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

/**
 * ============================================
 * 任务一：泛型类 Pair<K, V>
 * - 包含构造方法、getter/setter
 * - swap() 方法用于交换 K 和 V 的值
 * ============================================
 */
class Pair<K, V> {
    private K key;
    private V value;

    public Pair() {
    }

    public Pair(K key, V value) {
        this.key = key;
        this.value = value;
    }

    public K getKey() {
        return key;
    }

    public void setKey(K key) {
        this.key = key;
    }

    public V getValue() {
        return value;
    }

    public void setValue(V value) {
        this.value = value;
    }

    /**
     * 交换 key 和 value 的值
     */
    public void swap() {
        K temp = this.key;
        this.key = (K) this.value;
        this.value = (V) temp;
    }

    @Override
    public String toString() {
        return "Pair{key=" + key + ", value=" + value + "}";
    }

    public static void main(String[] args) {
        System.out.println("========== Pair 测试 ==========");

        Pair<String, Integer> pair = new Pair<>("Hello", 100);
        System.out.println("交换前: " + pair);
        pair.swap();
        System.out.println("交换后: " + pair);

        Pair<Double, String> pair2 = new Pair<>(3.14, "PI");
        System.out.println("交换前: " + pair2);
        pair2.swap();
        System.out.println("交换后: " + pair2);
    }
}

/**
 * ============================================
 * 任务二：泛型缓存类 Cache<K, V>
 * - put(K key, V value): 添加缓存
 * - get(K key): 获取缓存
 * - remove(K key): 删除缓存
 * - clear(): 清空缓存
 * ============================================
 */
class Cache<K, V> {
    private final Map<K, V> cache;

    public Cache() {
        this.cache = new HashMap<>();
    }

    public void put(K key, V value) {
        if (key == null) {
            throw new IllegalArgumentException("key不能为null");
        }
        cache.put(key, value);
    }

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

    public V remove(K key) {
        if (key == null) {
            return null;
        }
        return cache.remove(key);
    }

    public void clear() {
        cache.clear();
    }

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

    public boolean containsKey(K key) {
        if (key == null) {
            return false;
        }
        return cache.containsKey(key);
    }

    @Override
    public String toString() {
        return "Cache{" + cache + "}";
    }

    public static void main(String[] args) {
        System.out.println("========== Cache 测试 ==========");
        Cache<String, Integer> cache = new Cache<>();

        cache.put("Java", 90);
        cache.put("Python", 85);
        cache.put("C++", 92);
        System.out.println("添加后缓存: " + cache);
        System.out.println("缓存大小: " + cache.size());

        System.out.println("获取Java成绩: " + cache.get("Java"));
        System.out.println("获取不存在的Key: " + cache.get("Go"));

        cache.put("Java", 95);
        System.out.println("更新Java成绩后: " + cache);

        System.out.println("删除Python: " + cache.remove("Python"));
        System.out.println("删除后缓存: " + cache);

        cache.clear();
        System.out.println("清空后缓存: " + cache);

        try {
            cache.put(null, 100);
        } catch (IllegalArgumentException e) {
            System.out.println("null key测试: " + e.getMessage());
        }
    }
}

/**
 * ============================================
 * 主类：运行测试
 * ============================================
 */
public class GenericHomework {
    public static void main(String[] args) {
        System.out.println("【任务一】Pair<K, V> 测试");
        System.out.println("========================");
        Pair.main(null);
        System.out.println();

        System.out.println("【任务二】Cache<K, V> 测试");
        System.out.println("========================");
        Cache.main(null);
    }
}