Number of lines in a file in Java

Question

I use huge data files  sometimes I only need to know the number of lines in these files  usually I open them up and read them line by line until I reach the end of the file  I was wondering if there is a smarter way to do that

User · Answer

I tested the above methods for counting lines and here are my observations for Different methods as tested on my system

File Size : 1.6 Gb Methods:

Using Scanner : 35s approx
Using BufferedReader : 5s approx
Using Java 8 : 5s approx
Using LineNumberReader : 5s approx

Moreover Java8 Approach seems quite handy :

Files.lines(Paths.get(filePath), Charset.defaultCharset()).count()
[Return type : long]

User · Answer

Only way to know how many lines there are in file is to count them  You can of course create a metric from your data giving you an average length of one line and then get the file size and divide that with avg  length but that won t be accurate

User · Answer

Scanner with regex   public int getLineCount         Scanner fileScanner   null      int lineCount   0      Pattern lineEndPattern   Pattern compile    m            try           fileScanner   new Scanner new File filename   useDelimiter lineEndPattern           while  fileScanner hasNext                  fileScanner next                  lineCount                    catch FileNotFoundException e            e printStackTrace            return lineCount            fileScanner close        return lineCount      Haven t clocked it

User · Answer

Best Optimized code for multi line files having no newline   n   character at EOF               param filename     return     throws IOException     public static int countLines String filename  throws IOException       int count   0      boolean empty   true      FileInputStream fis   null      InputStream is   null      try           fis   new FileInputStream filename           is   new BufferedInputStream fis           byte   c   new byte 1024           int readChars   0          boolean isLine   false          while   readChars   is read c      -1                empty   false              for  int i   0  i  lt  readChars    i                    if   c i       n                          isLine   false                        count                   else if  isLine  amp  amp  c i       n   amp  amp  c i       r        Case to handle line count where no New Line character present at EOF                     isLine   true                                                    if isLine                 count                 catch IOException e           e printStackTrace         finally           if is    null               is close                          if fis    null               fis close                            LOG info  count    count       return  count    0  amp  amp   empty    1   count

User · Answer

On Unix-based systems  use the wc command on the command-line

User · Answer

if you use this   public int countLines String filename  throws IOException       LineNumberReader reader    new LineNumberReader new FileReader filename        int cnt   0      String lineRead           while   lineRead   reader readLine       null          cnt   reader getLineNumber         reader close        return cnt      you cant run to big num rows  likes 100K rows  because return from reader getLineNumber is int  you need long type of data to process maximum rows

User · Answer

On Unix-based systems  use the wc command on the command-line

User · Answer

With java-8  you can use streams   try  Stream lt String gt  lines   Files lines path  Charset defaultCharset         long numOfLines   lines count

User · Answer

if you use this   public int countLines String filename  throws IOException       LineNumberReader reader    new LineNumberReader new FileReader filename        int cnt   0      String lineRead           while   lineRead   reader readLine       null          cnt   reader getLineNumber         reader close        return cnt      you cant run to big num rows  likes 100K rows  because return from reader getLineNumber is int  you need long type of data to process maximum rows

User · Answer

Best Optimized code for multi line files having no newline   n   character at EOF               param filename     return     throws IOException     public static int countLines String filename  throws IOException       int count   0      boolean empty   true      FileInputStream fis   null      InputStream is   null      try           fis   new FileInputStream filename           is   new BufferedInputStream fis           byte   c   new byte 1024           int readChars   0          boolean isLine   false          while   readChars   is read c      -1                empty   false              for  int i   0  i  lt  readChars    i                    if   c i       n                          isLine   false                        count                   else if  isLine  amp  amp  c i       n   amp  amp  c i       r        Case to handle line count where no New Line character present at EOF                     isLine   true                                                    if isLine                 count                 catch IOException e           e printStackTrace         finally           if is    null               is close                          if fis    null               fis close                            LOG info  count    count       return  count    0  amp  amp   empty    1   count

User · Answer

This is the fastest version I have found so far  about 6 times faster than readLines  On a 150MB log file this takes 0 35 seconds  versus 2 40 seconds when using readLines    Just for fun  linux  wc -l command takes 0 15 seconds   public static int countLinesOld String filename  throws IOException       InputStream is   new BufferedInputStream new FileInputStream filename        try           byte   c   new byte 1024           int count   0          int readChars   0          boolean empty   true          while   readChars   is read c      -1                empty   false              for  int i   0  i  lt  readChars    i                    if  c i       n                           count                                                    return  count    0  amp  amp   empty    1   count        finally           is close              EDIT  9 1 2 years later  I have practically no java experience  but anyways I have tried to benchmark this code against the LineNumberReader solution below since it bothered me that nobody did it  It seems that especially for large files my solution is faster  Although it seems to take a few runs until the optimizer does a decent job  I ve played a bit with the code  and have produced a new version that is consistently fastest   public static int countLinesNew String filename  throws IOException       InputStream is   new BufferedInputStream new FileInputStream filename        try           byte   c   new byte 1024            int readChars   is read c           if  readChars    -1                   bail out if nothing to read             return 0                        make it easy for the optimizer to tune this loop         int count   0          while  readChars    1024                for  int i 0  i lt 1024                     if  c i         n                           count                                              readChars   is read c                         count remaining characters         while  readChars    -1                System out println readChars               for  int i 0  i lt readChars    i                    if  c i       n                           count                                              readChars   is read c                      return count    0   1   count        finally           is close              Benchmark resuls for a 1 3GB text file  y axis in seconds  I ve performed 100 runs with the same file  and measured each run with System nanoTime    You can see that countLinesOld has a few outliers  and countLinesNew has none and while it s only a bit faster  the difference is statistically significant  LineNumberReader is clearly slower

User · Answer

The accepted answer has an off by one error for multi line files which don t end in newline  A one line file ending without a newline would return 1  but a two line file ending without a newline would return 1 too  Here s an implementation of the accepted solution which fixes this  The endsWithoutNewLine checks are wasteful for everything but the final read  but should be trivial time wise compared to the overall function   public int count String filename  throws IOException       InputStream is   new BufferedInputStream new FileInputStream filename        try           byte   c   new byte 1024           int count   0          int readChars   0          boolean endsWithoutNewLine   false          while   readChars   is read c      -1                for  int i   0  i  lt  readChars    i                    if  c i       n                         count                            endsWithoutNewLine    c readChars - 1       n                      if endsWithoutNewLine                  count                     return count        finally           is close

User · Answer

This funny solution works really good actually   public static int countLines File input  throws IOException       try  InputStream is   new FileInputStream input             int count   1          for  int aChar   0  aChar    -1 aChar   is read                count    aChar      n    1   0          return count

User · Answer

I know this is an old question  but the accepted solution didn t quite match what I needed it to do  So  I refined it to accept various line terminators  rather than just line feed  and to use a specified character encoding  rather than ISO-8859-n   All in one method  refactor as appropriate    public static long getLinesCount String fileName  String encodingName  throws IOException       long linesCount   0      File file   new File fileName       FileInputStream fileIn   new FileInputStream file       try           Charset encoding   Charset forName encodingName           Reader fileReader   new InputStreamReader fileIn  encoding           int bufferSize   4096          Reader reader   new BufferedReader fileReader  bufferSize           char   buffer   new char bufferSize           int prevChar   -1          int readCount   reader read buffer           while  readCount    -1                for  int i   0  i  lt  readCount  i                      int nextChar   buffer i                   switch  nextChar                        case   r                                The current line is terminated by a carriage return or by a carriage return immediately followed by a line feed                          linesCount                            break                                            case   n                             if  prevChar      r                                    The current line is terminated by a carriage return immediately followed by a line feed                                 The line has already been counted                            else                                  The current line is terminated by a line feed                              linesCount                                                      break                                                          prevChar   nextChar                            readCount   reader read buffer                     if  prevCh    -1                switch  prevCh                    case   r                   case   n                            The last line is terminated by a line terminator                         The last line has already been counted                      break                                    default                           The last line is terminated by end-of-file                      linesCount                                                    finally           fileIn close              return linesCount      This solution is comparable in speed to the accepted solution  about 4  slower in my tests  though timing tests in Java are notoriously unreliable

User · Answer

This is the fastest version I have found so far  about 6 times faster than readLines  On a 150MB log file this takes 0 35 seconds  versus 2 40 seconds when using readLines    Just for fun  linux  wc -l command takes 0 15 seconds   public static int countLinesOld String filename  throws IOException       InputStream is   new BufferedInputStream new FileInputStream filename        try           byte   c   new byte 1024           int count   0          int readChars   0          boolean empty   true          while   readChars   is read c      -1                empty   false              for  int i   0  i  lt  readChars    i                    if  c i       n                           count                                                    return  count    0  amp  amp   empty    1   count        finally           is close              EDIT  9 1 2 years later  I have practically no java experience  but anyways I have tried to benchmark this code against the LineNumberReader solution below since it bothered me that nobody did it  It seems that especially for large files my solution is faster  Although it seems to take a few runs until the optimizer does a decent job  I ve played a bit with the code  and have produced a new version that is consistently fastest   public static int countLinesNew String filename  throws IOException       InputStream is   new BufferedInputStream new FileInputStream filename        try           byte   c   new byte 1024            int readChars   is read c           if  readChars    -1                   bail out if nothing to read             return 0                        make it easy for the optimizer to tune this loop         int count   0          while  readChars    1024                for  int i 0  i lt 1024                     if  c i         n                           count                                              readChars   is read c                         count remaining characters         while  readChars    -1                System out println readChars               for  int i 0  i lt readChars    i                    if  c i       n                           count                                              readChars   is read c                      return count    0   1   count        finally           is close              Benchmark resuls for a 1 3GB text file  y axis in seconds  I ve performed 100 runs with the same file  and measured each run with System nanoTime    You can see that countLinesOld has a few outliers  and countLinesNew has none and while it s only a bit faster  the difference is statistically significant  LineNumberReader is clearly slower

User · Answer

If you don t have any index structures  you ll not get around the reading of the complete file  But you can optimize it by avoiding to read it line by line and use a regex to match all line terminators

User · Answer

I concluded that wc -l s method of counting newlines is fine but returns non-intuitive results on files where the last line doesn t end with a newline    And  er vikas solution based on LineNumberReader but adding one to the line count returned non-intuitive results on files where the last line does end with newline    I therefore made an algo which handles as follows    Test public void empty   throws IOException       assertEquals 0  count           Test public void singleNewline   throws IOException       assertEquals 1  count   n         Test public void dataWithoutNewline   throws IOException       assertEquals 1  count  one         Test public void oneCompleteLine   throws IOException       assertEquals 1  count  one n         Test public void twoCompleteLines   throws IOException       assertEquals 2  count  one ntwo n         Test public void twoLinesWithoutNewlineAtEnd   throws IOException       assertEquals 2  count  one ntwo         Test public void aFewLines   throws IOException       assertEquals 5  count  one ntwo nthree nfour nfive n         And it looks like this   static long countLines InputStream is  throws IOException       try LineNumberReader lnr   new LineNumberReader new InputStreamReader is              char   buf   new char 8192           int n  previousN   -1            Read will return at least one byte  no need to buffer more         while  n   lnr read buf      -1                previousN   n                    int ln   lnr getLineNumber            if  previousN    -1                  No data read at all  i e file was empty             return 0            else               char lastChar   buf previousN - 1               if  lastChar      n     lastChar      r                       Ending with newline  deduct one                 return ln                                    normal case  return line number   1         return ln   1            If you want intuitive results  you may use this  If you just want wc -l compatibility  simple use  er vikas solution  but don t add one to the result and retry the skip   try LineNumberReader lnr   new LineNumberReader new FileReader new File  File1            while lnr skip Long MAX VALUE   gt  0         return lnr getLineNumber

User · Answer

This is the fastest version I have found so far  about 6 times faster than readLines  On a 150MB log file this takes 0 35 seconds  versus 2 40 seconds when using readLines    Just for fun  linux  wc -l command takes 0 15 seconds   public static int countLinesOld String filename  throws IOException       InputStream is   new BufferedInputStream new FileInputStream filename        try           byte   c   new byte 1024           int count   0          int readChars   0          boolean empty   true          while   readChars   is read c      -1                empty   false              for  int i   0  i  lt  readChars    i                    if  c i       n                           count                                                    return  count    0  amp  amp   empty    1   count        finally           is close              EDIT  9 1 2 years later  I have practically no java experience  but anyways I have tried to benchmark this code against the LineNumberReader solution below since it bothered me that nobody did it  It seems that especially for large files my solution is faster  Although it seems to take a few runs until the optimizer does a decent job  I ve played a bit with the code  and have produced a new version that is consistently fastest   public static int countLinesNew String filename  throws IOException       InputStream is   new BufferedInputStream new FileInputStream filename        try           byte   c   new byte 1024            int readChars   is read c           if  readChars    -1                   bail out if nothing to read             return 0                        make it easy for the optimizer to tune this loop         int count   0          while  readChars    1024                for  int i 0  i lt 1024                     if  c i         n                           count                                              readChars   is read c                         count remaining characters         while  readChars    -1                System out println readChars               for  int i 0  i lt readChars    i                    if  c i       n                           count                                              readChars   is read c                      return count    0   1   count        finally           is close              Benchmark resuls for a 1 3GB text file  y axis in seconds  I ve performed 100 runs with the same file  and measured each run with System nanoTime    You can see that countLinesOld has a few outliers  and countLinesNew has none and while it s only a bit faster  the difference is statistically significant  LineNumberReader is clearly slower

User · Answer

On Unix-based systems  use the wc command on the command-line

User · Answer

A straight-forward way using Scanner  static void lineCounter  String path  throws IOException            int lineCount   0  commentsCount   0           Scanner input   new Scanner new File path            while  input hasNextLine                  String data   input nextLine                 if  data startsWith        commentsCount                 lineCount                       System out println  Line Count      lineCount     t Comments Count      commentsCount

User · Answer

Count file rows         param file file     return file row count     throws IOException     public static long getLineCount File file  throws IOException        try  Stream lt String gt  lines   Files lines file toPath               return lines count              Tested on JDK8 u31  But indeed performance is slow compared to this method          Count file rows         param file file     return file row count     throws IOException     public static long getLineCount File file  throws IOException        try  BufferedInputStream is   new BufferedInputStream new FileInputStream file   1024              byte   c   new byte 1024           boolean empty   true                  lastEmpty   false          long count   0          int read          while   read   is read c      -1                for  int i   0  i  lt  read  i                      if  c i       n                         count                        lastEmpty   true                    else if  lastEmpty                        lastEmpty   false                                              empty   false                     if   empty                if  count    0                    count   1                else if   lastEmpty                    count                                     return count            Tested and very fast

User · Answer

I have implemented another solution to the problem  I found it more efficient in counting rows   try      FileReader       input   new FileReader  input txt       LineNumberReader count   new LineNumberReader input          while  count skip Long MAX VALUE   gt  0                Loop just in case the file is  gt  Long MAX VALUE or skip   decides to not read the entire file          result   count getLineNumber     1                                         1 because line index starts at 0

User · Answer

With java-8  you can use streams   try  Stream lt String gt  lines   Files lines path  Charset defaultCharset         long numOfLines   lines count

User · Answer

The accepted answer has an off by one error for multi line files which don t end in newline  A one line file ending without a newline would return 1  but a two line file ending without a newline would return 1 too  Here s an implementation of the accepted solution which fixes this  The endsWithoutNewLine checks are wasteful for everything but the final read  but should be trivial time wise compared to the overall function   public int count String filename  throws IOException       InputStream is   new BufferedInputStream new FileInputStream filename        try           byte   c   new byte 1024           int count   0          int readChars   0          boolean endsWithoutNewLine   false          while   readChars   is read c      -1                for  int i   0  i  lt  readChars    i                    if  c i       n                         count                            endsWithoutNewLine    c readChars - 1       n                      if endsWithoutNewLine                  count                     return count        finally           is close

User · Answer

Scanner with regex   public int getLineCount         Scanner fileScanner   null      int lineCount   0      Pattern lineEndPattern   Pattern compile    m            try           fileScanner   new Scanner new File filename   useDelimiter lineEndPattern           while  fileScanner hasNext                  fileScanner next                  lineCount                    catch FileNotFoundException e            e printStackTrace            return lineCount            fileScanner close        return lineCount      Haven t clocked it

User · Answer

This funny solution works really good actually   public static int countLines File input  throws IOException       try  InputStream is   new FileInputStream input             int count   1          for  int aChar   0  aChar    -1 aChar   is read                count    aChar      n    1   0          return count

User · Answer

Only way to know how many lines there are in file is to count them  You can of course create a metric from your data giving you an average length of one line and then get the file size and divide that with avg  length but that won t be accurate

User · Answer

If you don t have any index structures  you ll not get around the reading of the complete file  But you can optimize it by avoiding to read it line by line and use a regex to match all line terminators

User · Answer

How about using the Process class from within Java code  And then reading the output of the command   Process p   Runtime getRuntime   exec  wc -l     yourfilename   p waitFor     BufferedReader b   new BufferedReader new InputStreamReader p getInputStream      String line       int lineCount   0  while   line   b readLine       null        System out println line       lineCount   Integer parseInt line       Need to try it though  Will post the results

User · Answer

If you don t have any index structures  you ll not get around the reading of the complete file  But you can optimize it by avoiding to read it line by line and use a regex to match all line terminators

User · Answer

I have implemented another solution to the problem  I found it more efficient in counting rows   try      FileReader       input   new FileReader  input txt       LineNumberReader count   new LineNumberReader input          while  count skip Long MAX VALUE   gt  0                Loop just in case the file is  gt  Long MAX VALUE or skip   decides to not read the entire file          result   count getLineNumber     1                                         1 because line index starts at 0

User · Answer

The answer with the method count   above gave me line miscounts if a file didn t have a newline at the end of the file - it failed to count the last line in the file   This method works better for me   public int countLines String filename  throws IOException       LineNumberReader reader    new LineNumberReader new FileReader filename    int cnt   0  String lineRead       while   lineRead   reader readLine       null      cnt   reader getLineNumber     reader close    return cnt

User · Answer

It seems that there are a few different approaches you can take with LineNumberReader  I did this  int lines   0   FileReader input   new FileReader fileLocation   LineNumberReader count   new LineNumberReader input    String line   count readLine     if count ready          while line    null            lines   count getLineNumber            line   count readLine                   lines  1         count close     System out println lines    Even more simply  you can use the Java BufferedReader lines   Method to return a stream of the elements  and then use the Stream count   method to count all of the elements  Then simply add one to the output to get the number of rows in the text file  As example  FileReader input   new FileReader fileLocation   LineNumberReader count   new LineNumberReader input    int lines    int count lines   count     1       count close     System out println lines

User · Answer

I know this is an old question  but the accepted solution didn t quite match what I needed it to do  So  I refined it to accept various line terminators  rather than just line feed  and to use a specified character encoding  rather than ISO-8859-n   All in one method  refactor as appropriate    public static long getLinesCount String fileName  String encodingName  throws IOException       long linesCount   0      File file   new File fileName       FileInputStream fileIn   new FileInputStream file       try           Charset encoding   Charset forName encodingName           Reader fileReader   new InputStreamReader fileIn  encoding           int bufferSize   4096          Reader reader   new BufferedReader fileReader  bufferSize           char   buffer   new char bufferSize           int prevChar   -1          int readCount   reader read buffer           while  readCount    -1                for  int i   0  i  lt  readCount  i                      int nextChar   buffer i                   switch  nextChar                        case   r                                The current line is terminated by a carriage return or by a carriage return immediately followed by a line feed                          linesCount                            break                                            case   n                             if  prevChar      r                                    The current line is terminated by a carriage return immediately followed by a line feed                                 The line has already been counted                            else                                  The current line is terminated by a line feed                              linesCount                                                      break                                                          prevChar   nextChar                            readCount   reader read buffer                     if  prevCh    -1                switch  prevCh                    case   r                   case   n                            The last line is terminated by a line terminator                         The last line has already been counted                      break                                    default                           The last line is terminated by end-of-file                      linesCount                                                    finally           fileIn close              return linesCount      This solution is comparable in speed to the accepted solution  about 4  slower in my tests  though timing tests in Java are notoriously unreliable

User · Answer

On Unix-based systems  use the wc command on the command-line

User · Answer

Only way to know how many lines there are in file is to count them  You can of course create a metric from your data giving you an average length of one line and then get the file size and divide that with avg  length but that won t be accurate

User · Answer

The answer with the method count   above gave me line miscounts if a file didn t have a newline at the end of the file - it failed to count the last line in the file   This method works better for me   public int countLines String filename  throws IOException       LineNumberReader reader    new LineNumberReader new FileReader filename    int cnt   0  String lineRead       while   lineRead   reader readLine       null      cnt   reader getLineNumber     reader close    return cnt

User · Answer

It seems that there are a few different approaches you can take with LineNumberReader  I did this  int lines   0   FileReader input   new FileReader fileLocation   LineNumberReader count   new LineNumberReader input    String line   count readLine     if count ready          while line    null            lines   count getLineNumber            line   count readLine                   lines  1         count close     System out println lines    Even more simply  you can use the Java BufferedReader lines   Method to return a stream of the elements  and then use the Stream count   method to count all of the elements  Then simply add one to the output to get the number of rows in the text file  As example  FileReader input   new FileReader fileLocation   LineNumberReader count   new LineNumberReader input    int lines    int count lines   count     1       count close     System out println lines

User · Answer

How about using the Process class from within Java code  And then reading the output of the command   Process p   Runtime getRuntime   exec  wc -l     yourfilename   p waitFor     BufferedReader b   new BufferedReader new InputStreamReader p getInputStream      String line       int lineCount   0  while   line   b readLine       null        System out println line       lineCount   Integer parseInt line       Need to try it though  Will post the results

User · Answer

This is the fastest version I have found so far  about 6 times faster than readLines  On a 150MB log file this takes 0 35 seconds  versus 2 40 seconds when using readLines    Just for fun  linux  wc -l command takes 0 15 seconds   public static int countLinesOld String filename  throws IOException       InputStream is   new BufferedInputStream new FileInputStream filename        try           byte   c   new byte 1024           int count   0          int readChars   0          boolean empty   true          while   readChars   is read c      -1                empty   false              for  int i   0  i  lt  readChars    i                    if  c i       n                           count                                                    return  count    0  amp  amp   empty    1   count        finally           is close              EDIT  9 1 2 years later  I have practically no java experience  but anyways I have tried to benchmark this code against the LineNumberReader solution below since it bothered me that nobody did it  It seems that especially for large files my solution is faster  Although it seems to take a few runs until the optimizer does a decent job  I ve played a bit with the code  and have produced a new version that is consistently fastest   public static int countLinesNew String filename  throws IOException       InputStream is   new BufferedInputStream new FileInputStream filename        try           byte   c   new byte 1024            int readChars   is read c           if  readChars    -1                   bail out if nothing to read             return 0                        make it easy for the optimizer to tune this loop         int count   0          while  readChars    1024                for  int i 0  i lt 1024                     if  c i         n                           count                                              readChars   is read c                         count remaining characters         while  readChars    -1                System out println readChars               for  int i 0  i lt readChars    i                    if  c i       n                           count                                              readChars   is read c                      return count    0   1   count        finally           is close              Benchmark resuls for a 1 3GB text file  y axis in seconds  I ve performed 100 runs with the same file  and measured each run with System nanoTime    You can see that countLinesOld has a few outliers  and countLinesNew has none and while it s only a bit faster  the difference is statistically significant  LineNumberReader is clearly slower

User · Answer

Count file rows         param file file     return file row count     throws IOException     public static long getLineCount File file  throws IOException        try  Stream lt String gt  lines   Files lines file toPath               return lines count              Tested on JDK8 u31  But indeed performance is slow compared to this method          Count file rows         param file file     return file row count     throws IOException     public static long getLineCount File file  throws IOException        try  BufferedInputStream is   new BufferedInputStream new FileInputStream file   1024              byte   c   new byte 1024           boolean empty   true                  lastEmpty   false          long count   0          int read          while   read   is read c      -1                for  int i   0  i  lt  read  i                      if  c i       n                         count                        lastEmpty   true                    else if  lastEmpty                        lastEmpty   false                                              empty   false                     if   empty                if  count    0                    count   1                else if   lastEmpty                    count                                     return count            Tested and very fast

User · Answer

I tested the above methods for counting lines and here are my observations for Different methods as tested on my system

File Size : 1.6 Gb Methods:

Using Scanner : 35s approx
Using BufferedReader : 5s approx
Using Java 8 : 5s approx
Using LineNumberReader : 5s approx

Moreover Java8 Approach seems quite handy :

Files.lines(Paths.get(filePath), Charset.defaultCharset()).count()
[Return type : long]

User · Answer

I concluded that wc -l s method of counting newlines is fine but returns non-intuitive results on files where the last line doesn t end with a newline    And  er vikas solution based on LineNumberReader but adding one to the line count returned non-intuitive results on files where the last line does end with newline    I therefore made an algo which handles as follows    Test public void empty   throws IOException       assertEquals 0  count           Test public void singleNewline   throws IOException       assertEquals 1  count   n         Test public void dataWithoutNewline   throws IOException       assertEquals 1  count  one         Test public void oneCompleteLine   throws IOException       assertEquals 1  count  one n         Test public void twoCompleteLines   throws IOException       assertEquals 2  count  one ntwo n         Test public void twoLinesWithoutNewlineAtEnd   throws IOException       assertEquals 2  count  one ntwo         Test public void aFewLines   throws IOException       assertEquals 5  count  one ntwo nthree nfour nfive n         And it looks like this   static long countLines InputStream is  throws IOException       try LineNumberReader lnr   new LineNumberReader new InputStreamReader is              char   buf   new char 8192           int n  previousN   -1            Read will return at least one byte  no need to buffer more         while  n   lnr read buf      -1                previousN   n                    int ln   lnr getLineNumber            if  previousN    -1                  No data read at all  i e file was empty             return 0            else               char lastChar   buf previousN - 1               if  lastChar      n     lastChar      r                       Ending with newline  deduct one                 return ln                                    normal case  return line number   1         return ln   1            If you want intuitive results  you may use this  If you just want wc -l compatibility  simple use  er vikas solution  but don t add one to the result and retry the skip   try LineNumberReader lnr   new LineNumberReader new FileReader new File  File1            while lnr skip Long MAX VALUE   gt  0         return lnr getLineNumber

User · Answer

Only way to know how many lines there are in file is to count them  You can of course create a metric from your data giving you an average length of one line and then get the file size and divide that with avg  length but that won t be accurate

User · Answer

A straight-forward way using Scanner  static void lineCounter  String path  throws IOException            int lineCount   0  commentsCount   0           Scanner input   new Scanner new File path            while  input hasNextLine                  String data   input nextLine                 if  data startsWith        commentsCount                 lineCount                       System out println  Line Count      lineCount     t Comments Count      commentsCount

User · Answer

If you don t have any index structures  you ll not get around the reading of the complete file  But you can optimize it by avoiding to read it line by line and use a regex to match all line terminators

[java] Number of lines in a file in Java

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