Java Replace a word in a file

import java.io.*;
import java.util.*;
class FileOperations{
    public static void main(String[] args)throws Exception{
        int count=0;
        File f=new File("C:/hello.txt");
        BufferedReader br=new BufferedReader(new FileReader(f));
        String str="",st;
        while((st=br.readLine())!=null){
            str+=st+" ";
        }
        Scanner input=new Scanner(System.in);
        System.out.print("Search Word: ");
        String word=input.next();
        String arr[]=str.split(" ");
        for(int i=0;i<arr.length;i++){
            if(arr[i].equals(word)){
        count++;
        }
        }
        System.out.print("Enter Word: ");
        String newword=input.next();
        System.out.println("The word "+word+" occurs "+count+" times in a file");
        String newstring=str.replace(word,newword);
        BufferedWriter bw=new BufferedWriter(new FileWriter(f));
        bw.write(newstring);
        bw.close();
    }
}

Thank u very much... this work fine....

Thank u very much... this work fine....