Question

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What’s the implementation of hashCode in java Object?

While I was browsing through the Object class, I found that there is only a declaration of the hashCode() method. Where is the implementation part? If there is no implementation how does the hashCode() method return me a result?

Answer 1

It’s implemented in the native code. As for implementation, it’s a bit more tricky – you can alter default implementation. If you look at the “Open JDK” sources you will see the following options:

-XX:hashCode=n (from 0 to 5).

• 0 – Park-Miller RNG (default)
• 1 – function of address and some global state
• 2 – const 1
• 3 – sequenatial counter
• 4 – address of an object
• 5 – thread specific xor-shift

You can find a detailed implmenetation here: http://hg.openjdk.java.net/jdk7/jdk7/hotspot/file/tip/src/share/vm/runtime/synchronizer.cpp

Consider source code and comments of static inline intptr_t get_next_hash() function.