Search and return lines of text file

I wrote the following code, that found lines of txt file with contains certain word, but it returns only the first or the last line! (However, the System.out.println show me all the lines with the word! ) Where is the problem?!

Thank U :)

"import java.io.*;

public class SearchString {

String RL;

public String SearchString(String pathFile, String str) {

try {

BufferedReader in = new BufferedReader(new FileReader(pathFile));

String line = in.readLine();

while (line != null) {

if (line.contains(str)) {

System.out.println(line);

RL = line;

//return "Lines: "+RL; (if I write it the program copy only the first line) }

line = in.readLine();

}

} catch (IOException ex) { ex.printStackTrace(); }

return "Lines: "+RL;

} } "