Md5Hash.java

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;

public class Md5Hash {
    private final byte[] digest;
    private final int hashCode;

    private Md5Hash(byte[] digest) {
        if (digest == null || digest.length != 16) {
            throw new IllegalArgumentException();
        }
        this.hashCode = Arrays.hashCode(digest);
        this.digest = digest;
    }

    public static Md5Hash fromByteArray(byte[] array) {
        if (array == null) {
            throw new IllegalArgumentException();
        }
        MessageDigest messageDigest = getMd5Digest();
        messageDigest.update(array);
        return new Md5Hash(messageDigest.digest());
    }

    public static Md5Hash fromString(String string) {
        if (string == null) {
            throw new IllegalArgumentException();
        }
        MessageDigest messageDigest = getMd5Digest();
        messageDigest.update(string.getBytes());
        return new Md5Hash(messageDigest.digest());
    }

    public static Md5Hash fromDigest(byte[] digest) {
        return new Md5Hash(digest);
    }
    
    public byte[] getDigest() {
        return Arrays.copyOf(digest, digest.length);
    }

    @Override
    public int hashCode() {
        return hashCode;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Md5Hash hash = (Md5Hash) o;
        return hashCode == hash.hashCode &&
                Arrays.equals(digest, hash.digest);
    }

    private static MessageDigest getMd5Digest() {
        try {
            return MessageDigest.getInstance("MD5");
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder(32);
        for (int i = 0; i < digest.length; i++) {
            if (digest[i] > 9) {
                sb.append('0');
            }
            sb.append(Integer.toUnsignedString(digest[i], 16));
        }
        return sb.toString();
    }
}