Get the value from the file in Java

Question

How do I fetch a value corresponding to i (if i=5, I must get 57.05698808926067) from a text file myFile.txt ? The values may continue till 25000.

0->37.6715587270802
1->40.02056806304368
2->351.65161070935005
3->54.74486689415533
4->86.12063488461266
5->57.05698808926067
6->0.0
7->56.078343612293374

If the the file was like `0=37.6715587270802` it was ideal candidate for Properties file :) — Subir Kumar Sao, Mar 18 '14 at 07:51
@Subir Kumar Sao : property file with 25K of lines will be inefficient to handle just for one line — rkosegi, Mar 18 '14 at 07:53
You can also create Hashmap after reading the values from file and split("->"). Let the key be 0,1,...,25000 and value be the corresponding one. — Jigar, Mar 18 '14 at 08:32

fge · Answer 1 · 2014-03-18T07:56:31.723

Use this code:

// Note: bounds checking left as an exercise
public double getDoubleFromFile(final String filename, final int index)
    throws IOException
{
    final List<String> list = Files.readAllLines(paths.get(filename),
        StandardCharsets.UTF_8);

    return Double.parseDouble(list.get(index));
}

However, this slurps the whole file. Why not, if you have to query several times. Another solution:

public double getDoubleFromFile(final String filename, final int index)
    throws IOException
{
    final Path path = Paths.get(filename);
    int i = 0;
    String line;
    try (
        final BufferedReader reader = Files.newBufferedReader(path,
            StandardCharsets.UTF_8);
    ) {
        while ((line = reader.readLine()) != null) {
            if (index == i)
                return Double.parseDouble(line);
            i++;
        }
        return Double.NaN;
    }
}

score 0 · Answer 2 · answered Mar 18 '14 at 07:50

Read the file line by line and check characters before "->" and parse them to int and compare with i. Then get the values after "->".

You could also just read line by line and increment the value of an index variable every time you read a line and when the index is equal to i, get the string after "->".

Mzf · Answer 3 · 2014-03-18T07:58:17.207

0

This simple way is to read line by line til you get to your line

BufferedReader br = new BufferedReader(new FileReader(file));
String line;
int i=0;
while ((line = br.readLine()) != null) {
   if (i == muLineNumber){
       // process the line. 
       break;
   }

   i++;
}
br.close();

If the size of each line const, you can use BufferedReader.skip

If the file size of small, use FileUtils.readLines

Choose what is best for you

edited Mar 18 '14 at 07:58

answered Mar 18 '14 at 07:51

Mzf

5,210
2
24
37

`FileUtils.readLines()` is basically superseded by Java 7's `Files.readAllLines()` – fge Mar 18 '14 at 08:00
Heh, Java 8 is out today (Thu Mar 18th) and people still don't use the new Java 7 file API ;) It is miles better than `File`. – fge Mar 18 '14 at 08:03
You may want to read [this](http://fgaliegue.blogspot.com/2014/03/working-with-files-java-6-versus-java-7.html), in the hope you will find it useful – fge Mar 18 '14 at 08:05
java 7 highlight : http://stackoverflow.com/questions/213958/new-features-in-java-7 – Mzf Mar 18 '14 at 08:09

score 0 · Answer 4 · answered Mar 18 '14 at 08:03

Since 25'000 lines is not that huge, I would load the whole file in an array, and use i as an index into the array. If, however, I had harsh constraints on memory usage, I would use an RandomAccessFile, set position somewhere around i*average-line-length, find the next '\n', then read the index, if the index is the one that I was seeking, I would read the rest of the line, otherwise, move up if the index is greater or down if it's smaller, and repeat the process.

Get the value from the file in Java

4 Answers4