I am reading the Hashtable's code, and I learned that both Hashtable's key and value can not be null, but its equals method test the situation that value is null or not.
public synchronized boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Map))
return false;
Map<K,V> t = (Map<K,V>) o;
if (t.size() != size())
return false;
try {
Iterator<Map.Entry<K,V>> i = entrySet().iterator();
while (i.hasNext()) {
Map.Entry<K,V> e = i.next();
K key = e.getKey();
V value = e.getValue();
if (value == null) { // Can Hashtable's value be null?
if (!(t.get(key)==null && t.containsKey(key)))
return false;
} else {
if (!value.equals(t.get(key)))
return false;
}
}
} catch (ClassCastException unused) {
return false;
} catch (NullPointerException unused) {
return false;
}
return true;
}
It is kind of a pattern that is followed throughout to handle the NPE. Consider a simple class
public class HelloWorld {
String data;
}
If you generate hashCode() and equals() you will see this general pattern followed. As in this case
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
HelloWorld that = (HelloWorld) o;
if (data != null ? !data.equals(that.data) : that.data != null) return false;
return true;
}
@Override
public int hashCode() {
return data != null ? data.hashCode() : 0;
}
As you can see we always check for null. It is not mandatory but a good programming practice. I understand it makes no sense in the case of Hashtable's but as I mentioned earlier developers must have added this check to maintain a uniform pattern.
Update : As Tim has suggested Since Hashtable is subclassable, it is possible for a subclass to try to support null keys or values
. So it is safe to do a null check.
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