java学习笔记

import java.util.Scanner ;class WorkDay3 {public static void main(String[] args) {//1.求1到100能被7整除的数的个数int sum = 0 ;for(int i = 1;i <= 100; i++ ){if(i % 7 == 0){sum ++ ;}}System.out.println("能被7整除的个数为"+sum+"个") ;//2.打印实心菱形/*******************//*class PrintRhombus{public static void main(String[] args) {for(int i = 1;i <= 7;i++){if(i <= 4){for(int j = 1; j <= (4-i); j++){   //上半部分空格打印System.out.print(" ");}for(int k = 1; k <= (2*i-1);k++){ //打印*System.out.print("*");}}///////////////////////以上为上半部分打印///////////////////////////////////////////////////////以下为下半部分打印//////////////////////////////////////if(i>4){for(int m = 1; m <= (i-4);m++){  //下半部分空格打印System.out.print(" ");}for(int k = 1 ; k <= 2*(7-i+1)-1;k++){ //用总数减，反向计数，打印*System.out.print("*");}}System.out.println();}}} *//********************************/for(int i = 1;i <= 7;i++){if(i <= 4){for(int k = 1;k <= 4 - i;k ++){System.out.print(" ");}for(int j = 1;j <= i * 2 - 1; j++){System.out.print("*");}}else{for(int k = 1; k <= i - 4; k++){System.out.print(" ");}for(int j = 1;j <= 2*(7-i)+1;j++){System.out.print("*");}}System.out.println();}//3. 1到100所有奇数的和int sum1 = 0 ;//时刻注意变量的初始化for (int i = 1;i <= 100 ;i = i + 2 ){sum1 += i;}System.out.println(sum1);//4. 计算1-100内能被3或17整除的数的和int sum2 = 0;for(int i = 1;i < 100; i++ ){if(i % 3 == 0 && i % 17 == 0){sum2 += i ;}}System.out.println("1-100内能被3或17整除的数的和:"+sum2);//5. 计算1到100以内能被7或者3整除但不能同时被这两者整除的数的个数int num1 = 0 ;for(int i = 1;i < 100; i++ ){if((i % 7 == 0)||(i % 3 == 0)){if(i % 21 != 0){num1 ++ ;}}}System.out.println("1到100以内能被7或者3整除但不能同时被这两者整除的数的个数为："+ num1) ;//6.计算1到500以内能被7整除但不是偶数的数的个数int num2 = 0;for(int i = 1;i <= 500; i += 2){if(i % 7 == 0){num2 ++ ;}}System.out.println("计算1到500以内能被7整除但不是偶数的数的个数:"+ num2);//7.从键盘输入一个数n，判断是不是一个质数（质数是只能被1和它自身整除的数）。System.out.println("请输入一个数：") ;Scanner input = new Scanner(System.in);int num3 = input.nextInt() ;boolean flag = true ;for(int i =2;i < num3;i++){if(num3 % i ==0){flag = false ;}}if(flag == true ){ //为什么要加一个（i<=num3）的判断System.out.println("是质数！");}else{System.out.println("不是质数！");}//8.根据（1-7）的数值不同。打印对应的星期英文。System.out.println("今天是星期几？");Scanner day = new Scanner(System.in);int num4 = day.nextInt();switch(num4){case 1: System.out.println("今天是monday");break;case 2: System.out.println("今天是Thesday");break;case 3: System.out.println("今天是wednesday");break;case 4: System.out.println("今天是Thursday");break;case 5: System.out.println("今天是firday");break;case 6: System.out.println("今天是saturday");break;case 7: System.out.println("今天是sunday");break;default:System.out.println("您输入的信息有误！");break;}//中级//1.3000米长的绳子，每天减一半。问多少天这个绳子会小于5米？不考虑小数//法1 ：while循环int l = 3000 ;int num5 = 0;while(l >= 5){l = l / 2 ;num5 ++;}System.out.println("天数："+num5);//法2：for循环int day1 = 0;for(int i=3000;i>=5;i/=2){day1++;}System.out.println("天数="+day1);//2.求2～100之内的素数 System.out.print("素数：");for(int i = 2;i <= 100;i++){boolean flag2 = true ;for(int j = 2; j < i;j++){if(i % j == 0 ){flag2 = false ;break ;}    }if(flag2){System.out.print(i+"\t");}}///////////不会//3.打印空心菱形for(int i = 1;i <= 7;i++){  if(i <= 4 ){   for(int k = 1;k <= 4 - i;k++){  System.out.print(" ");   }  for(int k = 1;k <= i;k++){  System.out.print("* ");    }  System.out.println();  }else{  for(int k =1;k <= (i -7);k++){  System.out.print(" ");   }   for(int k = 1;k <= (2 *7 - i);k++){   System.out.print("* ");   }  System.out.println();   } }//4.判断某一年是否为闰年Scanner scan = new Scanner(System.in);System.out.println("请输入一个年份：");int year = scan.nextInt();if(year % 4==0 && year % 100 != 0 ||year % 400 ==0){System.out.println(year+"年是闰年");}else{System.out.println(year+"不是闰年");}//高级//1.求1000以内的水仙花数：（1~1000）int b,s,g ;System.out.print("水仙花数为：");for(int i = 100;i <=999;i++){b = i / 100;s = i % 100 /10 ;g = i % 10;if(b*b*b+s*s*s+g*g*g==i)System.out.print(i+"\t");}//2.有1、2、3、4四个数字，输出所有能组成的互不相同且无重复的三位数int b1,s1,g1;System.out.print("无重复的三位数：");for(b1=1;b1 <=4;b1++){for(s1=1;s1<=4;s1++){for(g1=1;g1<=4;g1++){if(b1 != s1 && s1 != g1 && b1 != g1){System.out.print(b1*100 + s1*10 +g1+"\t");}}}}System.out.println("Hello World!");}}