Remove Comment Lines From Input Java File?

I am trying to write a code to remove comment lines from the input java file.

I am getting an error -Exception in thread "main" java.lang.NullPointerException.

Whats wrong in the code. It is showing error- Exception in thread "main" java.lang.NullPointerException in- return new String(buffer),, and the word "String" everywhere present in the program.....

public static void main(String[] args) {String source = readFile("sorce.java");
System.out.println(source.replaceAll("(?:/*(?:[^*]|(?:*+[^*/]))**+/)| (?://.*)",""));
}
static String readFile(String fileName) {
File file = new File(fileName);

[Code] ....