Generic byte arrays

[UrbanCyborg]

This is really a question on how to use GetStateInformation() and SetStateInformation(). How do you pass different information into and out of the byte arrays those routines use for data transfer? In C or C++ the method would involve casting to a pointer type, but that's not an option in Java, and I can't find any sample code that shows how to do it. For instance, how would you save (or restore) say, a double, or a String to/from a byte array. I suppose for a string you could split out the characters one at a time and save them (although I'd bet character typing would get in the way), and it just don't seem right to break up a double by masking to get at byte-sized chunks. So I'm guessing there's some idiosyncratic syntax for doing this, and I'd really appreciate it if some kind soul would take pity on a poor ignorant code monkey and tell me what it is. Thanks.

Reid

[honki-bobo]

Hi Reid,

I find using java.nio.ByteBuffer is very convenient for me when I'm implementing GetStateInformation() and SetStateInformation().

In GetStateInformation() I would do something like

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int bufSize = 2048; // calculate the buffer size according to your needs; int := 4 bytes, double := 8 bytes
ByteBuffer buf = ByteBuffer.allocate(bufSize);

Then I use the putInt(), putDouble(), put() and similar methods of the ByteBuffer to fill it with the data I want to store. Strings can be stored like

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byte[] stringBytes = myString.getBytes();

and add its length + 4 byte (for an int to store the length of stringBytes) to bufSize.
The final line in GetStateInformation() is then

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return buf.array();

In SetStateInformation(byte[] stateInfo) I basically do it the other way around. I use

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ByteBuffer buf = ByteBuffer.wrap(stateInfo);

to turn the byte[] into a ByteBuffer. The I use the getInt(), getDouble(), get() methods of the ByteBuffer to retrieve the information stored in the byte[].

In general you need to think of a "format" or "order" in which you want/need to store your data so you can easily get it back from the byte[], e.g. store the length of strings or data with variable size in general.

Hope this helps. Best regards,
Martin

[UrbanCyborg]

Idiosyncratic I said, and idiosyncratic it is. Don't think that solution would have occurred to me, as that's a part of the library I'm not familiar with. Thanks mucho, mi amigo!

Reid

[ColinP]

Good advice from Martin.

ByteBuffer is a great class, it all works and makes sense and saves you having to worry about a lot of things but it still feels a little clumsy converting between byte[] and ByteBuffer on top of the other conversions.

I still consider myself a beginner in Java so might be missing an easier way but I ended up with ByteBuffer.allocate( Integer.BYTES ).putInt( i ).array() as the correct way to convert an integer to a byte array. Which does look a little idiosyncratic. I have a small number of helper functions that wrap such weird stuff up, so for instance the above becomes just intAsByteArray( i ) in my main code.

For simple modules I just write String asString() and void fromString( String s ) methods to convert module state to a string and a string to module state and then use ...

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@Override
public byte[] GetStateInformation()
{
   return asString().getBytes( StandardCharsets.UTF_8 );
}

@Override
public void SetStateInformation(byte[] stateInfo)
{
   fromString( new String( stateInfo, StandardCharsets.UTF_8 ) );
}

I use the same methods to perform undo and redo.

Using strings for state is also handy for debugging as you can just look at the string.

[UrbanCyborg]

Thanks, to both of you! I'm busy reading up on that end of the library, now that I'm aware of it. Since I'm coming from C++, idiosyncratic doesn't bother me.