How to Convert Between Byte Arrays and Integers in Java

What is the correct way to convert between byte arrays and integers in Java?

public static void main(String[] args) {
    int a = 123;
    byte[] aBytes = intToByteArray(a);
    int a2 = byteArrayToInt(aBytes);

    System.out.println(a);         // prints '123'
    System.out.println(aBytes);    // prints '[B@459189e1'
    System.out.println(a2);        // prints '123'
    System.out.println(intToByteArray(a2));  // prints '[B@459189e1'
}

In Java, converting integers to byte arrays and back can lead to discrepancies if not handled correctly. This guide provides a detailed explanation of how to perform this conversion accurately using custom functions, as well as common pitfalls and their solutions.

public static int byteArrayToInt(byte[] b) {
    int value = 0;
    for (int i = 0; i < 4; i++) {
        int shift = (4 - 1 - i) * 8;
        value += (b[i] & 0x000000FF) << shift;
    }
    return value;
}

public static byte[] intToByteArray(int a) {
    byte[] ret = new byte[4];
    ret[0] = (byte) (a & 0xFF);
    ret[1] = (byte) ((a >> 8) & 0xFF);
    ret[2] = (byte) ((a >> 16) & 0xFF);
    ret[3] = (byte) ((a >> 24) & 0xFF);
    return ret;
}

Causes

• The byte array representation may not be interpreted correctly when converting back to an integer if the order of bytes is not handled appropriately.
• Byte data type is signed in Java. When dealing with larger integers, this can cause unexpected results due to sign extension.

Solutions

• Ensure that you are interpreting the byte array in the same order that it was created.
• Use appropriate data size to avoid overflow and ensure the correct transformation from byte array to int.

Mistake: Not checking the size of the byte array before conversion.

Solution: Ensure that your byte array is exactly 4 bytes long before attempting to convert it to an integer.

Mistake: Assuming the byte order is the same during conversion.

Solution: Be consistent with your byte order (endianness) when converting both ways.