面向对象基础

public class Main {
    static class Dog {
        String name;
        int age;
 
        void bark() {
            System.out.println(name + " 汪汪叫");
        }
    }
 
    public static void main(String[] args) {
        Dog d1 = new Dog();
        d1.name = "旺财";
        d1.age = 3;
        d1.bark();
        System.out.println(d1.name + " 今年 " + d1.age + " 岁");
    }
}

public class Main {
    static class User {
        String name;
        int age;
 
        User() {
            this("匿名", 0);
        }
        User(String name, int age) {
            this.name = name;
            this.age = age;
        }
        void info() {
            System.out.printf("%s (%d 岁)%n", name, age);
        }
    }
 
    public static void main(String[] args) {
        new User().info();
        new User("Alice", 30).info();
        new User("Bob", 25).info();
    }
}

public class Main {
    static class BankAccount {
        private double balance;
 
        public BankAccount(double initial) {
            if (initial < 0) throw new IllegalArgumentException("初始金额不能为负");
            this.balance = initial;
        }
 
        public void deposit(double amount) {
            if (amount <= 0) throw new IllegalArgumentException("存款必须为正");
            balance += amount;
        }
 
        public boolean withdraw(double amount) {
            if (amount <= 0 || amount > balance) return false;
            balance -= amount;
            return true;
        }
 
        public double getBalance() {
            return balance;
        }
    }
 
    public static void main(String[] args) {
        BankAccount acc = new BankAccount(1000);
        acc.deposit(500);
        System.out.println("存款后余额：" + acc.getBalance());
 
        boolean ok = acc.withdraw(200);
        System.out.println("取款 200 成功？" + ok);
        System.out.println("最终余额：" + acc.getBalance());
 
        System.out.println(acc.withdraw(10000) ? "成功" : "失败（余额不足）");
    }
}

public class Main {
    static class Counter {
        static int total = 0;       // 类变量
        int id;                     // 实例变量
 
        Counter() {
            total++;
            id = total;
        }
    }
 
    public static void main(String[] args) {
        new Counter();
        new Counter();
        new Counter();
        System.out.println("创建了 " + Counter.total + " 个对象");
    }
}

public class Main {
    static class Student {
        private String name;
        private int[] scores;
        private static int count = 0;
 
        Student(String name, int[] scores) {
            this.name = name;
            this.scores = scores;
            count++;
        }
 
        double average() {
            if (scores.length == 0) return 0;
            int s = 0;
            for (int x : scores) s += x;
            return (double) s / scores.length;
        }
 
        void report() {
            System.out.printf("%s 平均分 %.2f%n", name, average());
        }
    }
 
    public static void main(String[] args) {
        new Student("Alice", new int[]{85, 92, 78}).report();
        new Student("Bob",   new int[]{90, 88, 95}).report();
        new Student("Carol", new int[]{76, 82, 80}).report();
    }
}

// 任务：实现 Book 类，包含 title、author、isBorrowed 状态
// 提供 borrow()/returnBook()/status() 方法
// 期望输出：
//   《Java 编程》- 凯·霍斯特曼 状态：可借
//   《Java 编程》- 凯·霍斯特曼 状态：已借出
//   《Java 编程》- 凯·霍斯特曼 状态：可借
 
public class Main {
    static class Book {
        // 补全字段和方法
    }
 
    public static void main(String[] args) {
        Book b = new Book("Java 编程", "凯·霍斯特曼");
        b.status();
        b.borrow();
        b.status();
        b.returnBook();
        b.status();
    }
}