Java platform libraries Hashcode

Question

In Effective Java Item 9 (Always override hashCode when you override equals) where it says

Many classes in the Java platform libraries, such as String, Integer, and Date, include in their specifications the exact value returned by their hashCode method as a function of the instance value. This is generally not a good idea, as it severely limits your ability to improve the hash function in future releases.

What does it mean ?

It means that the Java sources are not always a model of good programming style. — Ted Hopp
– Ted Hopp, Commented Dec 10, 2012 at 18:26

Louis Wasserman · Accepted Answer · 2012-12-10 18:25:13Z

4

It means that you can't rewrite the hash function in later versions of your code to have better hashing properties. For example, the String.hashCode() function is fast...but not very good. But it can't be changed anymore, because the hash code was specified and people have depended on that implementation in their own code.

answered Dec 10, 2012 at 18:25

Louis Wasserman

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4 Comments

Hiro2k Over a year ago

What do you mean by, "because the hash code was specified and people have depended on that implementation in their own code"? Is it like someone does this: int hash = string.hashCode() And then later they do something stupid like assert(hash == 12345) ?

Ted Hopp Over a year ago

@Hiro2k - The API docs specify that String.hashCode() is computed by a specific formula. Client code is free to independently compute the hash code using that exact formula and assume it will be the same as that returned by String.hashCode(). This might seem perverse for pure Java code, but does make some sense with JNI. There are probably other cases where it would make sense to take advantage of the extra knowledge that the API specifies.

peter Over a year ago

Can you explain a little bit why people think String.hashCode() is fast but not very good ? Does that mean the traditional way of calculating the hashcode by 31 is not good ?

Louis Wasserman Over a year ago

There are a variety of important measures of hash functions that measure how good they are at spreading inputs evenly, such as the avalanche property, at which the java.lang.String implementation doesn't do very well.

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