substring() method for linked list object

I'm having some trouble writing a substring() method for a class I'm building called LString. This class creates a linked list object called LString that builds strings. It mimics the String and StringBuilder objects in Java.

substring(int start, int end) creates a new LString out of the given this LString, from the index provided by start to end. It returns type LString.

Here is my code:

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

public class LString    {

     node   front;
     int size;

     //Creating a node class
     private    class   node {
          char data;
          node next;

          public    node (){
          }

          public    node    (char   newData){
                this.data = newData;
          }

          public    node    (char   newData,    node newNext){
                this.data = newData;
                this.next = newNext;
          }


     }
     //Constructors
     public LString(){
          this.size =   0;
          this.front =  null;
     }
     public LString(String original)    {
          this.size = original.length();
          if (original.length() > 0){

              this.front =  new node(original.charAt(0));
              node curr = this.front;

              for   (int i =1; i <  original.length(); i++) {
                    curr.next = new node(original.charAt(i));
                    curr = curr.next;
              }
          }



     }

    //  Length method,  returns the length of LString
     public int length()    {
        return this.size;
    }

    //  compareTo method,   compares    this LString to anotherLString, returns 0   if  equal,
    //  -1  if  lexicogrpahically   less,   and 1   if  lexicographically   greater
    public int compareTo(LString anotherLString)    {
        int len1    = length();
        int len2    = anotherLString.length();
        int lim = Math.min(len1, len2);

        node cn1    = front;
        node cn2    = anotherLString.front;

        int k   = 0;
        while   (k  < lim) {
            char c1 = cn1.data;
            char c2 = cn2.data;
            if  (c1 != c2) {
                return c1-c2;
            }
            k++;
            cn1 =   cn1.next;
            cn2 =   cn2.next;
        }
        return len1 - len2;

    }

    //  a boolean equals method that returns true   if  LString and other   are the same, false if not
    public boolean  equals(Object other)    {
        if  (this   ==  other) {
            return true;
        }
        if  (other instanceof   LString)    {
            LString otherLString    = (LString)other;
            int n   = length();
            if  (n  ==  otherLString.length()) {
                node n1 = front;
                node n2 = otherLString.front;
                while   (n1 != null) {
                    if  (n1.data    !=  n2.data)    {
                        return false;
                    }
                    n1  = n1.next;
                    n2  = n2.next;
                }
                return true;
            }
        }

        return false;
    }


    //  charAt returns  the character of LString at the argument index
    public char charAt(int index)   {

        if  ((index < 0) || (index >= this.length()))   {
            throw   new IndexOutOfBoundsException();
        }
        node curNode =  front;
        for (int    i = 0; i    < this.length(); i++, curNode   = curNode.next) {
            if  (i  ==  index) {
                return curNode.data;
            }
        }
        throw   new IllegalStateException();
    }

    //
    public void setCharAt(int index,    char ch)    {
      if (index < 0 || index >= this.length()) {
         throw new IndexOutOfBoundsException();
      }
      else {
         node currNode = front;
         for (int i = 0; i <this.length(); i++, currNode = currNode.next) {
            if (i == index) {
            currNode.data = ch;
            }
         }
      }
   }

    public LString  substring(int start,    int end)    {
      if (start == end == this.length()) {
           return null;
      }
      if (start < 0 || end > this.length() || start > end) {
         throw new IndexOutOfBoundsException();
      }
      // construct a string out of the characters in this LString?
      // turn string in LString with constructor : LString(String original)?
      //what I have so far:
      node currentNode = front;
      for (int i = start; i < end; i++, currentNode = currentNode.next) {
         currentNode.data = charAt(i);
      }//?


    }
}

I'm just caught up in how to write the method efficiently, and I've been working on this for awhile so I need some fresh eyes. Thanks for any advice, trying to learn Java.