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

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

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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 lt Integer gt  readIntegers File file  throws IOException       List lt Integer gt  result   new ArrayList lt  gt         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  lt  len  i                if   buf i   gt   48   amp  amp   buf i   lt   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

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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   -

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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

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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

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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

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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

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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

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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

[java] Java: How to read a text file

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