Java: Is there a performance difference between using n variables or using hard coded array elements?

Question

I'm making a program for myself and this question popped up. This program deals with graphics so I have to keep performance in mind.

Is there a difference in performance if I use several variables or if I use an array with hard coded indexes? If there is, which is better?

To illustrate:

R = (X *  3.2406) + (Y * -1.5372) + (Z * -0.4986);
G = (X * -0.9689) + (Y *  1.8758) + (Z *  0.0415);
B = (X *  0.0557) + (Y * -0.2040) + (Z *  1.0570);

or

RGB[0] = (XYZ[0] *  3.2406) + (XYZ[1] * -1.5372) + (XYZ[2] * -0.4986);
RGB[1] = (XYZ[0] * -0.9689) + (XYZ[1] *  1.8758) + (XYZ[2] *  0.0415);
RGB[2] = (XYZ[0] *  0.0557) + (XYZ[1] * -0.2040) + (XYZ[2] *  1.0570);

Thanks in advance.

If there is a performance difference (I doubt it), it is unlikely to be the bottleneck in your program. Use the easiest and most readable and move on to where the real performance issues are. — assylias
– assylias, Commented Dec 6, 2012 at 17:16
The variables are better in terms of maintenance. While using the array, you might use a wrong index to fetch a value and very hard to identify the issue. — muruga
– muruga, Commented Dec 6, 2012 at 17:16
@muruga It happened several times. But I changed to individual variables because something was terribly slowing the program down. But I'm not sure what it was. — GuiRitter
– GuiRitter, Commented Dec 6, 2012 at 20:25
@GuiRitter I really suspect that. I don't think that the variables would really slow things down (there could a very little over head), but you have to look at your whole program very closely, rather you can try some profiling. — muruga
– muruga, Commented Dec 19, 2012 at 15:47

akuhn · Accepted Answer · 2012-12-07 01:06:54Z

3

You are most likely faster with separate variables.

Why? The JVM optimizes your code at runtime to make it faster. It tracks the flow and values of each variable to learn how to optimize code. For example it might realize that a parameter to a method is never assigned and thus constant. But it does not do so for each element of an array. An array is just considered as one huge mutable variable. So you are more likely to get optimal code when using separate variables.

answered Dec 7, 2012 at 1:06

akuhn

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user1877337 · Accepted Answer · 2012-12-06 17:22:18Z

3

There definitely is a memory difference.

Your first scenario uses (assuming doubles):

8    8               8               8  = 24 bytes
R = (X *  3.2406) + (Y * -1.5372) + (Z * -0.4986);
8                                       = 32 bytes
G = (X * -0.9689) + (Y *  1.8758) + (Z *  0.0415);
8                                       = 40 bytes
B = (X *  0.0557) + (Y * -0.2040) + (Z *  1.0570);

Second scenario uses:

12 + 8    12 + 8               8                    8 = 56 bytes
RGB[0] = (XYZ[0] *  3.2406) + (XYZ[1] * -1.5372) + (XYZ[2] * -0.4986);
8                                                     = 64 bytes
RGB[1] = (XYZ[0] * -0.9689) + (XYZ[1] *  1.8758) + (XYZ[2] *  0.0415);
8                                                     = 72 bytes
RGB[2] = (XYZ[0] *  0.0557) + (XYZ[1] * -0.2040) + (XYZ[2] *  1.0570);

Referenced: http://www.javamex.com/tutorials/memory/object_memory_usage.shtml

answered Dec 6, 2012 at 17:22

user1877337

2 Comments

GuiRitter Over a year ago

Memory wise, you are right. There's the overhead for the arrays. But I'm most interested in speed.

user1877337 Over a year ago

How about this: docs.oracle.com/javase/1.5.0/docs/guide/language/…

AlexWien · Accepted Answer · 2012-12-06 17:32:30Z

0

You should think one step further:

Where does the x,y,z values come from?

Do you have an x[] an y[] and an z[] array for the coordinates? Where do you iterate?

for (int i = 0; i < len; i++) {
   process(x[i], y[i], z[i];
}

In doubt this would be faster to iterate inside:

public void process(double x[], double y[], double z[]) {

   for (int i = 0; i < len; i++) {
       rbgb[0] = (x[i] * 3.24606 +  y[i] * 1.5372 + z[i] * -0.4986);

    }

}

answered Dec 6, 2012 at 17:32

AlexWien

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1 Comment

GuiRitter Over a year ago

Again I failed to pick an example (: XYZ refers to CIEXYZ, the color space, not coordinates. So the array XYZ refers to a color, and the individual variables to components of the color. And I only work with one color at the time, so the array indexes will always be hard coded.

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