How to use String.hashCode to generate primary keys

Question

I understand that this seems to be already discussed and the answer is yes, String.hashCode can generate equal vales for different strings, but quite unlikely (Can Java's hashCode produce same value for different strings?). However it does happen in my application.

The following code will produce the same hashcode: -347019262 (jave 1.7.25)

String string1 = "/m/06qw_";
String string2="/m/0859_";
System.out.println(string1+","+string1.hashCode());
System.out.println(string2+","+string2.hashCode());

I do need hashcode in this case, and I want to use it to generate a unique primary key for a string. it seems that I am not doing it right. Any suggestions please?

Many thanks!

Answer 1

You misunderstand .hashCode().

One part of the contract is that objects who are equals() must have the same hashCode(). However, the reverse is not true: two objects who have the same hashCode() do not have to be equals().

This is a valid, albeit perfectly useless, hashCode() implementation:

@Override
public int hashCode()
{
    return 42; // universal answer
}

You should use the string itself as the "primary key". If you want a "more efficient" key, you should consider what format the input string is and, if possible, extract a significant part of this input.

Answer 2

The sensible option is to use the string as the primary key. (Another choice would be to associate a GUID with your data record and have that as the primary key.)

Hashing is meant to be (1) fast and (2) such that two equal strings will have the same hash code.

I'd submit it's likely that you'll get hashing clashes; after all an int (the hash return type) only has about 4 billion distinct values.