How to read odd number of lines from a text file using Java?

Question

I have a text file where each odd line holds an integer number (String ofcourse since its in a text file), even lines has a time. I only want to read the numbers, therefore the odd number of lines from a text file. How do I do that?

import java.io.*; 

public class File { 

BufferedReader in; 
String read; 
int linenum =12;


public File(){ 
try { 
in = new BufferedReader(new FileReader("MAP_allData.txt")); 

for (linenum=0; linenum<20; linenum++){

read = in.readLine();
if(read==null){} 
else{
System.out.println(read);  }
}
in.close(); 
}catch(IOException e){ System.out.println("There was a problem:" + e); 

} 
} 

public static void main(String[] args){ 
File File = new File(); 
} 
}

As of now, it will read all the (odd and even) lines until there is not any more to read from (null)

Since my even number lines is a time stamp like 13:44:23 so can I do something like

if(read==null OR if read includes a time or semi colons ){} else { SOP(read);}

Answer 1

Surely putting a simple in.readLine () just before the close of your for-loop will sort this out?

i.e.:

for (linenum=0; linenum<20; linenum++) {

    read = in.readLine();
    if(read==null){} 
    else{
        System.out.println(read);  
    }
    in.readLine ();
}

Answer 2

Read in all lines, and ignore every other one.

int lineNum = 0;
String line = null;
while ( (line = reader.readLine() ) != null ) {
   lineNum++;
 if ( lineNum % 2 == 0 ) continue;
   //else deal with it
}

Or just call readLine() twice per loop, ignoring the second time, and avoid having a counter (safe since all calls to readLine return null after end-of-stream has been reached).

Edit If efficiency is absolutely key and the date lines had a fixed-length format, you could use skip(15) or similar to efficiently skip the lines you don't care about.

Answer 3

You already have a linecounter, so if you want to use every other line, use the modulo check

if((linenum % 2) == 1){
     System.out.println(read);
}

Answer 4

Just perform something like this each time you read in a line:

String str = read;
char[] all = str.toCharArray();
bool isInt = true;
for(int i = 0; i < all.length;i++) {
    if(!Character.isDigit(all[i])) {
        isInt = false;
    }
    else {
        isInt = true;
    }
}
if isInt {
    //INTEGER LINE
} else {
    //DO SOMETHING WITH THE TIME
}

This makes the code dynamic. If the text file changes, and integers appear on different numbered lines instead, your code can stay the same.