第 7 章：面向对象编程（上）—— 类的"蓝图"¶

/**
 * Student 类 —— 描述一个学生应该有什么、能做什么
 */
public class Student {
    // ===== 属性（成员变量）=====
    String name;      // 姓名
    String studentId; // 学号
    int age;          // 年龄
    double score;     // 成绩

    // ===== 方法（行为）=====

    // 自我介绍
    public void introduce() {
        System.out.println("我叫" + name + "，学号" + studentId + "，今年" + age + "岁。");
    }

    // 考试
    public void takeExam(double newScore) {
        this.score = newScore;  // this 指代"当前这个对象"
        System.out.println(name + " 参加了考试，成绩: " + score);
    }

    // 判断是否及格
    public boolean isPassed() {
        return score >= 60;
    }
}

public class StudentTest {
    public static void main(String[] args) {
        // 创建对象：用 new 关键字 + 构造方法
        Student zhangsan = new Student();
        Student lisi = new Student();

        // 给属性赋值
        zhangsan.name = "张三";
        zhangsan.studentId = "2024001";
        zhangsan.age = 20;

        lisi.name = "李四";
        lisi.studentId = "2024002";
        lisi.age = 21;

        // 调用方法
        zhangsan.introduce();
        zhangsan.takeExam(85);
        System.out.println("张三及格了吗？ " + zhangsan.isPassed());

        System.out.println();

        lisi.introduce();
        lisi.takeExam(55);
        System.out.println("李四及格了吗？ " + lisi.isPassed());
    }
}

public class StudentV2 {
    String name;
    String studentId;
    int age;
    double score;

    // 构造方法：名字与类名相同，没有返回类型
    public StudentV2(String name, String studentId, int age) {
        this.name = name;           // this.name 是属性，name 是参数
        this.studentId = studentId;
        this.age = age;
        this.score = 0;             // 默认成绩为 0
    }

    // 方法重载：另一个构造方法（只传姓名和学号）
    public StudentV2(String name, String studentId) {
        this(name, studentId, 18);  // 调用上面的构造方法
    }

    public void introduce() {
        System.out.println(name + " | " + studentId + " | " + age + "岁 | " + score + "分");
    }
}

public class ConstructorTest {
    public static void main(String[] args) {
        // 创建对象时直接传入属性值
        StudentV2 s1 = new StudentV2("张三", "2024001", 20);
        StudentV2 s2 = new StudentV2("李四", "2024002");  // 使用默认年龄 18

        s1.introduce();
        s2.introduce();
    }
}

public class BankAccount {
    // 属性设为 private，外部不能直接访问
    private String owner;
    private double balance;
    private String password;

    public BankAccount(String owner, String password) {
        this.owner = owner;
        this.password = password;
        this.balance = 0;
    }

    // getter：获取余额（只读）
    public double getBalance() {
        return balance;
    }

    // getter：获取户主名
    public String getOwner() {
        return owner;
    }

    // setter：存款（有验证逻辑）
    public void deposit(double amount) {
        if (amount <= 0) {
            System.out.println("存款金额必须大于 0！");
            return;
        }
        balance += amount;
        System.out.println("存入 " + amount + " 元，余额: " + balance);
    }

    // setter：取款（需要密码验证）
    public boolean withdraw(double amount, String pwd) {
        if (!pwd.equals(password)) {
            System.out.println("密码错误！");
            return false;
        }
        if (amount > balance) {
            System.out.println("余额不足！");
            return false;
        }
        balance -= amount;
        System.out.println("取出 " + amount + " 元，余额: " + balance);
        return true;
    }
}

public class BankTest {
    public static void main(String[] args) {
        BankAccount account = new BankAccount("张三", "123456");

        System.out.println("户主: " + account.getOwner());
        System.out.println("初始余额: " + account.getBalance());

        account.deposit(1000);
        account.deposit(-50);  // 被拦截

        account.withdraw(300, "wrong");   // 密码错误
        account.withdraw(300, "123456");  // 成功
        account.withdraw(800, "123456");  // 余额不足
    }
}