public class WeakHashMapTest {
    public static void main(String[] args){
        testWeakHashMap();
    }

    private static void testWeakHashMap() {
        // 建立3個String物件用來做key
        String w1 = new String("key1");
        String w2 = new String("key2");
        String w3 = new String("key3");

        
 
// 新建WeakHashMap
        Map weakHashMap = new WeakHashMap();

        // 新增鍵值對
        weakHashMap.put(w1, "v1");
        weakHashMap.put(w2, "v2");
        weakHashMap.put(w3, "v3");

        // 打印出weakHashMap
        System.out.printf("weakHashMap:%s\n", weakHashMap);

        // containsKey(Object key) :是否包含鍵key
 

        System.out.printf("contains key key1 : %s\n",weakHashMap.containsKey("key1"));
        System.out.printf("contains key key4 : %s\n",weakHashMap.containsKey("key4"));

        // containsValue(Object value) :是否包含值value
        System.out.printf("contains value v1 : %s\n",weakHashMap.containsValue("v1"));
        System.out.printf(
 
"contains value 0 : %s\n",weakHashMap.containsValue(0));

        // remove(Object key) ： 刪除鍵key對應的鍵值對
        weakHashMap.remove("three");

        System.out.printf("weakHashMap: %s\n", weakHashMap);


        // ---- 測試 WeakHashMap 的自動回收特性 ----

        // 將w1設定null。
        // 這意味著“弱鍵”w1再沒有被其它物件引用，呼叫gc時會回收WeakHashMap中與“w1”對應的鍵值對
        w1 = null;

        // 記憶體回收。這裡，會回收WeakHashMap中與“w1”對應的鍵值對
        System.gc();

        // 遍歷WeakHashMap
        Iterator iter = weakHashMap.entrySet().iterator();
        while (iter.hasNext()) {
            Map.Entry en = (Map.Entry)iter.next();
            System.out.printf("next : %s - %s\n",en.getKey(),en.getValue());
        }
        // 列印WeakHashMap的實際大小
        System.out.printf("after gc WeakHashMap size:%s\n", weakHashMap.size());
    }
}