Converting Hex Strings to Byte Arrays in Java

In this tutorial, we will explore how to convert a string representation of a hex dump into a byte array using Java. This is a common requirement when working with binary data or parsing hexadecimal strings.

Understanding Hex Strings

A hex string is a sequence of characters that represent hexadecimal values. Each character in the string corresponds to a 4-bit value, ranging from 0 to F (or f for lowercase). For example, the string "00A0BF" represents the following byte values:

• 0x00
• 0xA0
• 0xBF

To convert this hex string into a byte array, we need to process each pair of characters and extract their corresponding byte values.

Using Java’s Built-in Functions (Java 17 and later)

Starting from Java 17, the java.util.HexFormat class provides a convenient method for converting hex strings to byte arrays:

import java.util.HexFormat;

public static byte[] hexStringToByteArray(String s) {
    return HexFormat.of().parseHex(s);
}

This approach is straightforward and eliminates the need for manual parsing or external libraries.

Manual Parsing (Java versions prior to 17)

For earlier Java versions, we can implement a simple function to manually parse the hex string:

public static byte[] hexStringToByteArray(String s) {
    int len = s.length();
    if (len % 2 != 0) {
        throw new IllegalArgumentException("Input string must have an even length");
    }

    byte[] data = new byte[len / 2];
    for (int i = 0; i < len; i += 2) {
        int highNibble = Character.digit(s.charAt(i), 16);
        int lowNibble = Character.digit(s.charAt(i + 1), 16);

        data[i / 2] = (byte) ((highNibble << 4) | lowNibble);
    }
    return data;
}

This function works by iterating over each pair of characters in the input string, extracting their corresponding nibble values using Character.digit, and combining them into a single byte value.

Using External Libraries

Several external libraries provide functions for converting hex strings to byte arrays. Some popular options include:

• Apache Commons Codec (org.apache.commons.codec.binary.Hex):

import org.apache.commons.codec.binary.Hex;

public static byte[] hexStringToByteArray(String s) {
    return Hex.decodeHex(s);
}

• Guava (com.google.common.io.BaseEncoding):

import com.google.common.io.BaseEncoding;

public static byte[] hexStringToByteArray(String s) {
    return BaseEncoding.base16().decode(s);
}

When choosing an external library, consider factors such as performance, dependencies, and compatibility with your project’s requirements.

Conclusion

Converting hex strings to byte arrays is a common task in Java programming. By using built-in functions (Java 17 and later), manual parsing, or external libraries, you can efficiently process hexadecimal data and integrate it into your applications.