6
在以下代码示例中，如果将键设置为null并调用System.gc()，则WeakHashMap会丢失所有映射并被清空。WeakHashMap vs HashMap
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 :{}
当WeakHashMap与HashMap一起使用和按键被设置为null，则WeakHashMap不会失去它的键 - 值映射关系。
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}
我的问题是为什么不WeakHashMap失去其条目在第二个代码示例的按键被丢弃后也？
我甚至从来没有想过变量只是包含一个指针，这是非常丰富的知道，谢谢！ – xorinzor