Question

We can disassemble String to and from byte[] easily

        String s = "my string";
        byte[] b = s.getBytes();
        System.out.println(new String(b)); // my string

When compression is involved however there seem to be some issues. Suppose you have 2 methods, compress and uncompress (code below works fine)

public static byte[] compress(String data) 
             throws UnsupportedEncodingException, IOException {
    byte[] input = data.getBytes("UTF-8");
    Deflater df = new Deflater();
    df.setLevel(Deflater.BEST_COMPRESSION);
    df.setInput(input);

    ByteArrayOutputStream baos = new ByteArrayOutputStream(input.length);
    df.finish();
    byte[] buff = new byte[1024];
    while (!df.finished()) {
        int count = df.deflate(buff);
        baos.write(buff, 0, count);
    }
    baos.close();
    byte[] output = baos.toByteArray();

    return output;
}

public static String uncompress(byte[] input) 
            throws UnsupportedEncodingException, IOException,
        DataFormatException {
    Inflater ifl = new Inflater();
    ifl.setInput(input);

    ByteArrayOutputStream baos = new ByteArrayOutputStream(input.length);
    byte[] buff = new byte[1024];
    while (!ifl.finished()) {
        int count = ifl.inflate(buff);
        baos.write(buff, 0, count);
    }
    baos.close();
    byte[] output = baos.toByteArray();

    return new String(output);
}

My Tests work as follows (works fine)

String text = "some text";
byte[] bytes = Compressor.compress(text);
assertEquals(Compressor.uncompress(bytes), text); // works

For no reason other then, why not, i’d like to modify the first method to return a String instead of the byte[].

So i return new String(output) from the compress method and modify my tests to:

String text = "some text";
String compressedText = Compressor.compress(text);
assertEquals(Compressor.uncompress(compressedText.getBytes), text); //fails

This test fails with java.util.zip.DataFormatException: incorrect header check

Why is that? What needs to be done to make it work?

Answer 1

The String(byte[]) constructor is the problem. You cannot simply take arbitrary bytes, convert them to a string and then back to byte array. String class performs sophisticated encoding on this byte based on desired charset. If given byte sequence can’t be represented e.g. in Unicode it will be discarded or converted to something else. The conversion from bytes to String and back to bytes is lossless only if these bytes really represented some String (in some encoding).

Here is a simplest example:

new String(new byte[]{-128}, "UTF-8").getBytes("UTF-8")

The above returns -17, -65, -67 while 127 input returns the exact same output.