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    <p>A File is just a wrapper for the file path. It doesn't matter how big the file is only its file name.</p> <p>When you want to get the size of all the files in a directory, the OS needs to read the directory and then lookup each file to get its size. Each access takes about 10 ms (because that's a typical seek time for a hard drive) So if you have 100,000 file it will take you about 17 minutes to get all their sizes.</p> <p>The only way to speed this up is to get a faster drive. e.g. Solid State Drives have an average seek time of 0.1 ms but it would still take 10 second or more to get the size of 100K files.</p> <p>BTW: The size of each file doesn't matter because it doesn't actually read the file. Only the file entry which has it s size.</p> <hr> <p>EDIT: For example, if I try to get the sizes of a large directory. It is slow at first but much faster once the data is cached.</p> <pre><code>$ time du -s /usr 2911000 /usr real 0m33.532s user 0m0.880s sys 0m5.190s $ time du -s /usr 2911000 /usr real 0m1.181s user 0m0.300s sys 0m0.840s $ find /usr | wc -l 259934 </code></pre> <p>The reason the look up is so fast the fist time is that the files were all installed at once and most of the information is available continuously on disk. Once the information is in memory, it takes next to no time to read the file information.</p> <p>Timing FileUtils.sizeOfDirectory("/usr") take under 8.7 seconds. This is relatively slow compared with the time it takes du, but it is processing around 30K files per second.</p> <p>An alterative might be to run <code>Runtime.exec("du -s "+directory);</code> however, this will only make a few seconds difference at most. Most of the time is likely to be spent waiting for the disk if its not in cache.</p>
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    1. POhow to find out the size of file and directory in java without creating the object?
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    1. USPeter Lawrey
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    1. CO@Peter Lawrey: These numbers don't make sense. I have a standard Dell laptop and I| just asked windows to get me the size of the `Windows` directory - it got me 10.7GB, 57K files + 11K directories and this took less than 30 sec.
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    2. CO@RonK, Caching makes a big difference. It appears to me it was about to get all that information in under 3K access to the disk (a typical laptop drive take 12 ms to do a seek) because the OS often reads in more than it needs to in the hope you will need that information later. e.g. It often reads in say 32KB, even though a typical file entry might be 512 bytes long. This means for every read you have actually loaded 64 entries which may be useful later. If you do exactly the same thing again, it should be much faster.
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    3. COIf you down vote but don't comment, its like says I know what you are doing wrong but I'm keeping to myself. We are all hear to learn, and I appreciate constructive criticism.
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