BigInteger -> byte[] -> Biginteger. Looks equal but if statement fails

Question

I am playing around with an idea I have for storing a public key for myself. For this I would need to transform the BigInteger in some sort of a variable and then recreate the BigInteger from that value.

I have searched through Stackoverflow to find the best way to do this is with byte[].

This is my code in Eclipse:

import java.math.BigInteger;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.interfaces.RSAPublicKey;

public class Vaja2 {
    public static void main(String[] args){
        try {

            // Create RSA Keypair (to obtain a BigInteger)
            KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
            kpg.initialize(1024);
            KeyPair keypair = kpg.generateKeyPair();

            // Extract the public key
            RSAPublicKey publicKey = (RSAPublicKey) keypair.getPublic();

            // Get the public Exponent from the public key, I have BigInteger now.
            BigInteger pkPublicExBI = publicKey.getPublicExponent();

            //Try this:   BigInteger -> byte-> BigInteger
            byte[] pkModByte = pkPublicExBI.toByteArray();
            BigInteger pkPublicExBIrecreated = new BigInteger(pkModByte);


            // Show Results
            System.out.println("Original BigInteger:    " + pkPublicExBI);
            System.out.println("Recreated BigInteger:   " + pkPublicExBIrecreated);

            if (pkPublicExBI == pkPublicExBIrecreated) {
                System.out.println("\nThey are equal");
            }
            else {
                System.out.println("\nThey are NOT equal");
            }


        } catch (Exception e) {
            // Nothing happens
        }

    }
}

And this is the result in shown in the Eclipse console.

Original BigInteger:    65537
Recreated BigInteger:   65537

They are NOT equal

The if statement tells me, that the two BigIntegers are not equal. Yet in the console, I see them as both being equal to 65537.

My questions:

Why does the "if" statement fail?
How would you recommend me to make the code different. Assuming the program will demand public key to be stored in notepad.exe or similar text-encoding program (ASCII will be used).

They're objects. == compares references. To check for value equality, use .equals() instead. — resueman, Jul 29 '14 at 14:34
Learn the difference between equals() and == . Good answer here http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java — Revive, Jul 29 '14 at 14:34

Zach · Accepted Answer · 2014-07-31T12:07:46.373

2

Use .equals() as opposed to == when comparing objects. == will compare the objects' references, whereas .equals() will check to see if they have the same values. Since two objects will very rarely have the same reference you should never use == except for comparing primitive types (int, char, but String is not a primitive type!) where it doesn't matter.

So you want:

if (pkPublicExBI.equals(pkPublicExBIrecreated)) {

Instead of

if (pkPublicExBI == pkPublicExBIrecreated) {

edited Jul 31 '14 at 12:07

answered Jul 29 '14 at 14:34

Zach

4,652
18
22

Thank your for the correct answer and taking your time to explain to me why my it is correct. (I am stepping from C to Java programming and class-oriented programming) – KrNeki Jul 29 '14 at 14:46
1

Your reasoning is correct, but the explanation is wrong. The == operator compares object identity. The concept of "address in memory" is at a much lower level than what you can access using Java. Both in theory and in practice, the JVM moves objects around in the heap, so the same object may very well appear on different addresses. – jarnbjo Jul 31 '14 at 09:58

score 0 · Answer 2 · answered Jul 29 '14 at 14:36

0

pkPublicExBI == pkPublicExBIrecreated

use this instead pkPublicExBI.equals(pkPublicExBIrecreated).

answered Jul 29 '14 at 14:36

Deepanshu J bedi

1,530
1
11
23

BigInteger -> byte[] -> Biginteger. Looks equal but if statement fails

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