Java how to make the hash unique

Question

I'm trying to create an abstract class Coin, having instance variables name (a String), symbol (of type java.awt.Image), and hash (a String.) The problem is that I need to make the attribute hash not the same as the one being returned by the default hashcode() method, hence having the hash of a Coin unique.

abstract class Coin {

    private String name;
    private Image symbol; 
    private String hash;

}

A hash code is not a `String`. You could always try something of the form `return name.hashCode()+symbol.hashCode()`, but it is of the essence of hash codes that they are *not* unique. There are no [tag:generics] here. — user207421, Sep 14 '17 at 09:36
hash computing should be designed (coded) in the same philosophy like `equals()` - this is comparison to be equal. Hash can be duplicate - equals() tell the true — Jacek Cz, Sep 14 '17 at 09:43
https://stackoverflow.com/questions/113511/best-implementation-for-hashcode-method - Check this out. Explains a good methodology for creating your own hash — aksappy, Sep 14 '17 at 09:44
@EJP Yes you are right hash code is not a String, however in this specific assignment I'm required to make the hash a String and have the hash of a Coin unique. Well I might have interpreted wrongly the implementation of the assignment, but I would love to see the way you would implement it. (ps sorry for the sloppy code but I'm a beginner) — sny, Sep 14 '17 at 09:45
assignment 'unique hash' is incorrect. I dont know, this is school (poor understanding), or industry (XY problem) — Jacek Cz, Sep 14 '17 at 09:49
For uniqueness, you need an UUID (basically a large random number), not a hash. — Mick Mnemonic, Sep 14 '17 at 09:49
@JacekCz It's a university assignment and the teacher mentioned that "the hash of a Coin is unique." I just didn't know how to implement it, because he can also refer to the class name which is Coin. — sny, Sep 14 '17 at 09:54
You need to post the background material. A hash code is *not* a `String`, whatever you were told: it is a number. — user207421, Sep 14 '17 at 10:28
@EJP I've added the assignment description above, if that helps. — sny, Sep 14 '17 at 11:58

nafas · Answer 1 · 2017-09-14T11:42:32.617

before I move on just let you know, a hash value may never be 100% unique because of hash collision.

having said that, I assume you want a unique String for variable hash(note hashCode() function in java is different as it returns an int)

there are many hashing algorithms, the one I normally use for unification in such scenario is MD5

There is an Apache utility called DiguestUtility which make life easy.

Here is an example of the use:

DigestUtils.md5(byte[] bytes);// --> returns a string of 32 char long
DigestUtils.md5(String s);// --> returns a string of 32 char long
...

Read through the methods in the documentation to see which one suits you more.

score -2 · Accepted Answer · answered Sep 14 '17 at 09:41

-2

You can override the default hashCode() function in the following way:

 @Override
public int hashCode() {
    // Unique hashcode generating function goes here
    return hash;
}

A way could be using name.hashCode()+symbol.hashCode().

answered Sep 14 '17 at 09:41

Chinmay jain

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mathematics say, such hash has poor statistic parameters - but technically is OK – Jacek Cz Sep 14 '17 at 09:44
generally, hashCode() method is very poor for uniquness the collision is very frequent. – nafas Sep 14 '17 at 09:55
@nafas Rubbish. It depends *entirely* on how the method is written. You cannot make blanket statements like that about a mere method signature. – user207421 Sep 14 '17 at 10:29
@EJP hashCode() returns an int so the number of combinations is bound to 32bits. even MD5 (supposed to be insecure nowadays) got 128bits, – nafas Sep 14 '17 at 11:41

Java how to make the hash unique

2 Answers2