java - What is an object's hash code if hashCode() is not overridden?

Question

If the hashCode() method is not overridden, what will be the result of invoking hashCode() on any object in Java?

Answer 1

In HotSpot JVM by default on the first invocation of non-overloaded Object.hashCode or System.identityHashCode a random number is generated and stored in the object header. The consequent calls to Object.hashCode or System.identityHashCode just extract this value from the header. By default it has nothing in common with object content or object location, just random number. This behavior is controlled by -XX:hashCode=n HotSpot JVM option which has the following possible values:

• 0: use global random generator. This is default setting in Java 7. It has the disadvantage that concurrent calls from multiple threads may cause a race condition which will result in generating the same hashCode for different objects. Also in highly-concurrent environment delays are possible due to contention (using the same memory region from different CPU cores).
• 5: use some thread-local xor-shift random generator which is free from the previous disadvantages. This is default setting in Java 8.
• 1: use object pointer mixed with some random value which is changed on the "stop-the-world" events, so between stop-the-world events (like garbage collection) generated hashCodes are stable (for testing/debugging purposes)
• 2: use always 1 (for testing/debugging purposes)
• 3: use autoincrementing numbers (for testing/debugging purposes, also global counter is used, thus contention and race conditions are possible)
• 4: use object pointer trimmed to 32 bit if necessary (for testing/debugging purposes)

Note that even if you set -XX:hashCode=4, the hashCode will not always point to the object address. Object may be moved later, but hashCode will stay the same. Also object addresses are poorly distributed (if your application uses not so much memory, most objects will be located close to each other), so you may end up having unbalanced hash tables if you use this option.