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Using RandomAccessFile along with BufferedReader to speed up file read


By : a14325
Date : October 16 2020, 06:10 PM
With these it helps The reason for the observed behavior is that, as the name suggests, the BufferedReader is buffered. It reads a larger chunk of data at once (into a buffer), and returns only the relevant parts of the buffer contents - namely, the part up to the next \n line separator.
I think there are, broadly speaking, two possible approaches:
code :
byte buffer[] = new byte[8192];
...
// In a loop:
int read = randomAccessFile.read(buffer);
// Figure out where a line break `\n` appears in the buffer,
// return the resulting lines, and take the position of the `\n`
// into account when storing the "file pointer"
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.RandomAccessFile;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;

public class LargeFileRead {
    public static void main(String[] args) throws Exception {

        String fileName = "myBigFile.txt";

        long before = System.nanoTime();
        List<String> result = readBuffered(fileName);
        //List<String> result = readDefault(fileName);
        long after = System.nanoTime();
        double ms = (after - before) / 1e6;
        System.out.println("Reading took " + ms + "ms "
                + "for " + result.size() + " lines");
    }

    private static List<String> readBuffered(String fileName) throws Exception {
        List<String> lines = new ArrayList<String>();
        RandomAccessFile randomAccessFile = new RandomAccessFile(fileName, "r");
        BufferedReader brRafReader = new BufferedReader(
                new FileReader(randomAccessFile.getFD()));
        String line = null;
        long currentOffset = 0;
        long previousOffset = -1;
        while ((line = brRafReader.readLine()) != null) {
            long fileOffset = randomAccessFile.getFilePointer();
            if (fileOffset != previousOffset) {
                if (previousOffset != -1) {
                    currentOffset = previousOffset;
                }
                previousOffset = fileOffset;
            }
            int bufferOffset = getOffset(brRafReader);
            long realPosition = currentOffset + bufferOffset;
            System.out.println("Position : " + realPosition 
                    + " with FP " + randomAccessFile.getFilePointer()
                    + " and offset " + bufferOffset);
            lines.add(line);
        }
        return lines;
    }

    private static int getOffset(BufferedReader bufferedReader) throws Exception {
        Field field = BufferedReader.class.getDeclaredField("nextChar");
        int result = 0;
        try {
            field.setAccessible(true);
            result = (Integer) field.get(bufferedReader);
        } finally {
            field.setAccessible(false);
        }
        return result;
    }

    private static List<String> readDefault(String fileName) throws Exception {
        List<String> lines = new ArrayList<String>();
        RandomAccessFile randomAccessFile = new RandomAccessFile(fileName, "r");
        String line = null;
        while ((line = randomAccessFile.readLine()) != null) {
            System.out.println("Position : " + randomAccessFile.getFilePointer());
            lines.add(line);
        }
        return lines;
    }
}


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BufferedReader vs. RandomAccessFile in java


By : Mohammad Soltani Del
Date : March 29 2020, 07:55 AM
I hope this helps you . RandomAccessFile.readLine() might be slightly faster because it ignores character encoding. However it doesn't use buffering and still use StringBuffer :P so it could be slower on your system.
BufferedReader.readLine() is preferred because it handles character encoding e.g. UTF-8 or Windows-1252.

How the RandomAccessFile class returns bytes with randomAccessFile.read() method;


By : amit
Date : March 29 2020, 07:55 AM
Any of those help You should use readFully
code :
    try (RandomAccessFile raf = new RandomAccessFile("filename", "r")) {
        byte[] document = new byte[(int) raf.length()];
        raf.readFully(document);
    }
class Test {
    public static void main(String args[]) {
        // here is where chars will be stored. If printed now, will show random junk
        char[] buffer = new char[5];

        // call our method. It does not "return" data.
        // It puts data into an array we already created.
        putCharsInMyBuffer(buffer);

        // prints "hello", even though hello was never "returned"
        System.out.println(buffer);
    }

    static void putCharsInMyBuffer(char[] buffer) {
        buffer[0] = 'h';
        buffer[1] = 'e';
        buffer[2] = 'l';
        buffer[3] = 'l';
        buffer[4] = 'o';
    }
}

Does RandomAccessFile.read() from local file guarantee that exact number of bytes will be read?


By : user3266072
Date : March 29 2020, 07:55 AM
help you fix your problem The read method does not guarantee to read all requested bytes (even if they exist in the file). The actual behavior is going to be dependent on the underlying OS and file system. For example, when backed by NFS, you may be more likely to not get all the requested bytes in a single call.
If you want to guarantee to get all the requested bytes in a single call, you must use readFully.

Android speed BufferedReader read line and ArrayList add and get


By : Shyam Kumar Emandi
Date : March 29 2020, 07:55 AM
wish of those help BufferedReader and ArrayList are not the problems.
Your performance problems basically come down to two issues: allocation and parsing.
code :
    String[] tokens = line.split("[ ]+");
    if (tokens[1].matches("[0-9]+")) {// f: v
    Face face = new Face(vertices.get(Integer.parseInt(tokens[1]) - 1), vertices.get(Integer.parseInt(tokens[2]) - 1), vertices.get(Integer.parseInt(tokens[1]) - 1));
    if (normals.size() > 0) {
        face.setAn(normals.get(Integer.parseInt(tokens[1]) - 1));
        face.setBn(normals.get(Integer.parseInt(tokens[2]) - 1));
        face.setCn(normals.get(Integer.parseInt(tokens[3]) - 1));
    }

RandomAccessFile to read files in a Jar file


By : Nicholas Chan
Date : March 29 2020, 07:55 AM
like below fixes the issue The key information you seem to be missing is that Jar files are compressed, and you can't "seek" because of the compression (which is I believe the DEFLATE algorithm).
However, you could extract the file(s) into temp file(s) on start and then use that. Temp files would be removed on application exit, and are the best answer I can think of.
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