WeakHashMap vs HashMap

Question

在以下代码示例中,将键设置为null和 System.gc()调用时,将WeakHashMap丢失所有映射并清空.

class WeakHashMapExample {

public static void main(String[] args) {

    Key k1 = new Key("Hello");
    Key k2 = new Key("World");
    Key k3 = new Key("Java");
    Key k4 = new Key("Programming");

    Map<Key, String> wm = new WeakHashMap<Key, String>();


    wm.put(k1, "Hello");
    wm.put(k2, "World");
    wm.put(k3, "Java");
    wm.put(k4, "Programming");
    k1=null;
    k2=null;
    k3=null;
    k4=null;
    System.gc();
    System.out.println("Weak Hash Map :"+wm.toString());

}

}

class Key{

private String key;

public Key(String key) {
    this.key=key;
}

@Override
public boolean equals(Object obj) {
    return this.key.equals((String)obj);
}
@Override
public int hashCode() {
    return key.hashCode();
}
@Override
public String toString() {
    return key;
}

}

Output: Weak Hash Map :{}

何时WeakHashMap使用HashMap和键被设置为null,WeakHashMap不会丢失其键值映射.

class WeakHashMapExample {

public static void main(String[] args) {

    Key k1 = new Key("Hello");
    Key k2 = new Key("World");
    Key k3 = new Key("Java");
    Key k4 = new Key("Programming");

    Map<Key, String> wm = new WeakHashMap<Key, String>();
    Map<Key, String> hm=new HashMap<Key, String>();

    wm.put(k1, "Hello");
    wm.put(k2, "World");
    wm.put(k3, "Java");
    wm.put(k4, "Programming");

    hm.put(k1, "Hello");
    hm.put(k2, "World");
    hm.put(k3, "Java");
    hm.put(k4, "Programming");
    k1=null;
    k2=null;
    k3=null;
    k4=null;
    System.gc();
    System.out.println("Weak Hash Map :"+wm.toString());
    System.out.println("Hash Map :"+hm.toString());
}

}

class Key{

private String key;

public Key(String key) {
    this.key=key;
}

@Override
public boolean equals(Object obj) {
    return this.key.equals((String)obj);
}
@Override
public int hashCode() {
    return key.hashCode();
}
@Override
public String toString() {
    return key;
}

}

输出: Weak Hash Map :{Java=Java, Hello=Hello, World=World, Programming=Programming} Hash Map :{Programming=Programming, World=World, Java=Java, Hello=Hello}

我的问题是,为什么WeakHashMap即使丢弃密钥后,第二个代码示例中的条目也不会丢失？

Answer 1

一个WeakHashMap丢弃条目时的关键不再是强可从活的代码.由于HashMap维护了对密钥的硬引用,因此密钥仍然可以访问,WeakHashMap并且不会丢弃条目.

关键是行为与对关键对象的引用有关,而不是与曾经可能具有对键的引用的任何变量的值有关.