //判断素数
        Scanner scan = new Scanner(System.in);
        int num = scan.nextInt();
        int i = 2;
        while (i <= Math.sqrt(num)) {
            if (num % i == 0) {
                System.out.println(num + " 不是素数");
                return;//跳出循环
            }
            i++;
        }
        System.out.println(num + "是素数");
    }

6. 打印1-100的所有素数

import java.util.Scanner;
public class TestDemo2 {
    public static void main(String[] args) {

        int i = 2;
        while (i < 100) {
            boolean isPrime = true;
// System.out.println(i);
            int k = 2;
            while (k <= Math.sqrt(i)) {
                if (i % k == 0) {
                    System.out.println(i + "不是素数");
                    isPrime = false;
                    break;
                }

                k++;
            }
        if (isPrime) {
            System.out.println(i + " 是素数");
        }
        i++;
        }

    }

}

7. 编写程序数一下 1到 100 的所有整数中出现多少个含有数字9的数的个数

import java.util.Scanner;
public class TestDemo2 {
    public static void main(String[] args) {
        int i = 1;
        int sum = 0;
        while(i <= 100) {

            if (i / 10 == 9 || i % 10 == 9) {
                sum++;
            }
            i++;
        }
        System.out.println(sum);
    }

8. 编写程序数一下 1到 100 的所有整数中出现多少个数字9

import java.util.Scanner;
public class TestDemo2 {
    public static void main(String[] args) {
        int i = 1;
        int count = 0;
        while(i <= 100){
            //十位是9
            if(i / 10 == 9){
                count++;
            }
            //个位是9
            if(i % 10 == 9){
                count++;
            }
            i++;
        }
        System.out.println(count);
    }

---

for循环

public class TestDemo3 {
    public static void main(String[] args) {
        for(表达式①;表达式②;表达式③){
            表达式④;
        }
    }

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表达式①：初始化条件
表达式②：循环判断条件
表达式④：循环需要执行的业务逻辑

for循环执行顺序

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One

表达式3

表达式1

布尔表达式2

表达式4