泛型类与子类型：求PayloadList泛型接口可运行实现示例

我们可以基于JDK自带的ArrayList扩展实现，ArrayList已经实现了List接口的所有基础方法，只需额外补充payload的存储和读写逻辑即可，完整可运行代码如下：

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;

// Oracle官方示例定义的PayloadList接口
interface PayloadList<E,P> extends java.util.List<E> {
    void setPayload(int index, P val);
    // 补充getPayload方法方便读取负载数据
    P getPayload(int index);
}

// PayloadList实现类，复用ArrayList的List基础能力
class PayloadArrayList<E,P> extends ArrayList<E> implements PayloadList<E,P> {
    // 用Map存储索引对应的负载数据
    private final Map<Integer, P> payloadMap = new HashMap<>();

    @Override
    public void setPayload(int index, P val) {
        // 校验索引合法性
        if (index < 0 || index >= size()) {
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size());
        }
        payloadMap.put(index, val);
    }

    @Override
    public P getPayload(int index) {
        if (index < 0 || index >= size()) {
            throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size());
        }
        return payloadMap.get(index);
    }
}

// 测试类，验证不同参数化类型的使用
public class PayloadListDemo {
    public static void main(String[] args) {
        // 测试1：PayloadList<String,String> 类型
        System.out.println("=== 测试String-String类型 ===");
        PayloadList<String, String> strStrList = new PayloadArrayList<>();
        strStrList.add("第一条数据");
        strStrList.add("第二条数据");
        strStrList.setPayload(0, "第一条的字符串负载");
        strStrList.setPayload(1, "第二条的字符串负载");
        for (int i = 0; i < strStrList.size(); i++) {
            System.out.println("元素值：" + strStrList.get(i) + "，负载值：" + strStrList.getPayload(i));
        }

        // 测试2：PayloadList<String,Integer> 类型
        System.out.println("\n=== 测试String-Integer类型 ===");
        PayloadList<String, Integer> strIntList = new PayloadArrayList<>();
        strIntList.add("苹果");
        strIntList.add("香蕉");
        strIntList.setPayload(0, 15);
        strIntList.setPayload(1, 8);
        for (int i = 0; i < strIntList.size(); i++) {
            System.out.println("元素值：" + strIntList.get(i) + "，负载值：" + strIntList.getPayload(i) + "元/斤");
        }

        // 测试3：PayloadList<String,Exception> 类型
        System.out.println("\n=== 测试String-Exception类型 ===");
        PayloadList<String, Exception> strExList = new PayloadArrayList<>();
        strExList.add("执行任务A");
        strExList.add("执行任务B");
        strExList.setPayload(1, new RuntimeException("任务B执行失败，网络超时"));
        for (int i = 0; i < strExList.size(); i++) {
            System.out.print("任务内容：" + strExList.get(i));
            Exception ex = strExList.getPayload(i);
            if (ex != null) {
                System.out.println("，异常信息：" + ex.getMessage());
            } else {
                System.out.println("，执行成功无异常");
            }
        }
    }
}

内容的提问来源于stack exchange，提问作者Yauheni