Fastest possible byte array concatenation method
arrays, concatenation, java, performance
Solution
Since you can find out the size of the message by adding up the lengths of the pieces, I would:
- add up the lengths of the pieces, and allocate the output array;
- use a loop to `arraycopy()` each piece into the correct position in the output array.
This is likely to be memory-efficient and fast. However, only profiling can tell the full story.
Problem
I got a map containing n parts of a message as a byte array. After the last piece has found his way into the map, the message has to be concatenated. I found two solutions which should meet the requirements. First one is using System.arraycopy: ``` public byte[] getMessageBytes() throws IOException { byte[] bytes = new byte[0]; for (final Map.Entry<Short,byte[]> entry : myMap.entrySet()) { byte[] entryBytes = entry.getValue(); byte[] temp = new byte[bytes.length + entryBytes.length]; System.arraycopy(bytes, 0, temp, 0, bytes.length); System.arraycopy(entryBytes, 0, temp, bytes.length, entryBytes.length); bytes = temp; } return bytes; } ``` And second one is using ByteArrayOutputStream: ``` public byte[] getMessageBytes() throws IOException { final ByteArrayOutputStream baos = new ByteArrayOutputStream(); for (final Map.Entry<Short,byte[]> entry : myMap.entrySet()) { baos.write(entry.getValue()); } baos.flush(); return baos.toByteArray(); } ``` What is the better method seen from the angle of performance and memory usage? Is there an alternative way of doing the concatenation which is even better?