How Does the `hashCode()` Method Work for Non-Overridden Objects in Java?

Understanding Hash Codes for Non-Overridden Objects in Java

In Java, objects inherit the hashCode() method from the Object class. This method, if not overridden, provides a default implementation for generating a unique identifier for an object.

What is the Default Hash Code?

When the hashCode() method is not overridden, the HotSpot JVM follows a specific behavior to generate hash codes:

1. Random Number on First Invocation: On the initial call to hashCode() or System.identityHashCode, a random number is generated and stored in the object's header.
2. Subsequent Invocation: Subsequent calls retrieve this stored value.

JVM Hash Code Options

The behavior of the default hash code generation can be customized using the -XX:hashCode=n HotSpot JVM option. Possible values include:

• 0: Use global random generator (default in Java 7)
• 5: Use thread-local xor-shift random generator (default in Java 8)
• 1: Use object pointer mixed with random value (stable between GC cycles)
• 2: Always use 1
• 3: Use autoincrementing numbers
• 4: Use object pointer trimmed to 32 bits

Note: Even with -XX:hashCode=4, the hash code may not always point to the exact object address, as objects can be moved in memory. Additionally, object addresses are not evenly distributed, potentially leading to imbalanced hash tables.

Therefore, it's important to consider overriding the hashCode() method if you require specific hash code behavior for your objects.

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