How can I read a large text file line by line using Java

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I need to read a large text file of around 5-6 GB line by line using Java   How can I do this quickly

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I usually do the reading routine straightforward    void readResource InputStream source  throws IOException       BufferedReader stream   null      try           stream   new BufferedReader new InputStreamReader source            while  true                String line   stream readLine                if line    null                    break                              process line             System out println line                  finally           closeQuiet stream            static void closeQuiet Closeable closeable        if  closeable    null            try               closeable close              catch  IOException ignore

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For reading a file with Java 8  package com java java8   import java nio file Files  import java nio file Paths  import java util stream Stream          The Class ReadLargeFile         author Ankit Sood Apr 20  2017     public class ReadLargeFile                   The main method                 param args                   the arguments             public static void main String   args            try               Stream lt String gt  stream   Files lines Paths get  C   Users  System  Desktop  demoData txt                 stream forEach System out  println                     catch  Exception e                   TODO Auto-generated catch block             e printStackTrace

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The clear way to achieve this   For example   If you have dataFile txt on your current directory  import java io    import java util Scanner  import java io FileNotFoundException   public class readByLine       public readByLine   throws FileNotFoundException               Scanner linReader   new Scanner new File  dataFile txt              while  linReader hasNext                          String line   linReader nextLine                System out println line                     linReader close                public static void main String args     throws FileNotFoundException               new readByLine              The output like as below

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In Java 8  there is also an alternative to using Files lines    If your input source isn t a file but something more abstract like a Reader or an InputStream  you can stream the lines via the BufferedReaders lines   method   For example   try  BufferedReader reader   new BufferedReader           reader lines   forEach line - gt  processLine line        will call processLine   for each input line read by the BufferedReader

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You need to use the readLine   method in class BufferedReader  Create a new object from that class and operate this method on him and save it to a string   BufferReader Javadoc

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I documented and tested 10 different ways to read a file in Java and then ran them against each other by making them read in test files from 1KB to 1GB  Here are the fastest 3 file reading methods for reading a 1GB test file   Note that when running the performance tests I didn t output anything to the console since that would really slow down the test  I just wanted to test the raw reading speed   1  java nio file Files readAllBytes    Tested in Java 7  8  9  This was overall the fastest method  Reading a 1GB file was consistently just under 1 second   import java io  File  import java io IOException  import java nio file Files   public class ReadFile Files ReadAllBytes     public static void main String    pArgs  throws IOException       String fileName    c   temp  sample-1GB txt       File file   new File fileName        byte    fileBytes   Files readAllBytes file toPath         char singleChar      for byte b   fileBytes          singleChar    char  b        System out print singleChar                 2  java nio file Files lines    This was tested successfully in Java 8 and 9 but it won t work in Java 7 because of the lack of support for lambda expressions  It took about 3 5 seconds to read in a 1GB file which put it in second place as far as reading larger files   import java io File  import java io IOException  import java nio file Files  import java util stream Stream   public class ReadFile Files Lines     public static void main String   pArgs  throws IOException       String fileName    c   temp  sample-1GB txt       File file   new File fileName        try  Stream linesStream   Files lines file toPath             linesStream forEach line - gt            System out println line                           3  BufferedReader  Tested to work in Java 7  8  9  This took about 4 5 seconds to read in a 1GB test file   import java io BufferedReader  import java io FileReader  import java io IOException   public class ReadFile BufferedReader ReadLine     public static void main String    args  throws IOException       String fileName    c   temp  sample-1GB txt       FileReader fileReader   new FileReader fileName        try  BufferedReader bufferedReader   new BufferedReader fileReader           String line        while  line   bufferedReader readLine       null            System out println line                       You can find the complete rankings for all 10 file reading methods here

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You can read file data line by line as below  String fileLoc    quot fileLocationInTheDisk quot    List lt String gt  lines   Files lines Path of fileLoc   StandardCharsets UTF 8  collect Collectors toList

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You can use streams to do it more precisely   Files lines Paths get  input txt    forEach s - gt  stringBuffer append s

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In Java 7   String folderPath    C  folderOfMyFile   Path path   Paths get folderPath   myFileName csv      or any text file eg   txt  bat  etc Charset charset   Charset forName  UTF-8     try  BufferedReader reader   Files newBufferedReader path   charset       while   line   reader readLine       null           separate all csv fields into string array     String   lineVariables   line split              catch  IOException e        System err println e

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You can use this code   import java io BufferedReader  import java io File  import java io FileReader  import java io IOException   public class ReadTextFile        public static void main String   args  throws IOException            try                File f   new File  src com data txt                 BufferedReader b   new BufferedReader new FileReader f                 String readLine                    System out println  Reading file using Buffered Reader                 while   readLine   b readLine       null                    System out println readLine                            catch  IOException e                e printStackTrace

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A common pattern is to use  try  BufferedReader br   new BufferedReader new FileReader file          String line      while   line   br readLine       null              process the line            You can read the data faster if you assume there is no character encoding  e g  ASCII-7 but it won t make much difference   It is highly likely that what you do with the data will take much longer   EDIT  A less common pattern to use which avoids the scope of line leaking   try BufferedReader br   new BufferedReader new FileReader file          for String line   line   br readLine       null                 process the line               line is not visible here        UPDATE  In Java 8 you can do  try  Stream lt String gt  stream   Files lines Paths get fileName              stream forEach System out  println       NOTE  You have to place the Stream in a try-with-resource block to ensure the  close method is called on it  otherwise the underlying file handle is never closed until GC does it much later

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You can also use Apache Commons IO   File file   new File   home user file txt    try       List lt String gt  lines   FileUtils readLines file     catch  IOException e           TODO Auto-generated catch block     e printStackTrace

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BufferedReader br  FileInputStream fin  try       fin   new FileInputStream fileName       br   new BufferedReader new InputStreamReader fin           Path pathToFile   Paths get fileName       br   Files newBufferedReader pathToFile StandardCharsets US ASCII          String line   br readLine        while  line    null            String   attributes   line split               Movie movie   createMovie attributes           movies add movie           line   br readLine              fin close        br close      catch  FileNotFoundException e        System out println  Your Message      catch  IOException e        System out println  Your Message        It works for me  Hope It will help you too

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By using the org apache commons io package  it gave more performance  especially in legacy code which uses Java 6 and below   Java 7 has a better API with fewer exceptions handling and more useful methods   LineIterator lineIterator   null  try       lineIterator   FileUtils lineIterator new File   home username m log     windows-1256       The second parameter is optionnal     while  lineIterator hasNext              String currentLine   lineIterator next               Some operation         finally       LineIterator closeQuietly lineIterator       Maven   lt  -- https   mvnrepository com artifact commons-io commons-io -- gt   lt dependency gt       lt groupId gt commons-io lt  groupId gt       lt artifactId gt commons-io lt  artifactId gt       lt version gt 2 6 lt  version gt   lt  dependency gt

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Once Java 8 is out  March 2014  you ll be able to use streams   try  Stream lt String gt  lines   Files lines Paths get filename   Charset defaultCharset         lines forEachOrdered line - gt  process line        Printing all the lines in the file   try  Stream lt String gt  lines   Files lines file  Charset defaultCharset         lines forEachOrdered System out  println

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In Java 8  you could do   try  Stream lt String gt  lines   Files lines  file  StandardCharsets UTF 8         for  String line    Iterable lt String gt   lines  iterator                            Some notes  The stream returned by Files lines  unlike most streams  needs to be closed  For the reasons mentioned here I avoid using forEach    The strange code  Iterable lt String gt   lines  iterator casts a Stream to an Iterable

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Look at this blog    Java Read File Line by Line - Java Tutorial      The buffer size may be specified  or   the default size may be used  The   default is large enough for most   purposes       Open the file FileInputStream fstream   new FileInputStream  textfile txt    BufferedReader br   new BufferedReader new InputStreamReader fstream     String strLine     Read File Line By Line while   strLine   br readLine       null           Print the content on the console   System out println  strLine        Close the input stream fstream close

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What you can do is scan the entire text using Scanner and go through the text line by line  Of course you should import the following   import java io File  import java io FileNotFoundException  import java util Scanner  public static void readText throws FileNotFoundException       Scanner scan   new Scanner new File  samplefilename txt         while scan hasNextLine             String line   scan nextLine              Here you can manipulate the string the way you want           Scanner basically scans all the text  The while loop is used to traverse through the entire text   The  hasNextLine   function is a boolean that returns true if there are still more lines in the text  The  nextLine   function gives you an entire line as a String which you can then use the way you want  Try System out println line  to print the text   Side Note   txt is the file type text

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FileReader won t let you specify the encoding  use InputStreamReaderinstead if you need to specify it   try       BufferedReader br   new BufferedReader new InputStreamReader new FileInputStream filePath    Cp1252                   String line      while   line   br readLine       null               process the line            br close       catch  IOException e        e printStackTrace        If you imported this file from Windows  it might have ANSI encoding  Cp1252   so you have to specify the encoding

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Here is a sample with full error handling and supporting charset specification for pre-Java 7   With Java 7 you can use try-with-resources syntax  which makes the code cleaner   If you just want the default charset you can skip the InputStream and use FileReader   InputStream ins   null     raw byte-stream Reader r   null     cooked reader BufferedReader br   null     buffered for readLine   try       String s      ins   new FileInputStream  textfile txt        r   new InputStreamReader ins   UTF-8       leave charset out for default     br   new BufferedReader r       while   s   br readLine       null            System out println s           catch  Exception e        System err println e getMessage        handle exception   finally       if  br    null    try   br close      catch Throwable t       ensure close happens            if  r    null    try   r close      catch Throwable t       ensure close happens            if  ins    null    try   ins close      catch Throwable t       ensure close happens            Here is the Groovy version  with full error handling   File f   new File  textfile txt    f withReader  UTF-8     br - gt      br eachLine   line - gt          println line

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You can use Scanner class  Scanner sc new Scanner file   sc nextLine

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Java 9  try  Stream lt String gt  stream   Files lines Paths get fileName          stream forEach System out  println

[java] How can I read a large text file line by line using Java?

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