为什么要学习循环
在编写代码时，业务需求(项目要求)原地打转，符合条件结束循环。 马拉松 (操场
while循环
public class Demo01 { public static void main(String[] args) { /* while (条件) { 循环的体（内容）; 更新条件; } 1 +到 100，5050 */ int i = 1, sum = 0; while (i <= 100) { sum += i; i++; } System.out.println("和：" + sum); } }
dowhile循环
public class Demo02 { public static void main(String[] args) { /* do { 循环的体（内容）; 更新条件; } while(条件); 1 +到 100，5050 */ int i = 1, sum = 0; do { sum += i; i++; } while (i <= 100); System.out.println("和：" + sum); } }
偶数之和
学员操作一计算100以内的偶数之和 训练要点 while循环结构 程序调试 需求说明 编程实现: 计算100以内 (包括100) 的偶数之和设置断点并调试程序，观察每一次循环中变量值的变化
public class Demo03 { public static void main(String[] args) { int n = 1, sum = 0; while (n <= 100) { if (n % 2 == 0) sum += n; n++; } System.out.println("sum: " + sum); } }
断点调试
购物结算
import java.util.Scanner; public class Demo04 { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); System.out.println("购物结算"); System.out.println("请选择商品编号："); System.out.println("1.T恤 2.网球鞋 3.网球拍"); String jx = "y"; while (jx.equals("y")) { System.out.print("请输入商品编号："); int select = scanner.nextInt(); switch (select) { case 1: System.out.println("T恤 100元"); break; case 2: System.out.println("网球鞋 200元"); break; case 3: System.out.println("网球拍 300元"); break; } System.out.print("是否继续（y/n）："); jx = scanner.next(); } System.out.println("程序结束！"); } }
循环的选择
import java.util.Scanner; public class Demo05 { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); System.out.println("购物结算"); System.out.println("请选择商品编号："); System.out.println("1.T恤 2.网球鞋 3.网球拍"); String jx; do { System.out.print("请输入商品编号："); int select = scanner.nextInt(); switch (select) { case 1: System.out.println("T恤 100元"); break; case 2: System.out.println("网球鞋 200元"); break; case 3: System.out.println("网球拍 300元"); break; } System.out.print("是否继续（y/n）："); jx = scanner.next(); } while (jx.equals("y")); System.out.println("程序结束！"); } }
类名和全类名
摄氏华氏对照表
public class Demo06 { public static void main(String[] args) { /* 使用do-while实现：输出 *摄氏温度 与 *华氏温度的对照表，要求它从摄氏温度0度到250度，每隔20度为一项，对照表中的 *条目 不超过10条。 转换关系：华氏温度 = 摄氏温度 * 9 / 5.0 + 3 */ double huashidu, sheshidu = 0; int count = 0; do { huashidu = sheshidu * 9 / 5.0 + 32; System.out.println(sheshidu + " vs. " + huashidu); sheshidu += 20; count++; } while (sheshidu <= 250 && count <= 10); } }
for循环
public class Demo07 { public static void main(String[] args) { /* 1 +到 100，5050 特点：循环次数固定下来的。建议使用for循环。 for (声明初始化循环变量; 条件; 修改循环变量) { 循环体; } */ int sum = 0; for (int i=0; i<=100; i++) { sum += i; } System.out.println("和：" + sum); } }
for执行次序
五门功课成绩
import java.util.Scanner; public class Demo08 { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); System.out.print("请输入姓名："); String name = scanner.next(); // 记录姓名 int sum = 0; // 记录总成绩 for (int i=1; i<=5; i++) { System.out.print("请输入5门功课的第" + i + "门的成绩："); sum += scanner.nextInt(); } System.out.println(name + "的平均分是：" + sum / 5.0); } }
for的特殊写法
public class Demo09 { public static void main(String[] args) { for (int i=0,j=6; i<=6; i++,j--) { System.out.println(i + "+" + j + "=" + 6); } } }
break和continue
import java.util.Scanner; public class Demo10 { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); int sum = 0; for (int i=1; i<=5; i++) { System.out.print("成绩："); int cj = scanner.nextInt(); if (cj < 0) break; // 结束循环 } System.out.println("最后"); } }
import java.util.Scanner; public class Demo11 { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); int sum = 0; for (int i=1; i<=5; i++) { System.out.print("成绩："); int cj = scanner.nextInt(); if (cj < 0) continue; // 忽略当次 sum += cj; } System.out.println("和：" + sum); } }
录入客户信息_continue使代码优雅
import java.util.Scanner; public class Demo12 { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); System.out.println("> 添加客户信息"); for (int i=1; i<=3; i++) { System.out.print("会员号《4位》："); int id = scanner.nextInt(); if (id >= 1000 && id <= 9999) { // 满足条件1 System.out.print("生日《mm/dd》："); String birth = scanner.next(); if (birth.length() == 5) { // 满足条件2 System.out.print("积分："); int jifeng = scanner.nextInt(); if (jifeng >= 0) { // 满足条件3 System.out.println("会员信息是：\n" + id + "\t" + birth + "\t" + jifeng); } } } } System.out.println("程序结束！"); } }
使用continue
import java.util.Scanner; public class Demo13 { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); System.out.println("> 添加客户信息"); for (int i=1; i<=3; i++) { System.out.print("会员号《4位》："); int id = scanner.nextInt(); if (id < 1000 || id > 9999) continue; // 过滤 System.out.print("生日《mm/dd》："); String birth = scanner.next(); if (birth.length() != 5) continue; // 过滤 System.out.print("积分："); int jifeng = scanner.nextInt(); if (jifeng < 0) continue; // 过滤 System.out.println("会员信息是：\n" + id + "\t" + birth + "\t" + jifeng); } System.out.println("程序结束！"); } }
小数的比较不能用==或!=
import java.util.Scanner; public class Demo14 { public static void main(String[] args) { final double JINDU = 0.00001; /* 1. 录入一个小数1，小数2，小数3。 判断小数1+小数2是否等于小数3？ 小数是模拟出来的数，近似值。无法用== !=进行比较的。 */ Scanner scanner = new Scanner(System.in); System.out.print("小数1："); double d1 = scanner.nextDouble(); System.out.print("小数2："); double d2 = scanner.nextDouble(); System.out.print("小数3："); double d3 = scanner.nextDouble(); if (d1 + d2 <= d3+JINDU && d1 + d2 >= d3-JINDU) System.out.println("d1+d2==d3"); else System.out.println("d1+d2!=d3"); System.out.println("程序结束！"); } }
