Here is an example of how to use the ByteArrayBuilder class from the org.apache.commons.io package in Java:

import org.apache.commons.io.ByteArrayBuilder;

public class Main {
    public static void main(String[] args) {
        // Create a ByteArrayBuilder with an initial capacity of 10 bytes
        ByteArrayBuilder byteBuilder = new ByteArrayBuilder(10);

        // Add some bytes to the builder
        byte[] bytes1 = {1, 2, 3};
        byteBuilder.append(bytes1);
        byte[] bytes2 = {4, 5, 6};
        byteBuilder.append(bytes2);

        // Get the underlying byte array
        byte[] result = byteBuilder.toByteArray();
        //Printing the Byte array
        for (byte b : result) {
            System.out.print(b + " ");
        }
    }
}

In this example, we create a ByteArrayBuilder object with an initial capacity of 10 bytes. Then we append two byte arrays to it using the append() method. Finally, we retrieve the underlying byte array using the toByteArray() method.

It's worth noting that this class was removed from the apache.commons.io library in version 2.6 and later versions, you can use other libraries like ByteArrayOutputStream and ByteBuffer for buffer functionality.