2. 用java定义一个快递包裹类 Package,并由此派生出普通包裹类和加急包裹类。 要求: (1)Package 类应包含寄件人和收件人姓名、地址和包裹重量属性,按重量*10 计费。 (2)普通包裹增加首重计费和超首重后部分的

计费规则，加急包裹在普通包裹的基础上增加加急费用。具体的Java代码实现如下：

// Package 类
public class Package {
    private String senderName;
    private String senderAddress;
    private String recipientName;
    private String recipientAddress;
    private int weight; // 单位为 g

    public Package(String senderName, String senderAddress, String recipientName, String recipientAddress, int weight) {
        this.senderName = senderName;
        this.senderAddress = senderAddress;
        this.recipientName = recipientName;
        this.recipientAddress = recipientAddress;
        this.weight = weight;
    }

    public int getWeight() {
        return weight;
    }

    public double getCost() {
        return weight * 0.1;
    }
}

// 普通包裹类
public class OrdinaryPackage extends Package {
    private static final int FIRST_WEIGHT = 1000; // 首重 g
    private static final double FIRST_COST = 10.0; // 首重计费
    private static final double EXTRA_COST = 5.0; // 超首重后部分的计费

    public OrdinaryPackage(String senderName, String senderAddress, String recipientName, String recipientAddress, int weight) {
        super(senderName, senderAddress, recipientName, recipientAddress, weight);
    }

    @Override
    public double getCost() {
        int weight = getWeight();
        if (weight <= FIRST_WEIGHT) {
            return FIRST_COST;
        } else {
            return FIRST_COST + (weight - FIRST_WEIGHT) * EXTRA_COST;
        }
    }
}

// 加急包裹类
public class ExpressPackage extends OrdinaryPackage {
    private static final double EXPRESS_COST = 20.0; // 加急费用

    public ExpressPackage(String senderName, String senderAddress, String recipientName, String recipientAddress, int weight) {
        super(senderName, senderAddress, recipientName, recipientAddress, weight);
    }

    @Override
    public double getCost() {
        return super.getCost() + EXPRESS_COST;
    }
}

使用示例：

Package p1 = new Package("Alice", "123 Main St", "Bob", "456 Elm St", 500);
System.out.println(p1.getCost()); // 输出 50.0

OrdinaryPackage p2 = new OrdinaryPackage("Alice", "123 Main St", "Bob", "456 Elm St", 1500);
System.out.println(p2.getCost()); // 输出 15.0

ExpressPackage p3 = new ExpressPackage("Alice", "123 Main St", "Bob", "456 Elm St", 1500);
System.out.println(p3.getCost()); // 输出 35.0