一、引用分類

強引用：StrongReference：引用指向物件，gc(Garbage collection）執行時不回收

軟引用：SoftReference gc執行時可能回收（jvm記憶體不夠）

弱引用：WeakReference gc執行時立即回收

虛引用：PhantomReference 類似於無引用，主要跟蹤物件被回收的狀態，不能單獨使用，必須與引用佇列（ReferenceQueue）聯合使用

import java.lang.ref.WeakReference;

/**
 * 引用分類：強、軟、弱、虛
 * 強與弱的引用
 */
public class RefTest {
    public static void main(String[] args){
        String str="zhangrui";//常量池物件不會被回收
        WeakReference weakReference=new WeakReference(str);
        System.out.println("回收前："+weakReference.get());
        //斷開引用
        str=null;
        System.gc();
        System.runFinalization();
        System.out.println("回收後："+weakReference.get());
        System.out.println("***************************");
        String str1=new String("zhangrui");
        WeakReference weakReference2=new WeakReference(str1);
        System.out.println("回收前："+weakReference2.get());
        //斷開引用
        str1=null;
        System.gc();
        System.runFinalization();
        System.out.println("回收後："+weakReference2.get());

    }
}
 

二、三個Map介面實現

WeakHashMap

鍵為弱引用

/**
 * WeakHashMap的key為弱引用
 */

import java.util.WeakHashMap;

public class WeakHashMap1 {
    public static void main(String[] args){
        WeakHashMap<String,String> map=new WeakHashMap<>();
        //常量池物件，不會回收
        map.put("a","zhang");
        map.put("b","z");
        //gc執行，已被回收
        map.put(new String("ss"),"s");
        map.put(new String("dd"),"d");
        System.out.println(map.size());
        //準備回收
        System.gc();
        System.runFinalization();
        System.out.println(map.size());
    }


}
 

IdentityHashMap

鍵比較地址去重，注意常量池的物件

import java.util.IdentityHashMap;

/**
 * IdentityHashMap 鍵比較地址去重
 * 而HashMap比較key的hashcode和equals
 */
public class IdentityHashMap1 {
    public static void main(String[] args){
        IdentityHashMap<String,String> map=new IdentityHashMap<>();
        map.put("a","a1");
        map.put("a","a2");
        System.out.println(map.size());//IdentityHashMap只比較key的地址
        map.put(new String("a2"),"aa");
        map.put(new String("a2"),"bb");
        System.out.println(map.size());
    }
}
 

EnumMap

列舉Map，要求鍵為列舉的值

import java.util.EnumMap;

/**
 * EnumMap要求鍵為列舉
 */
public class EnumMap1 {
    public static void main(String[] args){
        EnumMap<Season,String> map=new EnumMap<Season, String>(Season.class);
        map.put(Season.SPRING,"春困");
        map.put(Season.SUMMER,"夏天");
        map.put(Season.AUTUMN,"秋乏");
        map.put(Season.WINTER,"冬天");
        System.out.println(map.size());
    }
}
enum Season{
    SPRING,SUMMER,AUTUMN,WINTER
}