Java 入门教程（三）：流程控制

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import java.util.Scanner;
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public class Grade {
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    public static void main(String[] args) {
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        Scanner sc = new Scanner(System.in);
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        int score = sc.nextInt();
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        if (score >= 90) {
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            System.out.println("优秀");
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        } else if (score >= 80) {
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            System.out.println("良好");
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        } else if (score >= 60) {
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            System.out.println("及格");
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        } else {
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            System.out.println("不及格");
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        }
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    }
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}

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public class Calculator {
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    public static void main(String[] args) {
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        char op = '+';
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        int a = 3, b = 4;
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        switch (op) {
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            case '+':
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                System.out.println(a + b);
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                break;                          // ⚠ 照样会贯穿！
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            case '-':
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                System.out.println(a - b);
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                break;
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            case '*':
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            case 'x':                           // 多标签共享分支
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                System.out.println(a * b);
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                break;
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            default:
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                System.out.println("不支持");
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        }
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    }
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}

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public class Week {
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    public static void main(String[] args) {
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        int day = 6;
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        // ① 箭头分支（无贯穿烦恼）
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        String type = switch (day) {
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            case 1, 2, 3, 4, 5 -> "工作日";
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            case 6, 7 -> "周末";
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            default -> "非法日期";           // 必须穷尽！有 default 才编译通过
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        };
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        System.out.println(type);            // 周末
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        // ② 带代码块的分支（yield 返回值）
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        String detail = switch (day) {
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            case 6, 7 -> {
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                String name = (day == 6) ? "周六" : "周日";
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                yield name + "休息";         // yield 产出值
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            }
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            default -> "上班";
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        };
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        System.out.println(detail);
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    }
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}

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// 1~100 求和
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int sum = 0, i = 1;
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while (i <= 100) {
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    sum += i;
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    i++;
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}
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System.out.println(sum);      // 5050
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// do-while：至少执行一次（菜单场景）
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int choice;
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do {
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    System.out.println("1.开始 0.退出");
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    // choice = 读入...
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    choice = 0;               // 演示用
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} while (choice != 0);

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static boolean isPrime(int n) {
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    if (n < 2) return false;
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    for (int i = 2; (long) i * i <= n; i++) {    // i*i 防溢出用 long 比
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        if (n % i == 0) return false;
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    }
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    return true;
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}
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public static void main(String[] args) {
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    int count = 0;
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    for (int n = 2; n < 100; n++) {
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        if (isPrime(n)) {
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            System.out.printf("%d ", n);
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            count++;
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        }
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    }
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    System.out.println("\n共 " + count + " 个");
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}

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import java.util.Random;
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import java.util.Scanner;
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public class Guess {
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    public static void main(String[] args) {
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        Random rand = new Random();
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        int answer = rand.nextInt(100) + 1;     // 1~100
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        Scanner sc = new Scanner(System.in);
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        int tries = 0;
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        while (true) {
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            System.out.print("猜(1-100): ");
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            if (!sc.hasNextInt()) {             // 输入校验
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                System.out.println("请输入数字！" + sc.next() + " 被忽略");
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                continue;
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            }
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            int guess = sc.nextInt();
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            tries++;
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            if (guess < answer)      System.out.println("小了");
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            else if (guess > answer) System.out.println("大了");
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            else {
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                System.out.println("对了！用了 " + tries + " 次");
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                break;
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            }
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        }
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    }
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}