在以下实施频率代码中出现错误

Hello EveryOne我想根据频率对文件的文本进行排序.我在运行时出现错误,内存错误.请尽快给出回复.

import java.io.*; 

import java.util.*;

import java.util.Collections;   

class SortedInAlphaOrder

 {

public static void main(String a[]) throws IOException{

          Scanner input = new Scanner(System.in);

          System.out.println("Enter the filename with path : " );

          //String filename = input.next();

          File f = new File("Ashish.txt");

          BufferedReader br = new BufferedReader(new FileReader(f));

          StringBuffer buffer = new StringBuffer();

          String str =br.readLine();

          while((str!= null)){

          buffer.append(str);

          buffer.append(" ");

          }

          ArrayList list = new ArrayList();


       StringTokenizer st = new StringTokenizer(buffer.toString().toLowerCase());

          while(st.hasMoreTokens()){

                String s = st.nextToken();

                list.add(s);

                }

            HashSet set = new HashSet(list);

            List arrayList = new ArrayList(set);

            Collections.sort(arrayList);

            for(Object ob : arrayList){

                    System.out.println(ob.toString());

                }

            }           }

获取错误OutofMemoryHeap Error.Help me out.