Reading a plain text file in Java

Question

It seems there are different ways to read and write data of files in Java   I want to read ASCII data from a file  What are the possible ways and their differences

User · Answer

The methods within org apache commons io FileUtils may also be very handy  e g           Reads the contents of a file line by line to a List    of Strings using the default encoding for the VM      static List readLines File file

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I don t see it mentioned yet in the other answers so far   But if  Best  means speed  then the new Java I O  NIO  might provide the fastest preformance  but not always the easiest to figure out for someone learning   http   download oracle com javase tutorial essential io file html

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Guava provides a one-liner for this   import com google common base Charsets  import com google common io Files   String contents   Files toString filePath  Charsets UTF 8

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Below is a one-liner of doing it in the Java 8 way  Assuming text txt file is in the root of the project directory of the Eclipse   Files lines Paths get  text txt    collect Collectors toList

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You can use readAllLines and the join method to get whole file content in one line   String str   String join   n  Files readAllLines Paths get  e   text txt        It uses UTF-8 encoding by default  which reads ASCII data correctly   Also you can use readAllBytes   String str   new String Files readAllBytes Paths get  e   text txt     StandardCharsets UTF 8     I think readAllBytes is faster and more precise  because it does not replace new line with  n and also new line may be  r n  It is depending on your needs which one is suitable

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What do you want to do with the text   Is the file small enough to fit into memory   I would try to find the simplest way to handle the file for your needs   The FileUtils library is very handle for this   for String line  FileUtils readLines  my-text-file        System out println line

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The most simple way to read data from a file in Java is making use of the File class to read the file and the Scanner class to read the content of the file   public static void main String args   throws Exception      File f   new File  input txt       takeInputIn2DArray f      public static void takeInputIn2DArray File f  throws Exception       Scanner s   new Scanner f       int a       new int 20  20       for int i 0  i lt 20  i                  for int j 0  j lt 20  j                          a i  j    s nextInt                        PS  Don t forget to import java util    for Scanner to work

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This is basically the exact same as Jesus Ramos  answer  except with File instead of FileReader plus iteration to step through the contents of the file   Scanner in   new Scanner new File  filename txt      while  in hasNext         Iterates each line in the file     String line   in nextLine           Do something with line    in close       Don t forget to close resource leaks       throws FileNotFoundException

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Cactoos give you a declarative one-liner   new TextOf new File  a txt    asString

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This code I programmed is much faster for very large files   public String readDoc File f        String text           int read  N   1024   1024      char   buffer   new char N        try           FileReader fr   new FileReader f           BufferedReader br   new BufferedReader fr            while true                read   br read buffer  0  N               text    new String buffer  0  read                if read  lt  N                    break                                catch Exception ex            ex printStackTrace               return text

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String fileName    yourFileFullNameWithPath   File file   new File fileName      Creates a new file object for your file FileReader fr   new FileReader file     Creates a Reader that you can use to read the contents of a file read your file BufferedReader br   new BufferedReader fr     Reads text from a character-input stream  buffering characters so as to provide for the efficient reading of characters  arrays  and lines    The above set of line can be written into 1 single line as   BufferedReader br   new BufferedReader new FileReader  file txt        Optional   Adding to string builder If you file is huge  it s advised to use string builder else use normal String object   try           StringBuilder sb   new StringBuilder            String line   br readLine             while  line    null            sb append line           sb append System lineSeparator             line   br readLine                      String everything   sb toString              finally           br close

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Probably not as fast as with buffered I O  but quite terse       String content      try  Scanner scanner   new Scanner textFile  useDelimiter    Z              content   scanner next            The  Z pattern tells the Scanner that the delimiter is EOF

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I documented 15 ways to read a file in Java and then tested them for speed with various file sizes - from 1 nbsp KB to 1 nbsp GB and here are the top three ways to do this    java nio file Files readAllBytes    Tested to work in Java 7  8  and 9   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-10KB 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                java io BufferedReader readLine    Tested to work in Java 7  8  9   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-10KB txt       FileReader fileReader   new FileReader fileName        try  BufferedReader bufferedReader   new BufferedReader fileReader           String line        while  line   bufferedReader readLine       null            System out println line                        java nio file Files lines    This was tested to work in Java 8 and 9 but won t work in Java 7 because of the lambda expression requirement   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-10KB txt       File file   new File fileName        try  Stream linesStream   Files lines file toPath             linesStream forEach line - gt            System out println line

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Here s another way to do it without using external libraries   import java io File  import java io FileReader  import java io IOException   public String readFile String filename        String content   null      File file   new File filename      For example  foo txt     FileReader reader   null      try           reader   new FileReader file           char   chars   new char  int  file length             reader read chars           content   new String chars           reader close          catch  IOException e            e printStackTrace          finally           if reader    null               reader close                        return content

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ASCII is a TEXT file so you would use Readers for reading  Java also supports reading from a binary file using InputStreams  If the files being read are huge then you would want to use a BufferedReader on top of a FileReader to improve read performance   Go through this article on how to use a Reader  I d also recommend you download and read this wonderful  yet free  book called Thinking In Java  In Java 7   new String Files readAllBytes          docs   or  Files readAllLines         docs   In Java 8   Files lines     forEach         docs

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Use Java kiss if this is about simplicity of structure   import static kiss API     class App     void run         String line      try  Close in   inOpen  file dat            while   line   readLine       null            println line

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Using BufferedReader   import java io BufferedReader  import java io FileNotFoundException  import java io FileReader  import java io IOException   BufferedReader br  try       br   new BufferedReader new FileReader   fileToRead txt         try           String x          while    x   br readLine       null                    Printing out each line in the file             System out println x                       catch  IOException e            e printStackTrace            catch  FileNotFoundException e        System out println e       e printStackTrace

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This might not be the exact answer to the question  It s just another way of reading a file where you do not explicitly specify the path to your file in your Java code and instead  you read it as a command-line argument   With the following code   import java io BufferedReader  import java io InputStreamReader  import java io IOException   public class InputReader       public static void main String   args throws IOException          BufferedReader br   new BufferedReader new InputStreamReader System in            String s             while  s br readLine     null               System out println s                       just go ahead and run it with   java InputReader  lt  input txt   This would read the contents of the input txt and print it to the your console   You can also make your System out println   to write to a specific file through the command line as follows   java InputReader  lt  input txt  gt  output txt   This would read from input txt and write to output txt

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The buffered stream classes are much more performant in practice  so much so that the NIO 2 API includes methods that specifically return these stream classes  in part to encourage you always to use buffered streams in your application   Here is an example   Path path   Paths get   myfolder myfile ext    try  BufferedReader reader   Files newBufferedReader path            Read from the stream     String currentLine   null      while   currentLine   reader readLine       null            do your code here   catch  IOException e           Handle file I O exception        You can replace this code   BufferedReader reader   Files newBufferedReader path     with   BufferedReader br   new BufferedReader new FileReader   myfolder myfile ext       I recommend this article to learn the main uses of Java NIO and IO

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The most intuitive method is introduced in Java 11 Files readString  import java io    import java nio file Files  import java nio file Paths   public class App       public static void main String args    throws IOException           String content   Files readString Paths get  D   sandbox  mvn  my-app  my-app iml             System out print content             PHP has this luxury from decades ago

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Here are the three working and tested methods   Using BufferedReader  package io  import java io    public class ReadFromFile2       public static void main String   args throws Exception           File file   new File  C   Users  pankaj  Desktop  test java            BufferedReader br   new BufferedReader new FileReader file            String st          while  st br readLine       null               System out println st                       Using Scanner  package io   import java io File  import java util Scanner   public class ReadFromFileUsingScanner       public static void main String   args  throws Exception           File file   new File  C   Users  pankaj  Desktop  test java            Scanner sc   new Scanner file           while sc hasNextLine                 System out println sc nextLine                         Using FileReader  package io  import java io    public class ReadingFromFile        public static void main String   args  throws Exception           FileReader fr   new FileReader  C   Users  pankaj  Desktop  test java            int i          while   i fr read       -1               System out print  char  i                       Read the entire file without a loop using the Scanner class  package io   import java io File  import java io FileNotFoundException  import java util Scanner   public class ReadingEntireFileWithoutLoop        public static void main String   args  throws FileNotFoundException           File file   new File  C   Users  pankaj  Desktop  test java            Scanner sc   new Scanner file           sc useDelimiter    Z            System out println sc next

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try     File f   new File  filename txt      Scanner r   new Scanner f       while  r hasNextLine          String data   r nextLine        JOptionPane showMessageDialog data         r close      catch  FileNotFoundException ex      JOptionPane showMessageDialog  Error occurred      ex printStackTrace

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My favorite way to read a small file is to use a BufferedReader and a StringBuilder  It is very simple and to the point  though not particularly effective  but good enough for most cases    BufferedReader br   new BufferedReader new FileReader  file txt     try       StringBuilder sb   new StringBuilder        String line   br readLine         while  line    null            sb append line           sb append System lineSeparator             line   br readLine              String everything   sb toString      finally       br close        Some has pointed out that after Java 7 you should use try-with-resources  i e  auto close  features   try BufferedReader br   new BufferedReader new FileReader  file txt           StringBuilder sb   new StringBuilder        String line   br readLine         while  line    null            sb append line           sb append System lineSeparator             line   br readLine              String everything   sb toString        When I read strings like this  I usually want to do some string handling per line anyways  so then I go for this implementation   Though if I want to actually just read a file into a String  I always use Apache Commons IO with the class IOUtils toString   method  You can have a look at the source here   http   www docjar com html api org apache commons io IOUtils java html  FileInputStream inputStream   new FileInputStream  foo txt    try       String everything   IOUtils toString inputStream     finally       inputStream close        And even simpler with Java 7   try FileInputStream inputStream   new FileInputStream  foo txt               String everything   IOUtils toString inputStream          do something with everything string

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For JSF-based Maven web applications  just use ClassLoader and the Resources folder to read in any file you want    Put any file you want to read in the Resources folder  Put the Apache Commons IO dependency into your POM    lt dependency gt       lt groupId gt org apache commons lt  groupId gt       lt artifactId gt commons-io lt  artifactId gt       lt version gt 1 3 2 lt  version gt   lt  dependency gt   Use the code below to read it  e g  below is reading in a  json file    String metadata   null  FileInputStream inputStream  try        ClassLoader loader   Thread currentThread   getContextClassLoader        inputStream    FileInputStream  loader              getResourceAsStream   metadata json        metadata   IOUtils toString inputStream       inputStream close      catch  FileNotFoundException e           TODO Auto-generated catch block     e printStackTrace      catch  IOException e           TODO Auto-generated catch block     e printStackTrace      return metadata     You can do the same for text files   properties files  XSD schemas  etc

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The easiest way is to use the Scanner class in Java and the FileReader object  Simple example   Scanner in   new Scanner new FileReader  filename txt       Scanner has several methods for reading in strings  numbers  etc    You can look for more information on this on the Java documentation page   For example reading the whole content into a String   StringBuilder sb   new StringBuilder    while in hasNext          sb append in next       in close    outString   sb toString      Also if you need a specific encoding you can use this instead of FileReader   new InputStreamReader new FileInputStream fileUtf8   StandardCharsets UTF 8

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Here is a simple solution   String content   content   new String Files readAllBytes Paths get  sample txt

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import java util stream Stream  import java nio file    import java io     class ReadFile     public static void main String   args         String filename    Test txt        try Stream lt String gt  stream   Files lines Paths get filename                 stream forEach System out   println          catch  IOException e             e printStackTrace                    Just use java 8 Stream

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I had to benchmark the different ways  I shall comment on my findings but  in short  the fastest way is to use a plain old BufferedInputStream over a FileInputStream  If many files must be read then three threads will reduce the total execution time to roughly half  but adding more threads will progressively degrade performance until making it take three times longer to complete with twenty threads than with just one thread   The assumption is that you must read a file and do something meaningful with its contents  In the examples here is reading lines from a log and count the ones which contain values that exceed a certain threshold  So I am assuming that the one-liner Java 8 Files lines Paths get   path to file txt    map line - gt  line split       is not an option   I tested on Java 1 8  Windows 7 and both SSD and HDD drives   I wrote six different implementations   rawParse  Use BufferedInputStream over a FileInputStream and then cut lines reading byte by byte  This outperformed any other single-thread approach  but it may be very inconvenient for non-ASCII files   lineReaderParse  Use a BufferedReader over a FileReader  read line by line  split lines by calling String split    This is approximatedly 20  slower that rawParse   lineReaderParseParallel  This is the same as lineReaderParse  but it uses several threads  This is the fastest option overall in all cases   nioFilesParse  Use java nio files Files lines    nioAsyncParse  Use an AsynchronousFileChannel with a completion handler and a thread pool   nioMemoryMappedParse  Use a memory-mapped file  This is really a bad idea yielding execution times at least three times longer than any other implementation   These are the average times for reading 204 files of 4 nbsp MB each on an quad-core i7 and SSD drive  The files are generated on the fly to avoid disk caching   rawParse                11 10 sec lineReaderParse         13 86 sec lineReaderParseParallel  6 00 sec nioFilesParse           13 52 sec nioAsyncParse           16 06 sec nioMemoryMappedParse    37 68 sec   I found a difference smaller than I expected between running on an SSD or an HDD drive being the SSD approximately 15  faster  This may be because the files are generated on an unfragmented HDD and they are read sequentially  therefore the spinning drive can perform nearly as an SSD   I was surprised by the low performance of the nioAsyncParse implementation  Either I have implemented something in the wrong way or the multi-thread implementation using NIO and a completion handler performs the same  or even worse  than a single-thread implementation with the java io API  Moreover the asynchronous parse with a CompletionHandler is much longer in lines of code and tricky to implement correctly than a straight implementation on old streams   Now the six implementations followed by a class containing them all plus a parametrizable main   method that allows to play with the number of files  file size and concurrency degree  Note that the size of the files varies plus minus 20   This is to avoid any effect due to all the files being of exactly the same size   rawParse  public void rawParse final String targetDir  final int numberOfFiles  throws IOException  ParseException       overrunCount   0      final int dl    int           StringBuffer lineBuffer   new StringBuffer 1024       for  int f 0  f lt numberOfFiles  f              File fl   new File targetDir filenamePreffix String valueOf f    txt            FileInputStream fin   new FileInputStream fl           BufferedInputStream bin   new BufferedInputStream fin           int character          while  character bin read     -1                if  character  dl                        Here is where something is done with each line                 doSomethingWithRawLine lineBuffer toString                     lineBuffer setLength 0                             else                   lineBuffer append  char  character                                   bin close            fin close             public final void doSomethingWithRawLine String line  throws ParseException          What to do for each line     int fieldNumber   0      final int len   line length        StringBuffer fieldBuffer   new StringBuffer 256       for  int charPos 0  charPos lt len  charPos              char c   line charAt charPos           if  c  DL0                String fieldValue   fieldBuffer toString                if  fieldValue length   gt 0                    switch  fieldNumber                        case 0                          Date dt   fmt parse fieldValue                           fieldNumber                            break                      case 1                          double d   Double parseDouble fieldValue                           fieldNumber                            break                      case 2                          int t   Integer parseInt fieldValue                           fieldNumber                            break                      case 3                          if  fieldValue equals  overrun                                overrunCount                            break                                              fieldBuffer setLength 0                     else               fieldBuffer append c                       lineReaderParse  public void lineReaderParse final String targetDir  final int numberOfFiles  throws IOException  ParseException       String line      for  int f 0  f lt numberOfFiles  f              File fl   new File targetDir filenamePreffix String valueOf f    txt            FileReader frd   new FileReader fl           BufferedReader brd   new BufferedReader frd            while   line brd readLine     null              doSomethingWithLine line           brd close            frd close             public final void doSomethingWithLine String line  throws ParseException          Example of what to do for each line     String   fields   line split           Date dt   fmt parse fields 0        double d   Double parseDouble fields 1        int t   Integer parseInt fields 2        if  fields 3  equals  overrun            overrunCount        lineReaderParseParallel  public void lineReaderParseParallel final String targetDir  final int numberOfFiles  final int degreeOfParalelism  throws IOException  ParseException  InterruptedException       Thread   pool   new Thread degreeOfParalelism       int batchSize   numberOfFiles   degreeOfParalelism      for  int b 0  b lt degreeOfParalelism  b              pool b    new LineReaderParseThread targetDir  b batchSize  b batchSize b batchSize           pool b  start              for  int b 0  b lt degreeOfParalelism  b            pool b  join       class LineReaderParseThread extends Thread        private String targetDir      private int fileFrom      private int fileTo      private DateFormat fmt   new SimpleDateFormat  yyyy-MM-dd HH mm ss        private int overrunCounter   0       public LineReaderParseThread String targetDir  int fileFrom  int fileTo            this targetDir   targetDir          this fileFrom   fileFrom          this fileTo   fileTo             private void doSomethingWithTheLine String line  throws ParseException           String   fields   line split DL           Date dt   fmt parse fields 0            double d   Double parseDouble fields 1            int t   Integer parseInt fields 2            if  fields 3  equals  overrun                overrunCounter                Override     public void run             String line          for  int f fileFrom  f lt fileTo  f                  File fl   new File targetDir filenamePreffix String valueOf f    txt                try               FileReader frd   new FileReader fl               BufferedReader brd   new BufferedReader frd               while   line brd readLine     null                    doSomethingWithTheLine line                             brd close                frd close                  catch  IOException   ParseException ioe                          nioFilesParse  public void nioFilesParse final String targetDir  final int numberOfFiles  throws IOException  ParseException       for  int f 0  f lt numberOfFiles  f              Path ph   Paths get targetDir filenamePreffix String valueOf f    txt            Consumer lt String gt  action   new LineConsumer            Stream lt String gt  lines   Files lines ph           lines forEach action           lines close              class LineConsumer implements Consumer lt String gt          Override     public void accept String line                What to do for each line         String   fields   line split DL           if  fields length gt 1                try                   Date dt   fmt parse fields 0                              catch  ParseException e                              double d   Double parseDouble fields 1                int t   Integer parseInt fields 2                if  fields 3  equals  overrun                    overrunCount                        nioAsyncParse  public void nioAsyncParse final String targetDir  final int numberOfFiles  final int numberOfThreads  final int bufferSize  throws IOException  ParseException  InterruptedException       ScheduledThreadPoolExecutor pool   new ScheduledThreadPoolExecutor numberOfThreads       ConcurrentLinkedQueue lt ByteBuffer gt  byteBuffers   new ConcurrentLinkedQueue lt ByteBuffer gt          for  int b 0  b lt numberOfThreads  b            byteBuffers add ByteBuffer allocate bufferSize         for  int f 0  f lt numberOfFiles  f              consumerThreads acquire            String fileName   targetDir filenamePreffix String valueOf f    txt           AsynchronousFileChannel channel   AsynchronousFileChannel open Paths get fileName   EnumSet of StandardOpenOption READ   pool           BufferConsumer consumer   new BufferConsumer byteBuffers  fileName  bufferSize           channel read consumer buffer    0l  channel  consumer             consumerThreads acquire numberOfThreads       class BufferConsumer implements CompletionHandler lt Integer  AsynchronousFileChannel gt             private ConcurrentLinkedQueue lt ByteBuffer gt  buffers          private ByteBuffer bytes          private String file          private StringBuffer chars          private int limit          private long position          private DateFormat frmt   new SimpleDateFormat  yyyy-MM-dd HH mm ss             public BufferConsumer ConcurrentLinkedQueue lt ByteBuffer gt  byteBuffers  String fileName  int bufferSize                buffers   byteBuffers              bytes   buffers poll                if  bytes  null                  bytes   ByteBuffer allocate bufferSize                file   fileName              chars   new StringBuffer bufferSize               frmt   new SimpleDateFormat  yyyy-MM-dd HH mm ss                limit   bufferSize              position   0l                     public ByteBuffer buffer                 return bytes                      Override         public synchronized void completed Integer result  AsynchronousFileChannel channel                 if  result  -1                    bytes flip                    final int len   bytes limit                    int i   0                  try                       for  i   0  i  lt  len  i                              byte by   bytes get                            if  by    n                                                                       The code used to process the line goes here                             chars setLength 0                                                     else                                   chars append  char  by                                                                                     catch  Exception x                        System out println                           Caught exception     x getClass   getName           x getMessage                               i     String valueOf i       limit     String valueOf len                               position   String valueOf position                                       if  len  limit                        bytes clear                        position    len                      channel read bytes  position  channel  this                                     else                       try                           channel close                                              catch  IOException e                                              consumerThreads release                        bytes clear                        buffers add bytes                                               else                   try                       channel close                                      catch  IOException e                                      consumerThreads release                    bytes clear                    buffers add bytes                                     Override         public void failed Throwable e  AsynchronousFileChannel channel                   FULL RUNNABLE IMPLEMENTATION OF ALL CASES  https   github com sergiomt javaiobenchmark blob master FileReadBenchmark java

[java] Reading a plain text file in Java

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