How to Compute the hashCode() from an Object's Address in Java

Discover the best practices for computing `hashCode()` in Java when overriding `equals()` for stricter equality, and learn how to utilize object addresses effectively.
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Understanding hashCode() and equals() in Java

When working with Java, particularly with data structures like HashMap, it's essential to define how objects are compared and hashed. In this guide, we will explore a specific scenario involving a subclass, Vertex, derived from Java3D's Point3f. The challenge lies in computing the hashCode() based on the object's address while ensuring that equals() is implemented with a stricter interpretation.

The Problem

In your Vertex class, you want two vertices to be considered equal only if they reference the same object, which overrides the default behavior of Point3f. You’ve already implemented the equals() method like this:

[[See Video to Reveal this Text or Code Snippet]]

This approach breaks the standard contract of equals(), but since you're using your class only to compare vertices against other vertices, you feel comfortable with this implementation.

However, to utilize Vertex objects in a HashMap, you'll need to ensure that the hashCode() method is consistent with your equals() method. Currently, if hashCode() is computed based on the fields of Point3f, it could lead to hash collisions for Vertex objects with the same coordinates.

Key Questions

Should I use such a shallow equals()?

How can I compute the object's address to get a unique hashCode()?

The Solution

To address your concerns, here are strategic approaches that maintain the integrity of your Vertex class's functionality while conforming to Java's requirements.

Benefits:

Guarantees a hash code based on the actual object reference, ensuring unique hash codes for distinct objects, even if their fields are identical.

Example implementation in your Vertex class:

[[See Video to Reveal this Text or Code Snippet]]

2. Employing IdentityHashMap

Another elegant solution is to use IdentityHashMap, which distinguishes object references rather than relying on the standard hashCode() and equals() methods for comparisons.

Usage:

Replace your regular HashMap with an IdentityHashMap when storing Vertex objects.

What About a Random int Value?

Generating a random int value upon object creation and using it as a hash code may not be advisable for several reasons:

Potential Collisions: The likelihood of generating the same random value for different objects is significant.

Consistency: The hashCode() must remain consistent during an object's lifetime, which will not be the case with a random value.

Violation of hashCode() Contract: This will lead to unpredictable behavior in hash-based collections.

Conclusion

By understanding these approaches, you'll maintain the integrity of your object comparisons while leveraging Java's powerful collection framework efficiently.