How to reduce the time taken for loops?

Question

While I was testing my java program, I found out that the first run in a loop takes more time that the later runs.

Each of loop's task is to make 5 thumbnails of same image and store them in a zip file. I am using zip4j and thumbnailator. All of the runs have the same code.

public static void main(String[] args) throws IOException {
    try {
        for(int i=0;i<10;i++){
            long start = System.currentTimeMillis();
            ZipFile zipFile =  new ZipFile(System.nanoTime()+".zip");
            ZipParameters parameters = new ZipParameters();
            parameters.setCompressionMethod(Zip4jConstants.COMP_STORE);
            parameters.setCompressionLevel(Zip4jConstants.DEFLATE_LEVEL_FASTEST);
            parameters.setIncludeRootFolder(false);
            ArrayList<File> files = new ArrayList<File>();
            for(int j=1;j<5;j++){
                files.add(new File("C:\\savedFile2\\1.jpg"));
            }
            zipFile.createZipFile(files, parameters);


            File zippedFile = zipFile.getFile();
            byte[] buffer = new byte[(int)zippedFile.length()];
            FileInputStream fis = new FileInputStream(zippedFile);
            fis.read(buffer);
            fis.close();
            zippedFile.delete();
            System.out.println("Time taken for "+(i+1)+"th run: "+(System.currentTimeMillis() - start));
        }
    } catch (ZipException e) {
        e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
    }
}

Here's my code.

Time taken for 1th run: 58
Time taken for 2th run: 24
Time taken for 3th run: 24
Time taken for 4th run: 24
Time taken for 5th run: 25
Time taken for 6th run: 24
Time taken for 7th run: 25
Time taken for 8th run: 25
Time taken for 9th run: 25
Time taken for 10th run: 29

As you can see from the result above, the first run from the loop takes twice the time as the rest.

What is happening here? And how can I reduce the time for the first run?

Answer 1

I don't think this is related to looping, rather it is related to the createZipFile() function that seems to do some initializing/loading that runs at first time it is called. Consider the following modified example that is producing identical run times in the loop:

   public static void main(String[] args) throws IOException {
    try {
        long _start = System.currentTimeMillis();
        ZipFile _zipFile =  new ZipFile(System.nanoTime()+".zip");
        ZipParameters _parameters = new ZipParameters();
        _parameters.setCompressionMethod(Zip4jConstants.COMP_STORE);
        _parameters.setCompressionLevel(Zip4jConstants.DEFLATE_LEVEL_FASTEST);
        _parameters.setIncludeRootFolder(false);
        ArrayList<File> _files = new ArrayList<File>();
        for(int j=1;j<5;j++){
            _files.add(new File("1.jpg"));
        }            
          System.out.println("Initializing files: "+(System.currentTimeMillis() - _start));
        _zipFile.createZipFile(_files, _parameters);
           System.out.println("Initial run: "+(System.currentTimeMillis() - _start));
        for(int i=0;i<10;i++){
            long start = System.currentTimeMillis();
            ZipFile zipFile =  new ZipFile(System.nanoTime()+".zip");
            ZipParameters parameters = new ZipParameters();
            parameters.setCompressionMethod(Zip4jConstants.COMP_STORE);
            parameters.setCompressionLevel(Zip4jConstants.DEFLATE_LEVEL_FASTEST);
            parameters.setIncludeRootFolder(false);
            ArrayList<File> files = new ArrayList<File>();
            for(int j=1;j<5;j++){
                files.add(new File("1.jpg"));
            }
            zipFile.createZipFile(files, parameters);


            File zippedFile = zipFile.getFile();
            byte[] buffer = new byte[(int)zippedFile.length()];
            FileInputStream fis = new FileInputStream(zippedFile);
            fis.read(buffer);
            fis.close();
            zippedFile.delete();
               System.out.println("Time taken for "+(i+1)+"tenter code hereh run: "+(System.currentTimeMillis() - start));
        }
    } catch (ZipException e) {
        e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
    }
}