Java 从零开始完整教程（小白也能看懂）#

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

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import java.util.Scanner;   // 引入 Scanner 类
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public class Input {
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    public static void main(String[] args) {
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        Scanner scanner = new Scanner(System.in);
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        System.out.print("请输入你的名字：");
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        String name = scanner.nextLine();   // 读一整行字符串
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        System.out.print("请输入年龄：");
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        int age = scanner.nextInt();        // 读整数
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        System.out.println("你好，" + name + "，你今年" + age + "岁");
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    }
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}

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public class Condition {
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    public static void main(String[] args) {
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        int score = 85;
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        if (score >= 90) {
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            System.out.println("优秀！");
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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 Loop {
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    public static void main(String[] args) {
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        // for
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        for (int i = 1; i <= 5; i++) {
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            System.out.print(i + " ");
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        }
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        System.out.println();   // 1 2 3 4 5
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        // while
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        int n = 0;
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        while (n < 3) {
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            System.out.println("第" + n + "次");
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            n++;
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        }
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        // 增强 for（遍历数组/集合，最常用）
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        int[] nums = {10, 20, 30};
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        for (int num : nums) {
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            System.out.print(num + " ");
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        }
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        System.out.println();   // 10 20 30
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    }
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}

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public class Array {
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    public static void main(String[] args) {
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        // 声明数组
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        int[] scores = {90, 85, 78, 92, 88};
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        // 访问（下标从 0 开始）
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        System.out.println(scores[0]);   // 90
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        // 修改
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        scores[2] = 80;
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        // 长度
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        System.out.println(scores.length);   // 5
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        // 遍历
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        for (int i = 0; i < scores.length; i++) {
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            System.out.print(scores[i] + " ");
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        }
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        System.out.println();
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    }
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}

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public class StringDemo {
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    public static void main(String[] args) {
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        String s = "Hello World";
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        System.out.println(s.length());          // 11（长度）
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        System.out.println(s.toUpperCase());     // HELLO WORLD
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        System.out.println(s.toLowerCase());     // hello world
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        System.out.println(s.charAt(0));         // H（取第0个字符）
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        System.out.println(s.substring(0, 5));   // Hello（子串）
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        // 拼接（+ 号）
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        String name = "小明";
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        System.out.println("你好，" + name);
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        // 比较内容要用 equals，不能用 ==
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        String a = "abc";
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        String b = new String("abc");
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        System.out.println(a == b);            // false（比较的是地址）
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        System.out.println(a.equals(b));       // true（比较的是内容）
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    }
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}

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// 定义接口
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interface Flyable {
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    void fly();   // 接口里的方法只有声明，没有实现
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}
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// 类实现接口
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class Bird implements Flyable {
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    @Override
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    public void fly() {
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        System.out.println("鸟在飞");
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    }
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}
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class Plane implements Flyable {
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    @Override
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    public void fly() {
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        System.out.println("飞机在飞");
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    }
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}
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// 使用：都能飞，但实现不同
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public class Main {
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    public static void main(String[] args) {
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        Flyable[] things = { new Bird(), new Plane() };
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        for (Flyable f : things) {
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            f.fly();
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        }
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    }
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}

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public class ExceptionDemo {
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    public static void main(String[] args) {
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        try {
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            int[] nums = {1, 2, 3};
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            System.out.println(nums[5]);   // 越界，会抛异常
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        } catch (ArrayIndexOutOfBoundsException e) {
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            System.out.println("数组越界了：" + e.getMessage());
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        } finally {
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            System.out.println("无论对错都会执行（比如关闭资源）");
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        }
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    }
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}

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class AgeException extends Exception {
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    public AgeException(String msg) {
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        super(msg);
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    }
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}
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// 使用
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public static void checkAge(int age) throws AgeException {
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    if (age < 0) {
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        throw new AgeException("年龄不能为负数");
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    }
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}

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import java.util.ArrayList;
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import java.util.Scanner;
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public class ScoreManager {
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    public static void main(String[] args) {
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        Scanner scanner = new Scanner(System.in);
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        ArrayList<String> names = new ArrayList<>();
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        ArrayList<Integer> scores = new ArrayList<>();
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        while (true) {
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            System.out.println("\n==== 学生成绩管理 ====");
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            System.out.println("1. 录入成绩");
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            System.out.println("2. 查看全部");
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            System.out.println("3. 查看平均分");
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            System.out.println("4. 退出");
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            System.out.print("请选择：");
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            int choice = scanner.nextInt();
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            scanner.nextLine();   // 吃掉回车
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            if (choice == 1) {
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                System.out.print("输入姓名：");
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                String name = scanner.nextLine();
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                System.out.print("输入成绩：");
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                int score = scanner.nextInt();
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                scanner.nextLine();
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                names.add(name);
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                scores.add(score);
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                System.out.println("已录入！");
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            } else if (choice == 2) {
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                for (int i = 0; i < names.size(); i++) {
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                    System.out.println(names.get(i) + "：" + scores.get(i));
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                }
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            } else if (choice == 3) {
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                int sum = 0;
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                for (int s : scores) sum += s;
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                double avg = scores.size() > 0 ? (double) sum / scores.size() : 0;
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                System.out.println("平均分：" + avg);
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            } else if (choice == 4) {
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                System.out.println("再见！");
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                break;
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            } else {
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                System.out.println("输入无效");
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            }
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        }
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    }
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}