Using Java Deflater/Inflater with custom dictionary causes IllegalArgumentException

Question

The following code is based on the example given in the javadocs for java.util.zip.Deflater. The only changes I have made is to create a byte array called dict and then set

Answer 1

I actually figured this out while formulating the question but thought I should post the question anyway so others might benefit from my struggles.

It turns out you have to call inflate() once after setting the input but before setting the dictionary. The value returned will be 0, and a call to needsDictionary() will then return true. After that you can set the dictionary and call inflate again.

The amended code is as follows:

import java.util.zip.Deflater;
import java.util.zip.Inflater;

public class DeflateWithDictionary {
    public static void main(String[] args) throws Exception {
        String inputString = "blahblahblahblahblah??";
        byte[] input = inputString.getBytes("UTF-8");
        byte[] dict = "blah".getBytes("UTF-8");

        // Compress the bytes
        byte[] output = new byte[100];
        Deflater compresser = new Deflater();
        compresser.setInput(input);
        compresser.setDictionary(dict);
        compresser.finish();
        int compressedDataLength = compresser.deflate(output);

        // Decompress the bytes
        Inflater decompresser = new Inflater();
        decompresser.setInput(output, 0, compressedDataLength);
        byte[] result = new byte[100];
        decompresser.inflate(result);
        decompresser.setDictionary(dict);
        int resultLength = decompresser.inflate(result);
        decompresser.end();

        // Decode the bytes into a String
        String outputString = new String(result, 0, resultLength, "UTF-8");
        System.out.println("Decompressed String: " + outputString);
    }
}

This seems very counter intuitive and clunky from an API design perspective, so please enlighten me if there are any better alternatives.