Read several files at the same time Java

Question

I would like to read several files at the same time in Java. I have for example 5 files, and each file has 50 lines (but I cann't know it in advance).

I would like to read the line number 1 on each file, then the line number 2 on each file etc..

How can I do it ?

I have a BufferedReader array like :

BufferedReader[] readers = new BufferedReader[x];

You could use 5 threads. But if you do that to speed things up, all 5 files should be on different drives otherwise it's probably gonna be slower instead of faster. — Tesseract, Apr 14 '17 at 20:02
I don't want to use threads.. Is it not possible to do this with for and while loops ? — iAmoric, Apr 14 '17 at 20:04
Why do you need to read them at the same time? And how fast do you need to read them? — Tesseract, Apr 14 '17 at 20:05
Fill your array with reader responsible for each file, then iterate over them and make them read one line. Repeat until there is no more lines to read. — Pshemo, Apr 14 '17 at 20:06
@SpiderPig, I need to concatenate information of each file at the same line. The speed is not important. — iAmoric, Apr 14 '17 at 20:09

davidxxx · Accepted Answer · 2017-04-14T20:24:31.153

3

You may use a loop to iterate while you get a new line from the readers.
Inside the loop, add a nested loop to iterate over the readers. In this way, you can read the line matching to the same line number of each BufferedReader.

Here is the idea :

BufferedReader[] readers = new BufferedReader[5]; 
readers[0] = new BufferedReader(new FileReader("...));
...
readers[4] = new BufferedReader(new FileReader("...));


boolean noMoreLine = false;
while (!noMoreLine)       

   for (BufferedReader reader : readers){
        String line = reader.readLine();
        if (line == null){
           noMoreLine = true;
           System.out.println("no more line");
           break;
        }
        System.out.println("Line " + i + " = line);
       }
    }

}

edited Apr 14 '17 at 20:24

answered Apr 14 '17 at 20:07

davidxxx

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But how do I know when to stop if I don't know the number of line ? (10 in your example) – iAmoric Apr 14 '17 at 20:10
You check `line` value after `readLine()`. I was updated the code . – davidxxx Apr 14 '17 at 20:10
Yes it is ok. I will try this ! – iAmoric Apr 14 '17 at 20:11

score 1 · Answer 2 · edited May 23 '17 at 12:02

If I understand correctly, you have an array set up, with one buffered reader for each file you need to read. If you want to read the first line of every file, the the second of every file, etc., you need to stop when you've reached the last line of the longest file.

You can get this value by finding the number of lines in each file, and saving the maximum. Number of lines in a file in Java might be useful.

Then, write a for loop:

for (int i = 0; i < longestFileLength; i++) {
    String fileLine1 = readers[1].readLine();
    String fileLine2 = readers[2].readLine();
    //...
    String fileLine99 = readers[99].readLine();
}

score 1 · Answer 3 · answered Apr 14 '17 at 20:16

1

I'd use an ArrayList rather than an array.

ArrayList<BufferedReader> readers = new ArrayList<>();
...
while(!readers.isEmpty()) {
  for(BufferedReader reader: readers) {
    String line = reader.readLine();
    if(line == null) {
      readers.remove(reader);
    } else {
      ...
    }
  }
}

answered Apr 14 '17 at 20:16

Tesseract

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Yes, this solution seems good. I will try it ! – iAmoric Apr 14 '17 at 20:18
1

You shouldn't modify collection if you are iterating over it using enhanced for loop. Use `Iterator#remove` instead. – Pshemo Apr 14 '17 at 20:40

score 1 · Answer 4 · edited Aug 02 '18 at 15:48

1

You could use streams:

while(Arrays.stream(readers).filter(br -> br.ready()).count()>0)
    Arrays.stream(readers).map(br -> br.readLine()).toArray()

edited Aug 02 '18 at 15:48

jcaron

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answered Apr 14 '17 at 20:23

leoconco

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Read several files at the same time Java

4 Answers4