Java Programming

My requirement:

Your loan amount @loanamount is approved.

I wanted to replace @loanamount to 5,00,000. If i try with pdfbox api,i am not able to replace. i am getting empty space in pdf.
If i use same api and if try to replace @loanamount to abcd. now its replacing

replaceFirst("@loanamount","5,00,00"); - not able to replace. getting empty content in pdf

public void replaceString()
{
PDDocument doc = null;
try {
doc = PDDocument.load("D:/DOCS/Letter.pdf"); //Input PDF File Name
List pages = doc.getDocumentCatalog().getAllPages();
for (int i = 0; i < pages.size(); i++)
{
PDPage page = (PDPage) pages.get(i);
PDStream contents = page.getContents();
PDFStreamParser parser = new PDFStreamParser(contents.getStream());
parser.parse();
List tokens = parser.getTokens();
for (int j = 0; j < tokens.size(); j++)
{
Object next = tokens.get(j);
if (next instanceof PDFOperator)
{
PDFOperator op = (PDFOperator) next;
// Tj and TJ are the two operators that display strings in a PDF
if (op.getOperation().equals("Tj"))
{
// Tj takes one operator and that is the string
// to display so lets update that operator
COSString previous = (COSString) tokens.get(j - 1);

System.out.println("COSString::"+previous);

String string = previous.getString();

string = string.replaceFirst("@loanamount", "5,00,000");

//Word you want to change. Currently this code changes word "Solr" to "Solr123"
previous.reset();

previous.append(string.getBytes());
}

PDStream updatedStream = new PDStream(doc);
OutputStream out = updatedStream.createOutputStream();
ContentStreamWriter tokenWriter = new ContentStreamWriter(out);
tokenWriter.writeTokens(tokens);

System.out.println("updatedStream::"+updatedStream.getInputStreamAsString());
page.setContents(updatedStream);

//setItem(COSName.CONTENTS,updatedStream);

}
doc.save("D:/DOCS/loanamt_filled.pdf"); //Output file name

sample content in pdf:
Dear ABC,

Your loan amount @loanamount is approved.

Regards,
DEF

Output:Your loan amount is approved.
I m getting empty spaces instead of @loanamount


Could anyone help me out ?

Please suggest me java open source api to replace caption or content in existing pdf.

Thanks in advance...