Java String HashCode of n digits?

Question

I'm trying to get an ID from a String in Java, and I thought I would use hashcode (Yeah, two strings can have the same hashcode but I can live with that small probability). I want this ID to have a max of 4 digits. Is that possible?

This is the String default hashCode implementation:

public int hashCode() {
    int h = hash;
    if (h == 0 && value.length > 0) {
        char val[] = value;

        for (int i = 0; i < value.length; i++) {
            h = 31 * h + val[i];
        }
        hash = h;
    }
    return h;
}

Can I override it to produce a hash with 4 digits?

Just like you can have any number represented in 4 digits. The remainder. — Sid
– Sid, Commented Aug 3, 2018 at 15:50
Sure, you can @Override any inherited method that is not final nor private. However, what is the meaning of reducing the length of the hash code to 4 digits? — Nikolas
– Nikolas, Commented Aug 3, 2018 at 15:51

Mureinik · Accepted Answer · 2018-08-03 17:06:04Z

1

One simple trick is to just take the last four digits:

private static int myHash(String s) {
    return s.hashCode() % 10000;
}

EDIT:
As @Holger commented, hashCode() may return a negative value. If the requirement is to return a positive four-digit integer, you could take the absolute value:

private static int myHash(String s) {
    return Math.abs(s.hashCode() % 10000);
}

edited Aug 3, 2018 at 17:06

answered Aug 3, 2018 at 15:51

Mureinik

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9 Comments

Nikolas Over a year ago

Why the static?

Mureinik Over a year ago

@Nikolas why not?

user3458 Over a year ago

Of course, you now only have 10,000 has code values, so the probability of collision is much higher than before,

Mureinik Over a year ago

@Nikolas again - why not? You're not using any instance member here, so why push the this pointer needlessly in the stack?

Radiodef Over a year ago

@Nikolas The OP's code example is an instance method because it's the JDK source code for String.hashCode. (String is a final class, so we aren't talking about extending and overriding hashCode, nor should we ever override hashCode like this even in cases where we can, because of the Liskov Substitution Principle and contractual obligations in general.) With respect, your criticism doesn't make much sense. Utility methods like this are usually static.

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