[java] How to generate a unique hash code for string input in android...?

I wanted to generate a unique hash code for a string in put in android. Is there any predefined library is there or we have to generate manually. Please any body if knows please present a link or a code stuff.

This question is related to java android hash hashmap

The answer is


You can use this code for generating has code for a given string.

int hash = 7;
for (int i = 0; i < strlen; i++) {
    hash = hash*31 + charAt(i);
}

It depends on what you mean:

  • As mentioned String.hashCode() gives you a 32 bit hash code.

  • If you want (say) a 64-bit hashcode you can easily implement it yourself.

  • If you want a cryptographic hash of a String, the Java crypto libraries include implementations of MD5, SHA-1 and so on. You'll typically need to turn the String into a byte array, and then feed that to the hash generator / digest generator. For example, see @Bryan Kemp's answer.

  • If you want a guaranteed unique hash code, you are out of luck. Hashes and hash codes are non-unique.

A Java String of length N has 65536 ^ N possible states, and requires an integer with 16 * N bits to represent all possible values. If you write a hash function that produces integer with a smaller range (e.g. less than 16 * N bits), you will eventually find cases where more than one String hashes to the same integer; i.e. the hash codes cannot be unique. This is called the Pigeonhole Principle, and there is a straight forward mathematical proof. (You can't fight math and win!)

But if "probably unique" with a very small chance of non-uniqueness is acceptable, then crypto hashes are a good answer. The math will tell you how big (i.e. how many bits) the hash has to be to achieve a given (low enough) probability of non-uniqueness.


String input = "some input string";
int hashCode = input.hashCode();
System.out.println("input hash code = " + hashCode);

Let's take a look at the stock hashCode() method:

public int hashCode() {
    int h = hash;
    if (h == 0 && count > 0) {
        for (int i = 0; i < count; i++) {
            h = 31 * h + charAt(i);
        }
        hash = h;
    }
    return h;
}

The block of code above comes from the java.lang.String class. As you can see it is a 32 bit hash code which fair enough if you are using it on a small scale of data. If you are looking for hash code with more than 32 bit, you might wanna checkout this link: http://www.javamex.com/tutorials/collections/strong_hash_code_implementation.shtml


This is a class I use to create Message Digest hashes

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class Sha1Hex {

    public String makeSHA1Hash(String input)
            throws NoSuchAlgorithmException, UnsupportedEncodingException
        {
            MessageDigest md = MessageDigest.getInstance("SHA1");
            md.reset();
            byte[] buffer = input.getBytes("UTF-8");
            md.update(buffer);
            byte[] digest = md.digest();

            String hexStr = "";
            for (int i = 0; i < digest.length; i++) {
                hexStr +=  Integer.toString( ( digest[i] & 0xff ) + 0x100, 16).substring( 1 );
            }
            return hexStr;
        }
}

I use this i tested it as key from my EhCacheManager Memory map ....

Its cleaner i suppose

   /**
     * Return Hash256 of String value
     *
     * @param text
     * @return 
     */
    public static String getHash256(String text) {
        try {
            return org.apache.commons.codec.digest.DigestUtils.sha256Hex(text);
        } catch (Exception ex) {
            Logger.getLogger(HashUtil.class.getName()).log(Level.SEVERE, null, ex);
            return "";
        }
    }

am using maven but this is the jar commons-codec-1.9.jar


For me it worked

   public static long getUniqueLongFromString (String value){
       return  UUID.nameUUIDFromBytes(value.getBytes()).getMostSignificantBits();
    }

A few line of java code.

public static void main(String args[]) throws Exception{
       String str="test string";
       MessageDigest messageDigest=MessageDigest.getInstance("MD5");
       messageDigest.update(str.getBytes(),0,str.length());
       System.out.println("MD5: "+new BigInteger(1,messageDigest.digest()).toString(16));
}

Examples related to java

Under what circumstances can I call findViewById with an Options Menu / Action Bar item? How much should a function trust another function How to implement a simple scenario the OO way Two constructors How do I get some variable from another class in Java? this in equals method How to split a string in two and store it in a field How to do perspective fixing? String index out of range: 4 My eclipse won't open, i download the bundle pack it keeps saying error log

Examples related to android

Under what circumstances can I call findViewById with an Options Menu / Action Bar item? How to implement a simple scenario the OO way My eclipse won't open, i download the bundle pack it keeps saying error log getting " (1) no such column: _id10 " error java doesn't run if structure inside of onclick listener Cannot retrieve string(s) from preferences (settings) strange error in my Animation Drawable how to put image in a bundle and pass it to another activity FragmentActivity to Fragment A failure occurred while executing com.android.build.gradle.internal.tasks

Examples related to hash

php mysqli_connect: authentication method unknown to the client [caching_sha2_password] What is Hash and Range Primary Key? How to create a laravel hashed password Hashing a file in Python PHP salt and hash SHA256 for login password Append key/value pair to hash with << in Ruby Are there any SHA-256 javascript implementations that are generally considered trustworthy? How do I generate a SALT in Java for Salted-Hash? What does hash do in python? Hashing with SHA1 Algorithm in C#

Examples related to hashmap

How to split a string in two and store it in a field Printing a java map Map<String, Object> - How? Hashmap with Streams in Java 8 Streams to collect value of Map How to convert String into Hashmap in java Convert object array to hash map, indexed by an attribute value of the Object HashMap - getting First Key value The type java.util.Map$Entry cannot be resolved. It is indirectly referenced from required .class files Sort Go map values by keys Print all key/value pairs in a Java ConcurrentHashMap creating Hashmap from a JSON String