How to read a specific line using the specific line number from a file in Java

Question

In Java  is there any method to read a particular line from a file  For example  read line 32 or any other line number

User · Answer

You can also take a look at LineNumberReader  subclass of BufferedReader  Along with the readline method  it also has setter getter methods to access line number  Very useful to keep track of the number of lines read  while reading data from file

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You may try indexed-file-reader  Apache License 2 0   The class IndexedFileReader has a method called readLines int from  int to  which returns a SortedMap whose key is the line number and the value is the line that was read   Example   File file   new File  src test resources file txt    reader   new IndexedFileReader file    lines   reader readLines 6  10   assertNotNull  Null result    lines   assertEquals  Incorrect length    5  lines size     assertTrue  Incorrect value    lines get 6  startsWith   6      assertTrue  Incorrect value    lines get 7  startsWith   7      assertTrue  Incorrect value    lines get 8  startsWith   8      assertTrue  Incorrect value    lines get 9  startsWith   9      assertTrue  Incorrect value    lines get 10  startsWith   10              The above example reads a text file composed of 50 lines in the following format    1  The quick brown fox jumped over the lazy dog ODD  2  The quick brown fox jumped over the lazy dog EVEN   Disclamer  I wrote this library

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Use this code   import java nio file Files   import java nio file Paths   public class FileWork         public static void main String   args  throws IOException            String line   Files readAllLines Paths get  D  abc txt    get 1            System out println line

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Unless you have previous knowledge about the lines in the file  there s no way to directly access the 32nd line without reading the 31 previous lines   That s true for all languages and all modern file systems   So effectively you ll simply read lines until you ve found the 32nd one

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If you are talking about a text file  then there is really no way to do this without reading all the lines that precede it - After all  lines are determined by the presence of a newline  so it has to be read   Use a stream that supports readline  and just read the first X-1 lines and dump the results  then process the next one

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Another way   try  BufferedReader reader   Files newBufferedReader          Paths get  file txt    StandardCharsets UTF 8         List lt String gt  line   reader lines                                  skip 31                                 limit 1                                 collect Collectors toList          line stream   forEach System out  println

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public String readLine int line           FileReader tempFileReader   null          BufferedReader tempBufferedReader   null          try   tempFileReader   new FileReader textFile            tempBufferedReader   new BufferedReader tempFileReader             catch  Exception e              String returnStr    ERROR           for int i   0  i  lt  line - 1  i                 try   tempBufferedReader readLine      catch  Exception e                        try   returnStr   tempBufferedReader readLine       catch  Exception e               return returnStr

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It works for me   I have combined the answer of  Reading a simple text file  But instead of return a String I am returning a LinkedList of Strings  Then I can select the line that I want   public static LinkedList lt String gt  readFromAssets Context context  String filename  throws IOException       BufferedReader reader   new BufferedReader new InputStreamReader context getAssets   open filename         LinkedList lt String gt linkedList   new LinkedList lt  gt            do reading  usually loop until end of file reading     StringBuilder sb   new StringBuilder        String mLine   reader readLine        while  mLine    null            linkedList add mLine           sb append mLine      process line         mLine   reader readLine                reader close        return linkedList

User · Answer

Although as said in other answers  it is not possible to get to the exact line without knowing the offset   pointer   before  So  I ve achieved this by creating an temporary index file which would store the offset values of every line  If the file is small enough  you could just store the indexes   offset   in memory without needing a separate file for it    The offsets can be calculated by using the RandomAccessFile      RandomAccessFile raf   new RandomAccessFile  myFile txt   r           above  r  means open in read only mode     ArrayList lt Integer gt  arrayList   new ArrayList lt Integer gt         String cur line           while  cur line raf readLine     null            arrayList add raf getFilePointer                 Print the 32 line       Seeks the file to the particular location from where our  32  line starts     raf seek raf seek arrayList get 31        System out println raf readLine         raf close      Also visit the java docs for more information  https   docs oracle com javase 8 docs api java io RandomAccessFile html mode  Complexity   This is O n  as it reads the entire file once  Please be aware for the memory requirements  If it s too big to be in memory  then make a temporary file that stores the offsets instead of ArrayList as shown above   Note   If all you want in  32  line  you just have to call the readLine   also available through other classes  32  times  The above approach is useful if you want to get the a specific line  based on line number of course  multiple times   Thanks

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EASY WAY - Reading a line using line number  Let s say Line number starts from 1 till null   public class TextFileAssignmentOct            private void readData int rowNum  BufferedReader br  throws IOException           int n 1                                       Line number starts from 1         String row          while  row br readLine       null              Reads every line             if  n    rowNum                            When Line number matches with which you want to read                 System out println row                             n                                         This increments Line number                      public static void main String   args  throws IOException           File f   new File  quot    JavaPractice FileRead txt quot            FileReader fr   new FileReader f           BufferedReader br   new BufferedReader fr                    TextFileAssignmentOct txf   new TextFileAssignmentOct            txf readData 4  br        Read a Specific Line using Line number and Passing buffered reader

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Not that I know of  but what you could do is loop through the first 31 lines doing nothing using the readline   function of BufferedReader  FileInputStream fs  new FileInputStream  someFile txt    BufferedReader br   new BufferedReader new InputStreamReader fs    for int i   0  i  lt  31    i    br readLine    String lineIWant   br readLine

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Joachim is right on  of course  and an alternate implementation to Chris   for small files only because it loads the entire file  might be to use commons-io from Apache  though arguably you might not want to introduce a new dependency just for this  if you find it useful for other stuff too though  it could make sense     For example   String line32    String  FileUtils readLines file  get 31     http   commons apache org io api-release org apache commons io FileUtils html readLines java io File  java lang String

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you can use the skip   function to skip the lines from begining   public static void readFile String filePath  long lineNum        List lt String gt  list   new ArrayList lt  gt         long totalLines  startLine   0       try  Stream lt String gt  lines   Files lines Paths get filePath              totalLines   Files lines Paths get filePath   count            startLine   totalLines - lineNum             Stream lt String gt  line32   lines skip   startLine  1             list   lines skip startLine  collect Collectors toList                lines forEach list  add         catch  IOException e1               TODO Auto-generated catch block         e1 printStackTrace               list forEach System out  println

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In Java 8   For small files   String line   Files readAllLines Paths get  file txt    get n     For large files   String line  try  Stream lt String gt  lines   Files lines Paths get  file txt           line   lines skip n  findFirst   get        In Java 7  String line  try  BufferedReader br   new BufferedReader new FileReader  file txt           for  int i   0  i  lt  n  i            br readLine        line   br readLine        Source  Reading nth line from file

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They are all wrong I just wrote this in about 10 seconds  With this I managed to just call the object getQuestion  linenumber   in the main method to return whatever line I want   public class Questions    File file   new File  Question2Files triviagame1 txt     public Questions         public String getQuestion int numLine  throws IOException       BufferedReader br   new BufferedReader new FileReader file        String line           for int i   0  i  lt  numLine  i              line   br readLine              return line

User · Answer

No  unless in that file format the line lengths are pre-determined  e g  all lines with a fixed length   you ll have to iterate line by line to count them

User · Answer

For small files   String line32   Files readAllLines Paths get  file txt    get 32    For large files   try  Stream lt String gt  lines   Files lines Paths get  file txt           line32   lines skip 31  findFirst   get

User · Answer

You can use LineNumberReader instead of BufferedReader  Go through the api  You can find setLineNumber and getLineNumber methods

[java] How to read a specific line using the specific line number from a file in Java?

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