Decompressed using LZ77

Hi, I just started my first University and I don't know must of the IT stuff. I am really eager to learn. This code explains on how to Compressed and Decompressed the message using LZ77..

java code

package asslz77;
import java.io.*;
import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
import javax.swing.JOptionPane;
 
public class Main {

   public static void main(String args[]) throws FileNotFoundException, IOException
    {

        String input=null;
        try
        {
        FileInputStream file=new FileInputStream("input.txt");
        DataInputStream data=new DataInputStream(file);
        BufferedReader br=new BufferedReader(new InputStreamReader(data));

        input=br.readLine();
        System.out.println(input);

        FileOutputStream f1=new FileOutputStream("compress.txt");
        DataOutputStream d1=new DataOutputStream(f1);

        int pointer=0,len1=0;
        char o='0',nc='0';
        String w,y=null,x=null;
        int i,j=0,z,l=0;

        for(i=0;i<=input.length();i++)
        {
            w=null;
            w=input.substring(0,i);
            j=i+1;
           do{
               y=input.substring(i,j);
               z=w.lastIndexOf(y);
               if(z!=-1)
               {
                   l=z;
                    j++;

                    if(j>input.length())
                    {
                        i=input.length();//ye break mn elloop elkbera (i)
                        pointer=w.length()-l;
                        len1=y.length();// length of characters copied
                        nc='0';
                        d1.writeInt(pointer);
                        d1.writeInt(len1);
                        d1.writeChar(nc);
                        break;
                    }
               }
               else
               {
                   i=w.length()+(y.length()-1);
                   if(y.length()==1)
                   {
                       pointer=0;
                       len1=0;
                   }
                   else
                   {
                       pointer=w.length()-l;
                       len1=y.length()-1;// length of characters copied

                   }
                   nc=y.charAt(y.length()-1);//next character
                   d1.writeInt(pointer);
                   d1.writeInt(len1);
                   d1.writeChar(nc);
                   break;
               }
              }while(z!=-1);
        }
        d1.close();
        data.close();
        }
        catch (Exception ex)
        {}

        FileInputStream f2=new FileInputStream("compress.txt");
        DataInputStream d2=new DataInputStream(f2);
        BufferedReader b=new BufferedReader(new InputStreamReader(d2));

        FileOutputStream fos=new FileOutputStream("decompress.txt");
        DataOutputStream dos=new DataOutputStream(fos);

        int p=0,len=0;
        char n;
        String decomp = "";
        while(f2.available()!=0)
        {
            p=d2.readInt();//pointer
            System.out.println(p);
            len=d2.readInt();//length
            System.out.println(len);
            n=d2.readChar();//nextcharacter
            System.out.println(n);
            if(p==0)
                decomp+=n;
            else
            {
                if(n!='0')
                decomp+=(decomp.substring((decomp.length()-p),((decomp.length()-p)+len))+n);
                else
                    decomp+=(decomp.substring((decomp.length()-p),((decomp.length()-p)+len)));
            }
        }
        dos.writeChars(decomp);
        System.out.println(decomp);

        b.close();
        dos.close();

    }



}

Input it taken from a file :
input.txt

Code:

aaaaaaaaaaabbbbbbbbbaa


updated.