运算符

1.赋值运算符

x += 10;
System.out.println(x);
x *= y + 3;
System.out.println(x);

2.算数运算符

+-*/

3.关系运算符

< <= > >= == !=

4.逻辑运算符

& | !(逻辑取反)

boolean z = 5 > 3;
System.out.println(z);
System.out.println(!z);
System.out.println(z);

5.短路运算符

&& ||

5.其他运算符

• 三元条件运算符
  a > b ? a : b
• 自加自减运算符(尽量少用 千万不要乱用)

int a = 5;
int b = a++; // ++把自身加1

• new / instanceof / . / [] / ()
  () 其中一种可以用来强制转换

double x = 3.99999;
//强制类型转换
int a = (int) x;
System.out.println(a);
        
 int b = 5;
 double c = 10;
 int d = (int) (b + c);
        
 short e = 100;
 //e = (short) (e + 1);
 e += 1;

• 位运算

int a = 3;
System.out.println(a << 2); // 3 * 2^2
        
 int b = 90;
System.out.println(b >> 2); // 90 / 2^2
        
 int c = 3;
System.out.println(c << 33); // 33 % 32 余 1
        
 //乘以31最快
int d = 3;
d = (d << 5) - d;
System.out.println(d);

debug

debug - 调试 / 捉虫，先在对应行进行断点操作，然后按F11进入debug进行调试

分支结构 if

分支结构(选择结构) - 可以让程序有多条执行路径

练习

• 练习1： 输入一个年份判断是不是闰年

         Scanner input = new Scanner(System.in);
         System.out.println("输入一个年份: ");
         // 分支结构(选择结构) - 可以让程序有多条执行路径
         if (input.hasNextInt()) {
            int year = input.nextInt();
            if (year > 0) {
                if(year % 4 == 0 && year % 100 != 0 || 
                        year % 400 == 0) {
                            System.out.println(year + "年是闰年");
                        }
                else {
                    System.out.println(year + "年不是闰年");
                }
                    /*  boolean isLeapYear = (year % 4 == 0 && year % 100 != 0) || 
                     year % 400 == 0;
         System.out.println(year + "年" + (isLeapYear ? "是" : "不是") + "闰年");*/
            } 
            else {
                System.out.println("年份必须是一个正整数!");
            }
         } 
         else {
            System.out.println("输入错误");
         }
         input.close();

• 练习2： 输入三角形3条边，判断是否构成三角形，输出周长和面积

Scanner input = new Scanner(System.in);
        System.out.println("请输入三角形的三条边的长度: ");
        if (input.hasNextDouble()) {
            double a = input.nextDouble();
            double b = input.nextDouble();
            double c = input.nextDouble();
            boolean isValid = a > 0 && b > 0 && c > 0 && a + b > c && a + c > b && b + c > a;
            // if (a + b > c && a + c > b && b + c > a)
            if (isValid) {
                double perimeter = a + b + c;
                double half = perimeter / 2;
                double area = Math.sqrt(half * (half - a) * (half - b) * (half -c));
                System.out.printf("三角形的周长: %.2f\n", perimeter);
                System.out.printf("三角形的面积: %.2f\n", area);
                /*System.out.println("三角形的周长: " + a + b + c);
                double s = (a + b + c) * 0.5;
                System.out.println("三角形的面积: " + Math.sqrt(s * (s - a) * (s - b) * (s - c)));*/
            } else {
                System.out.println("这3条边不能构成三角形!");
            }
        } else {
            System.out.println("输入错误!");
        }
        input.close();

• 练习3： 输入身高 (cm) 和体重 (kg) 判断身材是否正常。判断标准 身高

/*Scanner input = new Scanner(System.in);
        System.out.println("请输入身高(m)和体重(kg): ");
        if(input.hasNextDouble()){
            double height = input.nextDouble();
            double weight = input.nextDouble();
            if(height > 0 && weight > 0){
                double bmi = weight / (height * height);
                if(bmi > 29.7){
                    System.out.println("肥胖");
                }
                else if (25.4 < bmi && bmi <29.7) {
                    System.out.println("超重");
                }
                else if (19.6 < bmi && bmi <25.4) {
                    System.out.println("正常");
                }
                else if (17.6 < bmi && bmi <19.6) {
                    System.out.println("偏瘦");
                }
                else {
                    System.out.println("消廋");
                }
            }
            else {
                System.out.println("身高和体重必须大于0");
            }
        }
        else {
            System.out.println("输入错误!");
        }
        input.close(); */
        
        Scanner input = new Scanner(System.in);
        System.out.print("请输入您的名字: ");
        String name = input.nextLine();
        System.out.print("请输入身高(cm): ");
        int height = input.nextInt();
        System.out.print("请输入体重(kg): ");
        int weight = input.nextInt();
    /*  System.out.println("请输入性别(1.男,2.女): ");
        int gender = input.nextInt();
        int refValue = gender == 1 ? 110 : 120; */
        System.out.print("请输入性别(\"男\"或者\"女\"): ");
        String gender = input.next(); //若使用nextLine()会处理前面输入体重后的回车符,实际没有性别输入结束
        int refValue = gender.equals("男") ? 110 : 120; 
        if (height - refValue >= weight) {
            System.out.println(name + "是标准身材!");
        } 
        else {
            System.out.println(name + "是肥胖身材!");
        } 

        input.close();

• 练习4： f(x) = { 3x+5 (x<-1) x-1 (-1<=x<=1) 5x-3 (x>1)

Scanner input =new Scanner(System.in);
        System.out.print("x = ");
        double x = input.nextDouble();
        double y;
        if (x < -1){
            y = 3 * x + 5;
        }
        else if (x <= 1) {
            y = x - 1;
        }
        else {
            y = 5 * x - 3;
        }
        System.out.println("y = " + y);
        input.close();

作业

• 作业1：输入年和月，输出这个月总共有几天

Scanner input = new Scanner(System.in);
        System.out.print("请输入年份: ");
        int year = input.nextInt();
        if (year > 0) {
            System.out.print("请输入第几月: ");
            int month = input.nextInt();
            if (1 <= month && month <= 12) {
                boolean Thrity = month == 4 || month == 6 || month == 9 || 
                        month == 11;
                if (Thrity) {
                    System.out.println(year + " 年 " + month + " 月 有30天");
                } else if (month == 2) {
                    boolean refValue = year % 4 == 0 && year % 100 != 0 || 
                            year % 400 == 0;
                    if (refValue) {
                        System.out.println(year + " 年 " + month + "月 有29天");
                    } else {
                        System.out.println(year + " 年 " + month + " 月 有28天");
                    }
                } else {
                    System.out.println(year + " 年 " + month + " 月 有31天");
                }
            } else {
                System.out.println("月份输入范围必须在:1~12");
            }
        } else {
            System.out.println("年份必须大于0");
        }
        input.close();

• 作业2：个人所得税计算。在我国，个人所得税的起征点是3500元。计算公式是:个人所得税 = (工资收入 - 五险一金 -个人起征点) * 税率 -速算扣除数

Scanner input = new Scanner(System.in);
        System.out.print("请输入你的工资: ");
        double wages = input.nextDouble();
        System.out.println("请输入你需要缴纳的五险一金: ");
        double pay = input.nextDouble();
        double tax,range;
        if(wages > 3500){
            range = wages - 3500;
            if(range < 1500){
                tax = (wages - pay - 3500) * 0.03;
            }
            else if (range < 4500){
                tax = (wages - pay - 3500) * 0.1 - 105;
            }
            else if (range < 9000) {
                tax = (wages - pay - 3500) * 0.2 - 555;
            }
            else if (range < 35000) {
                tax = (wages - pay - 3500) * 0.25 - 1005;
            }
            else if (range < 55000) {
                tax = (wages - pay - 3500) * 0.3 - 2755;
            }
            else if (range <80000) {
                tax = (wages - pay - 3500) * 0.35 - 5505;
            }
            else {
                tax = (wages - pay - 3500) * 0.45 - 13505;
            }
            System.out.println("你的个人所得税是: " + tax + " 元");
        }
        else {
            System.out.println("你不用缴纳个人所得税！");
        }
        input.close();

作业改正

• 作业1：

        Scanner input = new Scanner(System.in);
        System.out.print("请输入年和月: ");
        int year = input.nextInt();
        int month = input.nextInt();
        if (year > 0 && month >= 1 && month <=12) {
            int days;
            if (month == 2) {
/*              if (year % 4 ==0 && year % 100 !=0 || year % 400 == 0) {
                    days = 29;
                }
                else {
                    days = 28;
                }*/
                days = (year % 4 ==0 && year % 100 !=0 || year % 400 == 0) ? 29 : 28;
            }
            else if (month == 4 || month == 6 || month == 9 || month == 11) {
                days = 30;
            }
            else {
                days = 31;
            }
            System.out.println(year + "年" + month + "月有" + days + "天");
        }
        else {
            System.err.print("输入错误!!!"); // err 标准错误输出
        }
        input.close();

• 作业2：

        Scanner input = new Scanner(System.in);
        System.out.print("请输入月薪: ");
        double salary = input.nextDouble();
        System.out.print("请输入五险一金: ");
        double insurance = input.nextDouble();
        double diff = salary - insurance - 3500;
        double rate, deduction; // 税率，
        if (diff <= 0){
            rate = 0;
            deduction = 0;
        }
        else if (diff <= 1500) {
            rate = 0.03;
            deduction = 0;
        }
        else if (diff <= 4500) {
            rate = 0.1;
            deduction = 105;
        }
        else if (diff <= 9000) {
            rate = 0.2;
            deduction = 555;
        }
        else if (diff <= 35000) {
            rate = 0.25;
            deduction = 1005;
        }
        else if (diff <= 55000) {
            rate = 0.3;
            deduction = 2755;
        }
        else if (diff <= 80000) {
            rate = 0.35;
            deduction = 5505;
        }
        else {
            rate = 0.45;
            deduction = 13505;
        }
        double tax = diff * rate - deduction;
        System.out.printf("本月应缴纳税款: ￥%.2f元\n", tax);
        System.out.printf("本月实发工资: ￥%.2f元\n", salary - insurance -tax);
        input.close();