String In Java - Search Text On Document For A Word

Consider in a Document if a String " Hello" is Encoded and stored as "XYZAB"

I want to search the text on document for a word "Hello" and Replace the word with "HelloWorld"

The Program will encrypt the word "Hello" and Search the file then return the encrypted code as "XYZAB" Found

Now i have to replace the word "Hello" with "HelloWorld" in encrypted form so that the Letter "XYZABEFGHI" is replace in the place of Hello where "World" is encoded as "EFGHI"

Now the Problem is If there is more number of occurrence of the word "Helloworld" exist in the file... How can i Replace only one particular occurrence What can be done to select the particular occurrence.

I have attached my java program for Encryption along with this mail for your ease of use.