copy java array "over" another

Question

I wonder if there really - as my search shows - is no way to perform a bytewise copy of one array into another array of different primitive type. I have a byte[] from a file that represents a int[] in memory. I could do shifts like

myints[i] = (mybytes[0] << 24) || (mybytes[1] << 16) || (mybytes[2] << 8) || mybytes[0];

but this performance killer can't be the preferred way? Is there nothing like this?

byte[] mybytes = ... coming from file  
int[] myints = new int[mybytes.length / 4];  
MagicCopyFunction: copy mybytes.length bytes from 'mybytes' into 'myints'

One of the answers to memcpy function in C++ to Java equivalent might help, although if optimizing this is required for performance and/or you need this sort of low-level access, you probably shouldn't be using Java. — Bernhard Barker
– Bernhard Barker, Commented Sep 8, 2015 at 12:50
I can't imagine this assembling procedure with four value accesses and three shifts and ors is roughly as fast as a simple int copy that handles four bytes in one operation. Unfortunately I forgot to mention I do android programming so there is no unsafe class. — Droidum
– Droidum, Commented Sep 8, 2015 at 13:18

Kayaman · Accepted Answer · 2015-09-08 12:34:19Z

4

The easiest way to do this is with Buffers.

ByteBuffer b = ByteBuffer.allocate(1024);
// Fill the bytebuffer and flip() it
IntBuffer i = b.asIntBuffer();

answered Sep 8, 2015 at 12:34

Kayaman

73.8k6 gold badges91 silver badges128 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Eran Over a year ago

I don't think it improves the performance over the OPs original code. When you fetch an int from the IntBuffer, It ends up calling Bits.makeInt which does what the OP wanted to avoid - return (int)((((b3 & 0xff) << 24) | ((b2 & 0xff) << 16) | ((b1 & 0xff) << 8) | ((b0 & 0xff) << 0)));

Kayaman Over a year ago

While it may not affect performance as much (although with direct buffers it should be quicker), it'll at least prevent him from writing all that code himself and possibly making mistakes.

Droidum Over a year ago

Well ok even if the Buffer is not faster at least it keeps this terrible syntax away from me :-)

Collectives™ on Stack Overflow

copy java array "over" another

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related