How Can I Reset The File Pointer to the Beginning

Question

I am writing a program in Java that requires me to compare the data in 2 files. I have to check each line from file 1 against each line of file 2 and if I find a match write them to a third file. After I read to the end of file 2, how do I reset the pointer to the beginning of the file?

public class FiFo {
    public static void main(String[] args) 
    {
        FileReader file1=new FileReader("d:\\testfiles\\FILE1.txt");
        FileReader file2=new FileReader("d:\\testfiles\\FILE2.txt");
        try{
            String s1,s2;
            while((s1=file1.data.readLine())!=null){
                System.out.println("s1: "+s1);
                while((s2=file2.data.readLine())!=null){
                    System.out.println("s2: "+s2);
                }
            }
            file1.closeFile();
            file2.closeFile();
        }catch (IOException e) {
            e.printStackTrace();
        }
    }
}

class FileReader {
    BufferedReader data;
    DataInputStream in;

    public FileReader(String fileName)
    {
        try{
            FileInputStream fstream = new FileInputStream(fileName);
            data = new BufferedReader(new InputStreamReader(fstream));
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    } 

    public void closeFile()
    {
        try{
            in.close();
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Answer 1

As noted, there are better algorithms - investigate these

aside:

FileReader doesn't implement mark and reset, so trashgod's comments are inaccurate. You'd either have to implement a version of this (using RandomAccessFile or what not) or wrap in a BufferedReader. However, the latter will load the whole thing in memory if you mark it

Answer 2

As others have suggested, you should consider other approaches to the problem. For the specific question of returning to a previous point in a file, java.io.FileReader inherits mark() and reset() methods that address this goal.

Answer 3

If you can clearly indentify the dimension of your file you can use mark(int readAheadLimit) and reset() from the class BufferedReader. The method mark(int readAhedLimit) add a marker to the current position of your BufferedReader and you can go back to the marker using reset().

Using them you have to be careful to the number of characters to read until the reset(), you have to specify them as the argument of the function mark(int readAhedLimit).

Assuming a limit of 100 characters your code should look like:

class MyFileReader {
    BufferedReader data;
    int maxNumberOfCharacters = 100;

    public MyFileReader(String fileName)
    {
        try{
            FileInputStream fstream = new FileInputStream(fileName);
            data = new BufferedReader(new InputStreamReader(fstream));
            //mark the current position, in this case the beginning of the file
            data.mark(maxNumberOfCharacters);
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void resetFile(){
        data.reset();
    }

    public void closeFile()
    {
        try{
            in.close();
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Answer 4

well, Gennady S. answer is what I would use to solve your problem.

I am writing a program in Java that requires me to compare the data in 2 files

however, I would rather not code this up again.. I would rather use something like http://code.google.com/p/java-diff-utils/