Is Java Object hashCode() Value Mutable? Effects of Garbage Collection and How hashCode Is Generated

Answer: The default implementation of Java object's hashCode() is immutable; it does not change even after garbage collection.

Reasons:

1. If a class overrides hashCode() , the fields used in the calculation must remain unchanged; otherwise hash values used in hash‑based collections (e.g., HashMap ) can cause business errors, memory leaks, or OOM.

2. The default hashCode() is provided by the JVM as a native method. The computed value is stored in the object's header (MarkWord) and is not recomputed.

The OpenJDK source (jdk‑21‑ga) shows the native method registration and the generation strategy. The implementation ultimately uses a variation of Marsaglia's shift‑xor RNG (XORShift) to produce the hash code, with several possible strategies; the default strategy is the last one shown in the source.

Key source files include vmIntrinsics.hpp , javaClasses.cpp , jvm.cpp , and the runtime method ObjectSynchronizer::FastHashCode in synchronizer.cpp . The generated hash code follows the algorithm illustrated in the OpenJDK screenshots.