Direct access to part of an array

Question

To speed up my image processing application, I'm trying to address values in an array starting in the middle, and using indices starting at 0.

For example, when I work with two arrays, I often have a situation like this:

public void work(int[] array1, int[] array2, int offset)
{
    int len = array1.length;
    for (int i = 0; i < len; i++)
        array1[i] += array2[i + offset];
}

so I would like to create a new variable, array3 that directly maps into the middle of array2 (not a copy), so I can do this:

public void work(int[] array1, int[] array2, int offset)
{
    int[] array3 = array2[offset]...;
    int len = array1.length;
    for (int i = 0; i < len; i++)
        array1[i] += array3[i];
}

In effect, I want a Java equivalent to this c statement:

int *array3ptr = array2ptr + offset;

Note: I don't have any experience with JNI or anything, so if it boils down to using this, please provide working examples.

Is this a current performance issue? You're looking to replace a single add inside a loop? — Greg Kopff
– Greg Kopff, Commented May 14, 2012 at 9:06

aioobe · Accepted Answer · 2012-05-14 09:25:05Z

1

This is not possible to do by using pure Java arrays and the array[index] notation. In Java arrays can never overlap.

You could however wrap your arrays in IntBuffer. If you use a direct buffer (see allocateDirect) you should get real good performance.

A statement such as

int *array3ptr = array2ptr + offset;

would be written as

IntBuffer array3 = array2.slice().position(offset);

These

edited May 14, 2012 at 9:25

answered May 14, 2012 at 8:56

aioobe

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7 Comments

Mark Jeronimus Over a year ago

I read somewhere that it should be possible with JNI using IntBuffer or something

Michał Kosmulski Over a year ago

@Zom-B I don't think so. An array is an actual object which needs to be created whether by pure Java or by JNI. Even of you could re-use the data area, you would still have to create a new object with length field etc. The only working and elegant way IMO is to change your API so that apart from an array it accepts start and end indices, or use ByteBuffer and friends, which can wrap arrays for about the same effect.

Greg Kopff Over a year ago

A direct buffer will only give you a performance boost if you're shuffling the data somewhere (to disk, to the network), rather than just manipulating it in memory. But sure, if you're processing consists of read from disk, manipulate, write back then a direct buffer can help.

Mark Jeronimus Over a year ago

Too bad. I'm not shoving the data around as a whole, I'm manipulating it at the pel level (=individual colors in a pixel). Looking at the source of DirectByteBuffer, I see that the get method checks for out-of-bounds, calculates an index and gets the value from an Unsafe object. This can only be magnitudes slower than just array[x + variable]

aioobe Over a year ago

Try it out berfore drawing conclusions. array[x + variable] also performs.array bound checks.

|

Surender Thakran · Accepted Answer · 2012-05-14 08:59:19Z

0

In java even integer arrays are objects on the heap. Hence the int[] array2 is actually directly refering to the integer array in the memory. no copy is created of the object (i.e array), duplicate is only of the reference to the array.

answered May 14, 2012 at 8:59

Surender Thakran

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Comments

JB Nizet · Accepted Answer · 2012-05-14 09:01:29Z

0

I don't have much experience with them, but I think you could be able to do that by wrapping your arrays (and sub-arrays) into IntBuffer instances.

answered May 14, 2012 at 9:01

JB Nizet

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