Question

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Use cases for IdentityHashMap

What could be a practical use of the IdentityHashMap introduced in Java 5?

Answer 1

For adding dynamic fields to objects.

Some language directly support dynamic fields: anybody can add any field to any object any time.

This is handy when you want to associate some information to objects, unforseenable by object designer.

Java doesn’t have real dynamic field. We can simulate it by using an identity map to associate an object to some information of some kind.

WeakHashMap is better for the purpose; it is an identity map too, and it doesn’t add additional strong reference to the object. So it is much closer to the dynamic field concept.

Concurrency is the remaining problem. If two threads accessing the same dynamic field of two different objects, there shouldn’t be dependencies among two threads. We can solve it by some kind of concurrent weak hashmap. However the performance isn’t ideal compared to normal field access.

Think about java.lang.ThreadLocal, adding dynamic field to threads; and java.lang.ClassValue, adding dynamic field to classes. They aren’t strictly necessary – we can achieve the same thing with concurrent weak maps. They exist for performance reason. JDK can “hack” into Thread/Class to add supports to achieve faster lookup.