package collection;
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.HashMap;
import java.util.WeakHashMap;
class MyArray{
    byte[] bytes = new byte[3*1024*1024];  //即占用3M
}
class Test{
}
public class TestReference {
    public static void main(String[] args) {
//        //引用隊列
//    	//將軟引用對象關聯(lián)成一個key值，添加到隊列里，遍歷 隊列中每個元素分別找到其在 HashMap里的位置，然后把結(jié)點（key,value）完全刪除掉
//        ReferenceQueue<MyArray> queue  = new ReferenceQueue<>();
//        //創(chuàng)建一個軟引用對象
//        SoftReference<MyArray> softReference = new SoftReference<>(new MyArray(), queue);//當對象被回收時，會直接入到queue里
//        //獲取軟引用對象（softReference）
//        System.out.println(softReference.get());
//        //判斷是否被回收(true  被回收  入到隊列)
//        System.out.println(softReference.isEnqueued());
//   	   //   -Xms3M  -Xmx5M   即初始給堆3M空間，最大是5M空間     右鍵-》 run configurations——》設置虛擬機參數(shù)
//        //System.gc();//可模擬垃圾回收
//        byte[] array = new byte[3*1024*1024];
//
//        System.out.println(softReference.get());
//        System.out.println(softReference.isEnqueued());
//        //  弱引用對象
//        ReferenceQueue<Test> queue  = new ReferenceQueue<>();
//        WeakReference<Test> weakReference = new WeakReference<>(new Test(), queue);
//
//        System.out.println(weakReference.get());
//        System.out.println(weakReference.isEnqueued());
//
//        System.gc();
//
//        System.out.println(weakReference.get());
//        System.out.println(weakReference.isEnqueued());
//        
//        //虛引用對象
//        ReferenceQueue<Test> queue  = new ReferenceQueue<>();
//        PhantomReference<Test> phantomReference = new PhantomReference<>(new Test(), queue);
//        //進程的執(zhí)行，需要線程的支持
//  	  //垃圾回收作為一個線程（等CPU分配時間片）支持虛擬機的執(zhí)行
//    	  //JDK（或虛擬機）里垃圾回收線程是隨機啟動的，調(diào)用  System.gc();可以手動回收
//        System.gc();
//        if(phantomReference.isEnqueued()){
//            System.out.println("該對象已經(jīng)被回收");
//        }else{
//            System.out.println("該對象已經(jīng)被回收");
//        }
    }
}

    /**
     * Retrieve object hash code and applies a supplemental hash function to the
     * result hash, which defends against poor quality hash functions.  This is
     * critical because HashMap uses power-of-two length hash tables, that
     * otherwise encounter collisions for hashCodes that do not differ
     * in lower bits.
     */
    final int hash(Object k) {
        int h = k.hashCode();
        // This function ensures that hashCodes that differ only by
        // constant multiples at each bit position have a bounded
        // number of collisions (approximately 8 at default load factor).
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

//HashMap337行
   static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }