第一天：

求完数：

编程求出1000 以内的所有完数。

public class JavaBasic08 {

    public static void main(String[] args) {

        int sum = 0;

        for (int i = 2;i < 1000;i +=2) {    //最小的完数是2，并且完数都是偶数

            for (int j = 1; j < i; j++)

                if (i % j == 0)

                    sum += j;

            if (i == sum)

                System.out.print(i + "  ");

            sum = 0;

        }

    }

}

求不重复数字：

public class JavaBasic09 {

    public static void main(String[] args) {

        int count = 0 ;

        for (int Dig_1 = 1;Dig_1 <= 4;Dig_1 ++)

            for (int Dig_2 = 1;Dig_2 <= 4;Dig_2 ++)

                for (int Dig_3 = 1;Dig_3 <= 4;Dig_3 ++)

                    if(Dig_1 != Dig_2 && Dig_1 != Dig_3 && Dig_3 != Dig_2) {

                        count ++;

                        System.out.print(Dig_3 + "" + Dig_2 + "" +Dig_1 + "   ");

                    }

        System.out.print("\n一共可以组成 " + count + " 个符合条件的三位数。");

    }

}

第二天：

累加：

public class JavaBasic12 {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);

        System.out.print("求前 n 个数的和，输入 n 的值（n > 0）:");

        int n = input.nextInt();

        int sum = 0;

        for (;n > 0;n --)

            sum += n;

        System.out.print("和为 " + sum);

        input.close();

    }

}

累乘：

public class JavaBasic13 {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);

        System.out.print("求前 n 个数积，输入 n 的值（n > 0）:");

        int n = input.nextInt();

        int accumulate = 1;

        for (;n > 0;n --)

            accumulate *= n;

        System.out.print("积为 " + accumulate);

        input.close();

    }

}

第三天：

求最大值：

public class JavaBasic14 {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);

        System.out.print("请分别输入这三个数：");

        int num1 = input.nextInt(),num2 = input.nextInt(),num3 = input.nextInt();

        //if嵌套方式

        if(num1 > num2)

            if(num1 > num3)

                System.out.println("max = " + num1);

            else

                System.out.println("max = " + num2);

        else

            if(num2 > num3)

                System.out.println("max = " + num2);

            else

                System.out.println("max = " + num3);

        //if非嵌套方式

        int temp;

        if(num1 > num2)

            temp = num1;

        else

            temp = num2;

        if(temp > num3)

            System.out.println("max = " + temp);

        else

            System.out.println("max = " + num3);

        int result = num1 > num2 ?( num1 > num3 ? num1 :num3 ) : (num2  > num3 ? num2 : num3);

        System.out.println("max = " + result);

        input.close();

    }

}

第四天：

数列的前n项和：

public class JavaBasic11 {

    public static void main(String[] args) {

        int numerator = 2 , denominator = 1;  //第一个数字是2/1

        double result = 0;

        System.out.print(numerator + "/" + denominator);

        for (int num = 1;num <= 20; num ++) {    //计算前20项和--循环20次

            result += (double) numerator / denominator;

            numerator += denominator;

            denominator = numerator - denominator;

            System.out.print(" + " + numerator + "/" + denominator);      //分子：numerator，分母：denominator

        }

        System.out.println("\n结果是:" + result);

        input.close();

    }

}

第五天：

Swiitch练习 求某月天数：

public class JavaBasic29 {

    public static void main(String[] args) {

        //闰年是指能被4整除，但是不能被100整除的年份 或者 能被400整除的年份

        //平年2月有28天，闰年2月有29天

        Scanner input = new Scanner(System.in);

        System.out.print("请输入年份和月份:");

        int year = input.nextInt();

        int month = input.nextInt();

        int day = 28;

        switch (month) {

            case 1:

            case 3:

            case 5:

            case 7:

            case 8:

            case 10:

            case 12:

                day = 31;

                break;

            case 2:

                if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0)

                    day = 29;

                break;

            case 4:

            case 6:

            case 9:

            case 11:

                break;

        }

        System.out.print("在 " + year + " 年 " + month + " 月一共有 " + day + " 天。");

        input.close();

    }

}