compareTo does not work properly

Question

I want to sort a input.txt file and save it in output.txt for instance. I use the insertion sort algorithm. Now my problem: the compareTo method seems to work incorrectly (or at least not how I want to to work). It returns integer greater than 1 thus the algorithm does not really especially for negative numbers. I hope you guys can help me with that problem, thanks!

Thats my code:

import java.util.ArrayList;
import java.io.*;

class Isort
{
    public static void main(String[] args)
    {   
        if(args[0].equals("int"))
        {
            ArrayList<Integer> array = new ArrayList<Integer>();
            sort(array, args[1], args[2]);
        }
        else if(args[0].equals("float"))
        {
            ArrayList<Float> array = new ArrayList<Float>();
            sort(array, args[1], args[2]);
        }
        else if(args[0].equals("String"))
        {
            ArrayList<String> array = new ArrayList<String>();
            sort(array, args[1], args[2]);
        }
        else
        {
            //do nothing
        }
    }
    public static <T extends Comparable<T>> void sort(ArrayList<T> array, String input, String output)
    {   
        try
        {
            File file = new File(input);
            BufferedReader reader = new BufferedReader(new FileReader(file));       
            reader.mark((int)file.length() + 1); 
            int count = 0;          
            while(reader.readLine() != null)
            {
                count++;
            }
            reader.reset();
            for(int i = 0; i<count; i++)
            {
                array.add((T)(reader.readLine()));
            }
            reader.close();

            int j;
            T temp;
            for(int i = 1; i < array.size(); i++)
            {
                j = i;
                while(j > 0 && array.get(j-1).compareTo(array.get(j)) > 0)
                {
                    temp = array.get(j);
                    array.set(j,array.get(j-1));
                    array.set(j-1,temp);
                    j -= 1;
                    System.out.println(array);
                }
            }
            PrintWriter writer = new PrintWriter(output);
            for(int i = 0; i<array.size(); i++)
            {
                writer.write(String.valueOf(array.get(i)));
                writer.write(System.getProperty ("line.separator")); 
            }
            writer.flush();
            writer.close();
        }
        catch(FileNotFoundException e)
        {

        }
        catch(IOException e)
        {
            e.printStackTrace();
        }
        catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }
}

Answer 1

I believe you are confused by the use of generics. You are making generic ArrayLists of Integer, Long and String. You are then reading a line of text and attempting to cast it to T.

This will not do anything at runtime due to type-erasure. In all of the cases above (int, long and string) you will be passing an ArrayList<Object> and adding String to the list. When you read the String from the file the cast doesn't do anything except cast it to an Object which String already is. So unless the compareTo of String matches your requirements for int and long this will not work.

In reply to comment...

That's the point. Casting to T or really using generics at all in this case don't do what you need. In all cases you are reading and comparing String. Instead you need to have three methods readInt, readLong and readString and call the appropriate one based on what you are expecting. One option would be to use an interface of readNextValue and pass in an appropriate implementation depending on the situation.

Answer 2

I suggest you to using a "Comparator" class in "Collections.sort(...)" method. You can find an example here-> http://www.vogella.com/blog/2009/08/04/collections-sort-java/.