Java How to read a text file

Question

I want to read a text file containing space separated values  Values are integers  How can I read it and put it in an array list   Here is an example of contents of the text file   1 62 4 55 5 6 77   I want to have it in an arraylist as  1  62  4  55  5  6  77   How can I do it in Java

User · Answer

All the answers so far given involve reading the file line by line, taking the line in as a String, and then processing the String.

There is no question that this is the easiest approach to understand, and if the file is fairly short (say, tens of thousands of lines), it'll also be acceptable in terms of efficiency. But if the file is long, it's a very inefficient way to do it, for two reasons:

Every character gets processed twice, once in constructing the String, and once in processing it.
The garbage collector will not be your friend if there are lots of lines in the file. You're constructing a new String for each line, and then throwing it away when you move to the next line. The garbage collector will eventually have to dispose of all these String objects that you don't want any more. Someone's got to clean up after you.

If you care about speed, you are much better off reading a block of data and then processing it byte by byte rather than line by line. Every time you come to the end of a number, you add it to the List you're building.

It will come out something like this:

private List<Integer> readIntegers(File file) throws IOException {
    List<Integer> result = new ArrayList<>();
    RandomAccessFile raf = new RandomAccessFile(file, "r");
    byte buf[] = new byte[16 * 1024];
    final FileChannel ch = raf.getChannel();
    int fileLength = (int) ch.size();
    final MappedByteBuffer mb = ch.map(FileChannel.MapMode.READ_ONLY, 0,
            fileLength);
    int acc = 0;
    while (mb.hasRemaining()) {
        int len = Math.min(mb.remaining(), buf.length);
        mb.get(buf, 0, len);
        for (int i = 0; i < len; i++)
            if ((buf[i] >= 48) && (buf[i] <= 57))
                acc = acc * 10 + buf[i] - 48;
            else {
                result.add(acc);
                acc = 0;
            }
    }
    ch.close();
    raf.close();
    return result;
}

The code above assumes that this is ASCII (though it could be easily tweaked for other encodings), and that anything that isn't a digit (in particular, a space or a newline) represents a boundary between digits. It also assumes that the file ends with a non-digit (in practice, that the last line ends with a newline), though, again, it could be tweaked to deal with the case where it doesn't.

It's much, much faster than any of the String-based approaches also given as answers to this question. There is a detailed investigation of a very similar issue in this question. You'll see there that there's the possibility of improving it still further if you want to go down the multi-threaded line.

User · Answer

Look at this example  and try to do your own   import java io     public class ReadFile        public static void main String   args           String string               String file    textFile txt               Reading         try              InputStream ips   new FileInputStream file               InputStreamReader ipsr   new InputStreamReader ips               BufferedReader br   new BufferedReader ipsr               String line              while   line   br readLine       null                   System out println line                   string    line     n                             br close                      catch  Exception e               System out println e toString                           Writing         try               FileWriter fw   new FileWriter  file               BufferedWriter bw   new BufferedWriter  fw               PrintWriter fileOut   new PrintWriter  bw                   fileOut println  string   n test of read and write                   fileOut close                System out println  the file     file     is created                       catch  Exception e               System out println e toString

User · Answer

Using Java 7 to read files with NIO 2  Import these packages   import java nio charset Charset  import java nio file Files  import java nio file Path  import java nio file Paths    This is the process to read a file   Path file   Paths get  C   Java  file txt     if Files exists file   amp  amp  Files isReadable file          try              File reader         BufferedReader reader   Files newBufferedReader file  Charset defaultCharset              String line             read each line         while  line   reader readLine       null                System out println line                  tokenize each number             StringTokenizer tokenizer   new StringTokenizer line                    while  tokenizer hasMoreElements                         parse each integer in file                 int element   Integer parseInt tokenizer nextToken                                     reader close          catch  Exception e            e printStackTrace              To read all lines of a file at once   Path file   Paths get  C   Java  file txt    List lt String gt  lines   Files readAllLines file  StandardCharsets UTF 8

User · Answer

read the file and then do whatever you want  java8 Files lines Paths get  c   lines txt    collect Collectors toList

User · Answer

This example code shows you how to read file in Java   import java io            This example code shows you how to read file in Java       IN MY CASE RAILWAY IS MY TEXT FILE WHICH I WANT TO DISPLAY YOU CHANGE WITH YOUR   OWN             public class ReadFileExample         public static void main String   args                System out println  Reading File from Java code             Name of the file        String fileName  RAILWAY txt          try               Create object of FileReader           FileReader inputFile   new FileReader fileName                Instantiate the BufferedReader Class           BufferedReader bufferReader   new BufferedReader inputFile                Variable to hold the one line data           String line                Read file line by line and print on the console           while   line   bufferReader readLine       null                  System out println line                           Close the buffer reader           bufferReader close            catch Exception e             System out println  Error while reading file line by line     e getMessage

User · Answer

Use Apache Commons  IO and Lang  for simple common things like this   Imports   import org apache commons io FileUtils  import org apache commons lang3 ArrayUtils    Code   String contents   FileUtils readFileToString new File  path to your file txt     String   array   ArrayUtils toArray contents split          Done

User · Answer

Java 1 5 introduced the Scanner class for handling input from file and streams   It is used for getting integers from a file and would look something like this   List lt Integer gt  integers   new ArrayList lt Integer gt     Scanner fileScanner   new Scanner new File  c   file txt     while  fileScanner hasNextInt        integers add fileScanner nextInt         Check the API though  There are many more options for dealing with different types of input sources  differing delimiters  and differing data types

User · Answer

You can use Files readAllLines   to get all lines of a text file into a List lt String gt    for  String line   Files readAllLines Paths get   path to file txt                      Tutorial  Basic I O   File I O   Reading  Writing and Creating text files    You can use String split   to split a String in parts based on a regular expression   for  String part   line split    s                      Tutorial  Numbers and Strings   Strings   Manipulating Characters in a String    You can use Integer valueOf   to convert a String into an Integer   Integer i   Integer valueOf part     Tutorial  Numbers and Strings   Strings   Converting between Numbers and Strings    You can use List add   to add an element to a List   numbers add i     Tutorial  Interfaces   The List Interface    So  in a nutshell  assuming that the file doesn t have empty lines nor trailing leading whitespace    List lt Integer gt  numbers   new ArrayList lt  gt     for  String line   Files readAllLines Paths get   path to file txt           for  String part   line split    s               Integer i   Integer valueOf part           numbers add i               If you happen to be at Java 8 already  then you can even use Stream API for this  starting with Files lines     List lt Integer gt  numbers   Files lines Paths get   path to test txt         map line - gt  line split    s     flatMap Arrays  stream       map Integer  valueOf       collect Collectors toList       Tutorial  Processing data with Java 8 streams

User · Answer

Just for fun  here s what I d probably do in a real project  where I m already using all my favourite libraries  in this case Guava  formerly known as Google Collections    String text   Files toString new File  textfile txt    Charsets UTF 8   List lt Integer gt  list   Lists newArrayList    for  String s   text split    s          list add Integer valueOf s        Benefit  Not much own code to maintain  contrast with e g  this   Edit  Although it is worth noting that in this case tschaible s Scanner solution doesn t have any more code   Drawback  you obviously may not want to add new library dependencies just for this   Then again  you d be silly not to make use of Guava in your projects   -

[java] Java: How to read a text file

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