java/w8/Pair.java

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;

public class Pair<K, V> {
    private K first;
    private V second;

    public Pair(K first, V second) {
        this.first = first;
        this.second = second;
    }

    public K getFirst() {
        return first;
    }

    public void setFirst(K first) {
        this.first = first;
    }

    public V getSecond() {
        return second;
    }

    public void setSecond(V second) {
        this.second = second;
    }

    /**
     * 安全地交换 Pair 中的两个元素。
     * 注意：这将创建一个新的 Pair 对象并返回，而不是修改原对象。
     */
    public Pair<V, K> swapped() {
        return new Pair<>(this.second, this.first);
    }

    @Override
    public String toString() {
        return "Pair{first=" + first + ", second=" + second + '}';
    }

    public static class Cache<K, V> {
        private final Map<K, V> cache = new HashMap<>();

        public void put(K key, V value) {
            cache.put(key, value);
        }

        public V get(K key) {
            return cache.get(key);
        }

        public boolean containsKey(K key) {
            return cache.containsKey(key);
        }

        public void remove(K key) {
            cache.remove(key);
        }

        public int size() {
            return cache.size();
        }

        // 为了能够通过反射获取泛型信息，需要让子类继承它
        public Class<?> getKeyType() {
            return getGenericType(0);
        }

        public Class<?> getValueType() {
            return getGenericType(1);
        }

        private Class<?> getGenericType(int index) {
            Type genericSuperclass = this.getClass().getGenericSuperclass();
            if (genericSuperclass instanceof ParameterizedType) {
                ParameterizedType parameterizedType = (ParameterizedType) genericSuperclass;
                Type[] actualTypeArguments = parameterizedType.getActualTypeArguments();
                if (index >= 0 && index < actualTypeArguments.length) {
                    Type type = actualTypeArguments[index];
                    if (type instanceof Class) {
                        return (Class<?>) type;
                    }
                }
            }
            return Object.class;
        }
    }

    public static void main(String[] args) {
        // 演示 Pair 的用法
        Pair<String, Integer> pair = new Pair<>("Hello", 100);
        System.out.println("Original Pair: " + pair);
        Pair<Integer, String> swappedPair = pair.swapped();
        System.out.println("Swapped Pair: " + swappedPair);

        // 演示 Cache 的用法和泛型类型获取
        // 必须创建一个匿名子类才能保留泛型信息供反射使用
        Cache<String, Integer> stringIntCache = new Cache<String, Integer>() {};
        stringIntCache.put("answer", 42);
        stringIntCache.put("year", 2024);

        System.out.println("\n--- Cache Demo ---");
        System.out.println("Key type: " + stringIntCache.getKeyType().getSimpleName());
        System.out.println("Value type: " + stringIntCache.getValueType().getSimpleName());
        System.out.println("Size: " + stringIntCache.size());
        System.out.println("Value for 'answer': " + stringIntCache.get("answer"));
    }
}