未更新Java WeakHashMap引用

在下面的代码中,我创建了一个Pen对象,并将其颜色初始化为白色。在Pen的构造函数中,将字段“penColor”设置为传递给构造函数的值后,我更新了一个全局静态弱hashmap,其中键是“this pointer”——在我的例子中是Pen,值是另一个weakhashmap,其键是字符串“penColor”,其值是对penColor成员字段的引用。

package weakhashmaptest;


import java.awt.Color;
import java.util.WeakHashMap;
import java.util.Iterator;

public class Main {

    static WeakHashMap <Object, WeakHashMap>ownerMap = new WeakHashMap<Object, WeakHashMap>();

    public static void main(String[] args) {

        Pen r = new Pen(Color.WHITE);

        Iterator i = ownerMap.keySet().iterator();

        while(i.hasNext()) {
            Object key = i.next();
            System.out.println("\telement of hashmap is : " +ownerMap.get(key));
        }

        r.setColor(Color.BLACK);

        System.gc();

        i = ownerMap.keySet().iterator();

        while(i.hasNext()) {
            Object key = i.next();
            System.out.println("\telement of hashmap is : " +ownerMap.get(key));
        }

    }

    public static void mapUpdate(Object owner, Object reference, String field_name) {

        WeakHashMap reference_map = ownerMap.get(owner);

        if (reference_map == null) {         
            reference_map = new WeakHashMap();
            reference_map.put(field_name, reference);
        } else {         
            reference_map.put(field_name, reference);
        }

        ownerMap.put(owner, reference_map);

    }

}

class Pen {

    Color penColor;

    public Pen(Color c) {

        penColor = c;
        Main.mapUpdate(this, penColor, "penColor");

    }

    public void setColor(Color c) {

        penColor = c;

    }

}