Java 入门教程（五）：面向对象（上）——类与封装

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public class Account {
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    private String owner;
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    private double balance;
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    public Account(String owner) {
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        this.owner = owner;              // ① 区分字段与同名参数
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    }
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    public Account(String owner, double balance) {
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        this(owner);                     // ② this(...) 调用兄弟构造（必须第一行）
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        this.balance = balance;
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    }
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    public Account deposit(double amount) {
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        if (amount > 0) this.balance += amount;
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        return this;                     // ③ 返回自身 → 链式调用
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    }
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}
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new Account("小明", 100).deposit(50).deposit(30);

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// 反例：字段公开
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public class Bad {
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    public double balance;          // 任何代码都能写 account.balance = -999
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}
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// 正例：私有字段 + 校验入口
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public class Good {
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    private double balance;         // 外部不可见
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    public void deposit(double amount) {
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        if (amount <= 0) throw new IllegalArgumentException("金额必须为正");
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        balance += amount;          // 修改必经校验
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    }
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    public double getBalance() { return balance; }
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}

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public class GameCharacter {
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    // 静态字段：全类共享一份（不属于任何对象）
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    private static int createdCount = 0;
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    static final int MAX_LEVEL = 99;          // 静态常量
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    // 实例字段：每个对象一份
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    private String name;
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    private int level;
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    public GameCharacter(String name) {
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        this.name = name;
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        this.level = 1;
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        createdCount++;                        // 每创建一个 +1
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    }
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    // 静态方法：属于类，没有 this（不能访问实例成员！）
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    public static int getCreatedCount() {
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        // System.out.println(this.name);     // ❌ 编译错误，静态无 this
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        return createdCount;
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    }
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    public static void main(String[] args) {
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        new GameCharacter("勇者");
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        new GameCharacter("法师");
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        System.out.println(GameCharacter.getCreatedCount());   // 2（用类名调）
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        System.out.println(GameCharacter.MAX_LEVEL);
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    }
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}

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import java.util.ArrayList;
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import java.util.List;
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public class Library {
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    // ===== 内部数据类 =====
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    private static class Book {
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        private final String isbn;
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        private final String title;
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        private boolean borrowed;
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        Book(String isbn, String title) {
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            this.isbn = isbn;
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            this.title = title;
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        }
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        String getTitle() { return title; }
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        String getIsbn() { return isbn; }
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        boolean isBorrowed() { return borrowed; }
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    }
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    // ===== 业务异常 =====
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    public static class LibraryException extends RuntimeException {
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        LibraryException(String msg) { super(msg); }
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    }
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    // ===== 状态封装在 Library 内 =====
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    private final List<Book> books = new ArrayList<>();
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    public void addBook(String isbn, String title) {
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        if (findBook(isbn) != null)
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            throw new LibraryException("ISBN 已存在: " + isbn);
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        books.add(new Book(isbn, title));
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    }
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    public void borrow(String isbn) {
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        Book b = requireBook(isbn);
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        if (b.borrowed)
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            throw new LibraryException("《" + b.getTitle() + "》已借出");
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        b.borrowed = true;
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    }
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    public void giveBack(String isbn) {
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        requireBook(isbn).borrowed = false;
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    }
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    public void listAll() {
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        System.out.println("—— 馆藏 " + books.size() + " 本 ——");
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        for (Book b : books)
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            System.out.printf("%-14s %-20s %s%n",
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                    b.getIsbn(), b.getTitle(),
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                    b.isBorrowed() ? "[借出]" : "[在架]");
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    }
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    private Book findBook(String isbn) {          // 私有工具方法
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        for (Book b : books)
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            if (b.getIsbn().equals(isbn)) return b;
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        return null;
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    }
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    private Book requireBook(String isbn) {
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        Book b = findBook(isbn);
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        if (b == null) throw new LibraryException("无此书: " + isbn);
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        return b;
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    }
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    public static void main(String[] args) {
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        Library lib = new Library();
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        lib.addBook("978-7-111", "Java 核心技术");
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        lib.addBook("978-7-115", "算法（第4版）");
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        lib.borrow("978-7-111");
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        lib.listAll();
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        try {
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            lib.borrow("978-7-111");          // 重复借
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        } catch (LibraryException e) {
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            System.out.println("❌ " + e.getMessage());
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        }
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        lib.giveBack("978-7-111");
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        lib.listAll();
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    }
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}