Avoid String Allocation while using a LRU Cache

Question

I use a LRU and a disk cache to cache some images in the memory and on the disk. Previously I used

private LruCache<String, Bitmap> mMemoryCache;

to save the images but as I watch the allocation stack I see a lot of String allocations which is clear to me because everytime i use mMemoryCache.get(cs); it will allocate a new String object.

During my drawing of each bitmap i need to use the cache around 50-100 times each frame with will cause a lot of GC because of the short living strings.

How can i prevent this?

I looked at other questions on how to prevent String allocations, like this one and I use the solution by David for the Debug FPS showing, but i cant use it for this special cache version.

I split every image into 100x100 tiles, my Cache keys will look like this pageNumber-withcount-heightcount-zoomfactor

I tried using a StringBuffer but that will not work as toString() will allocate a new String and I need to compare the content. I could try to use a int/float with 000 as replacement for "-" or even work with a char[] array inside my cache class, but what is the best way to do this?

Why not use an Integer as the cache key and calculate it as follows (assuming the zoomfactor is < 100, too): int keyInt = (((pageNumber * 100 + withcount) * 100) + heightcount) * 100 + zoomfactor; Integer key = Integer.valueOf(keyInt); This will still allocate objects, but Integer.valueOf has a cache for some of them already, and the objects will be smaller. — Stefan Haustein
– Stefan Haustein, Commented Apr 27, 2013 at 11:19
That was the way i tried next, to get a number based on the given numbers. That also looks good, it will cause less often the GC to kick in. I think this is a good way, but one question: Why do we convert the int still one more time to int with valueOf? You said because of the internal cache? — Chumper
– Chumper, Commented Apr 27, 2013 at 11:45
Actually Java will call valueOf() automatically when auto-boxing in implicit conversions. I just wanted to point out that one should not use new Integer() to convert from 'int' to 'Integer'. — Stefan Haustein
– Stefan Haustein, Commented Apr 27, 2013 at 11:56
Thanks so far, that was easier than i thought... I use the formular slightly different with a sparse array now and i don't have any problems so far. If you don'T mind, you can answer the question and i will close this then. Thanks — Chumper
– Chumper, Commented Apr 27, 2013 at 12:16

Stefan Haustein · Accepted Answer · 2013-04-27 13:52:25Z

Why not use an Integer as the cache key and calculate it as follows (assuming the zoomfactor is < 100, too):

int keyInt = (((pageNumber * 100 + withcount) * 100) + heightcount) * 100 + zoomfactor; Integer key = Integer.valueOf(keyInt);

This will still allocate objects, but Integer.valueOf has a cache for some of them already, and the objects will be smaller. An auto-boxing will also use Integer.valueOf (which doesn't create a new instance for small numbers), just avoid new Integer().

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