How to detect first and last line during reader.readLine()?

Question

Hi,

I am reading each line of the file in the below way

BufferedReader in = new BufferedReader(new FileReader(inFile));

while (null != (line = in.readLine())) {


}

I want to do some validation in the first line and last line alone. Is there any way to check if it's a first line and last line inside the while loop

while (null != (line = in.readLine())) {    

    if(firstlineoffile) {
    }

    else if (lastlineoffile) {
    }

    else
    {
    }

}

Answer 1

Hi Arav, if you add a count, and rearrange your code a little, this should work (I haven't tested this so there may be syntax errros):

int count = 0;
String line = in.readLine();
while (line!=null) {
  String currLine = line;
  if(count==0){
     //currLine is the first line
  }

  line = in.readLine();
  if(line==null){
     //currLine is the last line
  }

  if(count>0 && line!=null){
     //do something with lines in between
     System.out.println(currLine);
  }
  count++;
}

Answer 2

String currentLine = in.readLine();
String nextLine = in.readLine();
boolean hasStarted = false;
while(null != currentLine){
    if(!hasStarted){
        //first line.
        hasStarted = true;
    }
    //all your processing here.
    if(null == nextLine){
        //last line, cause there's nothing else coming up
    }
    currentLine = nextLine;
    nextLine = in.readLine();
}

Answer 3

Someone might well come up with a more elegant solution than this, but here we go:

boolean isFirstLine = true;
do{
      String line = in.readLine();
      if(isFirstLine){
         //this is the first line
         isFirstLine = false; 
      }
      else if(line==null){ //previous line was the last line
         in.reset();
         line = in.readLine();
         //do last line specific stuff
         break;
      }
      else {
          //do stuff for lines in between
      }
      in.mark(100);

}while (line!=null);

I haven't tested this, so there might be minor errors. I haven't sorted out exception handling in this code. readLine(), mark() and reset() throw IOException and mark() can also throw IllegalArgumentException

Answer 4

Cool question. I played a bit round it and here's an SSCCE, just copy'n'paste'n'run it.

package com.stackoverflow.q2292917;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.PrintWriter;

public class Test {

    public static void main(String... args) throws IOException {
        // Create test file.
        File file = new File("/test.txt");
        PrintWriter writer = new PrintWriter(file);
        writer.println("line 1");
        writer.println("line 2");
        writer.println("line 3");
        writer.println("line 4");
        writer.println("line 5");
        writer.close();

        // Read test file.
        BufferedReader reader = null;
        try {
            reader = new BufferedReader(new FileReader(file));
            String next, line = reader.readLine();
            for (boolean first = true, last = (line == null); !last; first = false, line = next) {
                last = ((next = reader.readLine()) == null);

                if (first) {
                    System.out.println("first line: " + line);
                } else if (last) {
                    System.out.println("last line: " + line);
                } else {
                    System.out.println("normal line: " + line);
                }
            }
        } finally {
            if (reader != null) try { reader.close(); } catch (IOException logOrIgnore) {}
        }

        // Delete test file.
        file.delete();
    }

}

Output:

first line: line 1
normal line: line 2
normal line: line 3
normal line: line 4
last line: line 5

I however question the readability and interpretability by starters... ;)