The object your method decompressGZIP() needs is a byte[].

So the basic, technical answer to the question you have asked is:

byte[] b = string.getBytes();
byte[] b = string.getBytes(Charset.forName("UTF-8"));
byte[] b = string.getBytes(StandardCharsets.UTF_8); // Java 7+ only

However the problem you appear to be wrestling with is that this doesn't display very well. Calling toString() will just give you the default Object.toString() which is the class name + memory address. In your result [B@38ee9f13, the [B means byte[] and 38ee9f13 is the memory address, separated by an @.

For display purposes you can use:

Arrays.toString(bytes);

But this will just display as a sequence of comma-separated integers, which may or may not be what you want.

To get a readable String back from a byte[], use:

String string = new String(byte[] bytes, Charset charset);

The reason the Charset version is favoured, is that all String objects in Java are stored internally as UTF-16. When converting to a byte[] you will get a different breakdown of bytes for the given glyphs of that String, depending upon the chosen charset.

You used array.toString(), which is implemented like this:

return "[B@" + Integer.toString(this.hashCode(), 16);

(In fact it inherits the definition from Object, and the part before the @ simply is the result of getClass().getName().)

And the hashCode here does not depend on the content.

Instead, use new String(array, encoding).

Of course, this only works for byte-arrays which are really representable as Java strings (which then contain readable characters), not for arbitrary arrays. There better use base64 like Bozho recommended (but make sure to use it on both sides of the channel).