WeakHashMap、IdentityHashMap 、EnumMap简单了解——高淇JAVA300讲笔记之其他Map实现类

  

案例一：弱引用管理对象

package com.bjsxt.others.three;

import java.lang.ref.WeakReference;

/**
 * 引用分类：强、软、弱、虚
 * 强与弱引用
 *
 */
public class RefDemo {
    public static void main(String[] args) {
        //字符串常量池 存在堆中
        String str = new String("bjsxt is very good");
        //弱引用 管理 对象
        WeakReference<String> wr = new WeakReference<String>(str);
        System.out.println("gc运行前："+wr.get());
        //断开引用
        str = null;
        //通知回收
        System.gc();
        System.runFinalization();
        //对象被回收
        System.out.println("gc运行后："+wr.get());
    }
    
    public static void testStrong() {
        //字符串常量池 共享（不能回收）
        String str = "bjsxt is very good";
        //弱引用 管理 对象
        WeakReference<String> wr = new WeakReference<String>(str);
        System.out.println("gc运行前："+wr.get());
        //断开引用
        str = null;
        //通知回收
        System.gc();
        System.runFinalization();
        System.out.println("gc运行后："+wr.get());
    }
    
}

运行结果：如果执行testStrong()，不会被回收

gc运行前：bjsxt is very good
gc运行后：bjsxt is very good

执行main方法中的代码，会被回收

gc运行前：bjsxt is very good
gc运行后：null

案例二：WeakHashMap示例

package com.bjsxt.others.three;

import java.util.WeakHashMap;

/**
 * WeakHashMap 键为弱类型，gc运行立即回收
 *
 */
public class WeakHashMapDemo {
    public static void main(String[] args) {
        WeakHashMap<String,String> map = new WeakHashMap<String,String>();
        //测试数据
        //常量池对象，不会回收
        map.put("abc","a");
        map.put("d","test");
        //gc运行 已被回收
        map.put(new String("bjsxt"), "c");
        map.put(new String("dsf"), "d");
        
        //通知回收
        System.gc();
        System.runFinalization();
        
        System.out.println(map.size());
    }
    
    
}

运行结果：2

案例三：IdentityHashMap 键比较地址去重

package com.bjsxt.others.three;

import java.util.IdentityHashMap;

/**
 * IdentityHashMap 键比较地址去重
 *
 */
public class IdentityHashMapDemo {
    public static void main(String[] args) {
        IdentityHashMap<String,String> map = new IdentityHashMap<String,String>();
        //常量池中的"a"
        map.put("a","a1");
        map.put("a","a2");
        System.out.println(map.size());
        map.put(new String("a"), "a3");
        map.put(new String("a"), "a4");
        System.out.println(map.size());
    }
}

运行结果：

1
3

案例四：EnumMap要求键为枚举

package com.bjsxt.others.three;

import java.util.EnumMap;

/**
 * EnumMap要求键为枚举
 *
 */
public class EnumMapDemo {
    public static void main(String[] args) {
        EnumMap<Season,String> map = new EnumMap<Season,String>(Season.class);
        //存放值
        map.put(Season.SPRING, "春困");
        map.put(Season.SUMMER, "夏无力");
        map.put(Season.AUTUMN, "秋乏");
        map.put(Season.WINTER, "冬眠");
        
        System.out.println(map.size());
    }
}

//季节
enum Season{
    SPRING,SUMMER,AUTUMN,WINTER
}

运行结果：4