问题
Possible Duplicate:
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?
回答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.
回答2:
The native keyword indicates that it has been implemented in native code (the JVM).
回答3:
If you see the declaration of hashcode
public native int hashCode();
native in declaration indicates that it is implemented natively in jvm code.
回答4:
Where is the implementation part?
It's implemented by the framework already. Please see the documentation.
If there is no implementation how does the hashCode() method return me a result?
However, if you create a custom type you are responsible for generating an int value that is a good representation of the objects current state. Here is a good example of that.
来源：https://stackoverflow.com/questions/14521756/how-hashcode-method-in-object-class-is-implemented