How do I write to File in Java

//Reads an input file containing numbers or strings and sorts them and writes to a new file
import java.io.*;
import java.util.*;

public class FileSort
{
	private Scanner input;
	private Formatter output;
        private List<Integer> list;

	private void openFile(String fileName)
	{
		try{
			input = new Scanner(new File(fileName));
		}

		catch(FileNotFoundException fileNotFoundException)
		{
			System.err.println("Error opening the file. File Not Found");
			System.exit(1);
		}
	}

	private void readFile(String fileName)
	{
		try{
                    list = new ArrayList<Integer>();
                    while(input.hasNext())
                    {
                        list.add(Integer.valueOf(input.next()));
                    }
                    Collections.sort(list);
                    //System.out.printf("\nThe list is: %s",list);
		}
		
		catch(NoSuchElementException elementException)
		{
			System.err.println("File improperly formatted.");
			input.close();
			System.exit(1);
		}
	}

	private void writeFile(String fileName)
	{
		try{
                    output = new Formatter(fileName);
		    output.format("%s", "Hello");
		}
		
		catch(Exception e)
		{
			System.err.println(e.getMessage());
			output.close();
			System.exit(1);
		}
	}
	
	public static void main(String[] args)
	{
		FileSort fs = new FileSort();
		fs.openFile("input.txt");
		fs.readFile("input.txt");
		fs.writeFile("output.txt");
	}
}