Java Help

[Nazca]

public class Fraud {

    public static final int ARRSIZE=10000;
    private String [] niNumbers = new String[ARRSIZE];
 
    public Fraud(String file) {
        
        String thisLine;
        
        for (int i = 0; i <= 10000; i++){
            
            try{
                
            BufferedReader br = new BufferedReader (new FileReader (file));
            while ((thisLine = br.readLine()) != null){
                
                niNumbers[i] = thisLine;
                i++;
                System.out.println(thisLine);
            }
            }//end try
            
            catch (IOException e){
                System.err.println("Error: Cannot open " +file +" + e");
                System.exit(-1);
            }//end catch    
        
    
        }//end of for loop    
    } // end of constructor
   
public static void main(String args[]) {
    
        Fraud notts = new Fraud("notts.txt");
        Fraud derbs = new Fraud("derbs.txt");
        Fraud leics = new Fraud("leics.txt");
        
        
        
        String[] duplicates=notts.findDuplicates(derbs);

I'm having a bit of trouble with a bit of coding. I'm trying to compare 3 text files containing a list of strings. I'm trying to find duplicates between these files. Can't work out how to compare the objects once the've been read in.

Can anyone help? Cheers.