Java (BigInteger from byte array)

Question

I'm using following code to create a BigInteger from hexadecimal string and print in to output.

package javaapplication2;

import java.math.BigInteger;
import javax.xml.bind.DatatypeConverter;

public class JavaApplication2 {
    public static void main(String[] args) {
        // Number in hexadecimal form
        String HexString = "e04fd020ea3a6910a2d808002b30309d";
        // Convertation from string to byte array
        byte[] ByteArray = toByteArray(HexString);
        // Creation of BigInteger from byte array
        BigInteger BigNumber = new BigInteger(ByteArray);
        // Print result
        System.out.print(BigNumber + "\n");
    }
    public static String toHexString(byte[] array) {
        return DatatypeConverter.printHexBinary(array);
    }

    public static byte[] toByteArray(String s) {
        return DatatypeConverter.parseHexBinary(s);
    }
}

After execution of this code I'm get a following result:

-42120883064304190395265794005525319523

But I'm expected to see this result:

298161483856634273068108813426242891933

What I'm doing wrong?

Jon Skeet · Accepted Answer · 2015-08-28 14:40:41Z

25

You're passing in a byte array where the first byte has a top bit that is set - making it negative. From the constructor documentation:

Translates a byte array containing the two's-complement binary representation of a BigInteger into a BigInteger. The input array is assumed to be in big-endian byte-order: the most significant byte is in the zeroth element.

A two's-complement binary representation with a leading set bit is negative.

To get the result you want, you can do any of:

Prefix the hex string with "00" so that you'll always get a top byte of 0
Pass the hex string straight into the BigInteger(String, int) constructor, where the sign is inferred from the presence or absence of "-" at the start of the string. (Obviously you'd pass in 16 as the base.)
Use the BigInteger(int, byte[]) constructor, passing 1 as the signum value

If your real context is that you've already got the byte array, and you were only parsing it from a hex string for test purposes, I'd use the third option. If you've genuinely got a hex string as input, I'd use the second option.

edited Aug 28, 2015 at 14:40

answered Aug 28, 2015 at 14:35

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Kechup Over a year ago

Thank you! Following your answer I fixed this problem.

Jkike · Accepted Answer · 2015-08-28 14:34:36Z

5

try

BigInteger bigInt = new BigInteger(HexString, 16);

answered Aug 28, 2015 at 14:34

Jkike

8056 silver badges12 bronze badges

4 Comments

Davide Lorenzo MARINO Over a year ago

Very interesting. I didn't know the existence of this constructor. Well done!

Kechup Over a year ago

Thank you! I'm like your answer and will use it in future.

brso05 Over a year ago

Why did someone downvote the correct answer? You really shouldn't do that.

Jkike Over a year ago

I don't know why it was downgradred, but text of the question was stating; " create a BigInteger from hexadecimal string" ... it didn't not say it had to pass thought byte[]... if it was for the missing explanation, I really don't know how to explain that the method exist in the class :(

Collectives™ on Stack Overflow

Java (BigInteger from byte array)

2 Answers 2

1 Comment

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related