How do hashCode() and identityHashCode() work at the back end?
hashcode, java, object
Solution
How do Object.hashCode() and System.identityHashCode() work at the back end?
Assuming that it hasn't been overridden, the `Object.hashCode()` method simply calls `System.identityHashCode(this)`.
The exact behavior of `System.identityHashCode(Object)` depends on the JVM implementation. (The actual implementation on recent Hotspot JVMs is rather clever, but I digress.)
Does `identityHashCode()` return the reference of the object?
No. It returns an `int`, and an `int` cannot hold a reference.
That integer returned by `identityHashCode` may be related to the (a) machine address for the object, or it may not be1. The value returned by `identityHashCode()` is guaranteed not to change for the lifetime of the object. This means that if the GC relocates an object (after an `identityHashCode()` call) then it cannot use the new object address as the identity hashcode.
Does hashCode() depend on the `?` of the object `? ==` operator how to work in back end.
This doesn't make sense. There is no `? ==` or `?==` operator in Java.
What is the difference between hashCode() and identityHashCode()?
This is partly explained above. Other differences include:
The `hashCode()` method is a non-`final` instance method, and should be overridden in any class where the `equals(Object)` is overridden. By contrast, `identityHashCode(Object)` is a `static` method and therefore cannot be overridden.
The `identityHashCode(Object)` method gives you a identifier for an object which can (in theory) be used for other things than hashing and hash tables. (Unfortunately, it is not a unique identifier, but it is guaranteed to never change for the lifetime of the object.)
1 - For current generation JVMs, it is not related to the memory address at all. See @bestsss's answer.
Problem
How do `Object.hashCode()` and `System.identityHashCode()` work at the back end? Does `identityHashCode()` return the reference of the object? Does `hashCode()` depend on the content or address of the object? What is the difference between `hashCode()` and `identityHashCode()`?