Java入门代码练习

一、第一个Java程序

1、helloworld

public class Hello {
    public static void main(String[] args){
        System.out.println("Hello world!");
    }
}

2、变量

import java.util.Scanner;

public class Hello{
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        int price;
        System.out.println("请输入金额：");
        price = in.nextInt();
        System.out.println("找零："+(100-price));
    }
}

3、赋值

import java.util.Scanner;

public class Hello{
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        int amount;
        int price;
        System.out.print("请输入票面：");
        amount = in.nextInt();
        System.out.print("请输入金额：");
        price = in.nextInt();
        int change = amount - price ;
        System.out.println(amount+"-"+price+"="+change);
    }
}

4、浮点数

import java.util.Scanner;

public class Hello {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        int foot;
        double inch;
        foot = in.nextInt();
        inch = in.nextDouble();
        System.out.println("foot="+foot+" inch="+inch);
        System.out.println((foot + inch/12)*0.3048);
        /*
        int inch;
        inch = in.nextInt();
        inch/12.0
         */
        //浮点数计算有误差
        System.out.println(1.2-1.1);
    }
}

5、优先级

6、关系运算

import java.util.Scanner;

public class Hello {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        double a = 5.0;
        int b = 5;
        System.out.println(a==b);//比较浮点数和整数是否相等，结果为true
        double c = 1.0;
        double d = 0.1+0.1+0.1+0.1+0.1+0.1+0.1+0.1+0.1+0.1;
        System.out.println(c==b);//c和b不相等，结果为false,浮点运算有误差
        System.out.println("c="+c+" d="+d);
        //比较两个浮点数是否相等：
        System.out.println(Math.abs(c-d)<1e-6);//c和d差的绝对值是否小于一个比较小的数，10的-6次方

    }
}

7、判断

import java.util.Scanner;

public class Hello {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        System.out.print("请投币：");
        int amount = in.nextInt();
        if (amount >= 10){
            //打印车票
            System.out.println("*****************");
            System.out.println("****Java城际铁轨***");
            System.out.println("***无指定座位票*****");
            System.out.println("****票价10元******");
            System.out.println("*****************");
            //计算并打印找零
            System.out.println("找零："+(amount-10));
        }
    }
}

8、做判断

import java.util.Scanner;

public class Hello {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        final int MINOR = 35;
        System.out.print("请输入你的年龄：");
        int age = in.nextInt();
        System.out.println("你的年龄是："+age);
        if (age < MINOR){
            System.out.println("年轻是美好的！");
        }
    }
}

import java.util.Scanner;

public class Hello {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        int x = in.nextInt();
        int y = in.nextInt();
        int max;
        if (x > y){
            max = x;
        }else{
            max = y;
        }
        System.out.println(max);

    }
}

import java.util.Scanner;

public class Hello {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
       final double RATE = 8.25;
       final int STANDARD = 40;
       double pay = 0.0;
       System.out.print("Enter the number of hours worked: ");
       int hours = in.nextInt();
       System.out.println();
       if (hours > STANDARD){
           pay = STANDARD * RATE + (hours - STANDARD) * (RATE * 1.5);
       }else{
           pay = hours * RATE;
       }
       System.out.println("Gross earnings:" + pay);
    }
}

import java.util.Scanner;

public class Hello {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        final int PASS = 60;
        System.out.print("请输入成绩：");
        int score = in.nextInt();
        System.out.println("你的成绩是："+score);
        if (score >= PASS){
            System.out.println("恭喜你，成绩合格！");
        }else{
            System.out.println("很遗憾，成绩不及格！");
        }
        System.out.println("再见！");
    }
}

9、嵌套和级联判断

import java.util.Scanner;

public class Hello {
    public static void main(String[] args){
        System.out.println("请输入三位数：");
        Scanner scan = new Scanner(System.in);
        int x = scan.nextInt();
        int y = scan.nextInt();
        int z = scan.nextInt();
        int max = 0;
        if (x > y){
            if(x > z){
                max = x;
            }else{
                max = z;
            }
        }else{
            if (y > z){
                max = y;
            }else{
                max = z;
            }
        }
        System.out.println(max);
    }
}

import java.util.Scanner;

public class Hello {
    public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        int x = scan.nextInt();
        int y;
        if (x < 0){
            y = -1;
        }else{
            if (x == 0){
                y = 0;
            }else{
                y = 2 * x;
            }
        }
        System.out.println("x="+x+" y="+y);
    }
}

10、多路分支 Switch Case语句

import java.util.Scanner;

public class Hello {
    public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        int type = scan.nextInt();
        switch (type){
            case 1:
            case 2:
                System.out.println("你好");
                break;
            case 3:
                System.out.println("晚上好");
            case 4:
                System.out.println("再见");
                break;
            default:
                System.out.println("what are you doing?");
                break;
        }
    }
}