CODEWITHSUNDEEP
javajava-8nullpointerexception
Ulrich Scholz
Ulrich Scholz

Reputation: 2111

NPE in Java HashSet.isEmpty()

I got an NPE in java.util.HashSet.isEmpty() on line 191. I guess, this line just calls isEmpty() on the internal map field.

To my surprise, this field is transient, so after deserialization, it would be null. But isn't that the purpose of deserializing a set, to get the values back? To my eyes, being deserialized is the only way this field can be null for a HashSet instance.

Probably, I'm missing something here. Can someone explain?

Java 1.8.0_151 on Linux (3.13.0-61-generic) on platform amd64

Here is the implementation of HashMap.isEmpty from the JDK:

/**
 * Returns <tt>true</tt> if this set contains no elements.
 *
 * @return <tt>true</tt> if this set contains no elements
 */
public boolean isEmpty() {
    return map.isEmpty(); // line 191
}

Edit / additional info:

Upvotes: 1

Views: 430

Answers (1)

Holger
Holger

Reputation: 298459

Since the map field is not final, there is no guarantee to see it in its initialized state when publishing a newly constructed HashSet instance improperly in multi-threaded code. Whereas “newly constructed” means that no other action ensuring memory visibility between the involved threads has happened since the construction, which in principle may be an arbitrary long time.

During object deserialization, the readObject method takes care of re-initializing the map field and putting back the elements. This is necessary to hide these implementation details from the persistent form. Further, deserialized objects may have a different hash code than they had upon serializing (take the hash code inherited from java.lang.Object, for example). So they have to be re-inserted anyway. It’s a general principle of most collections to hide the implementation details and just serialize and deserialize each contained element one after another in dedicated writeObject and readObject methods.

So a future HashSet implementation could be a real hash set, not using a HashMap behind the scenes, without affecting serialization compatibility.

Upvotes: 3

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