How to initialize a ByteBuffer if you don't know how many bytes to allocate beforehand?

Question

Is this:

ByteBuffer buf = ByteBuffer.allocate(1000);

...the only way to initialize a ByteBuffer?

What if I have no idea how many bytes I need to allocate..?

Edit: More details:

I'm converting one image file format to a TIFF file. The problem is the starting file format can be any size, but I need to write the data in the TIFF to little endian. So I'm reading the stuff I'm eventually going to print to the TIFF file into the ByteBuffer first so I can put everything in Little Endian, then I'm going to write it to the outfile. I guess since I know how long IFDs are, headers are, and I can probably figure out how many bytes in each image plane, I can just use multiple ByteBuffers during this whole process.

Answer 1

The idea is that it's only a buffer - not the whole of the data. It's a temporary resting spot for data as you read a chunk, process it (possibly writing it somewhere else). So, allocate yourself a big enough "chunk" and it normally won't be a problem.

What problem are you anticipating?

Answer 2

The types of places that you would use a ByteBuffer are generally the types of places that you would otherwise use a byte array (which also has a fixed size). With synchronous I/O you often use byte arrays, with asynchronous I/O, ByteBuffers are used instead.

If you need to read an unknown amount of data using a ByteBuffer, consider using a loop with your buffer and append the data to a ByteArrayOutputStream as you read it. When you are finished, call toByteArray() to get the final byte array.

Any time when you aren't absolutely sure of the size (or maximum size) of a given input, reading in a loop (possibly using a ByteArrayOutputStream, but otherwise just processing the data as a stream, as it is read) is the only way to handle it. Without some sort of loop, any remaining data will of course be lost.

Answer 3

Depends.

Known Quantities

• Reading a file? Allocate file.size() bytes.
• Copying a string? Allocate string.length() bytes.
• Copying a TCP packet? Allocate 1500 bytes, for example.

Unknown Quantities

When the number of bytes is truly unknown, you can do a few things:

• Make a guess.
• Analyze example data sets to buffer; use the average length.

Example

Java's StringBuffer, unless otherwise instructed, uses an initial buffer size to hold 16 characters. Once the 16 characters are filled, a new, longer array is allocated, and then the original 16 characters copied. If the StringBuffer had an initial size of 1024 characters, then the reallocation would not happen as early or as often.

Optimization

Either way, this is probably a premature optimization. Typically you would allocate a set number of bytes when you want to reduce the number of internal memory reallocations that get executed.

It is unlikely that this will be the application's bottleneck.