Regular expression in java in String class

Hello everyone,

I'm in a middle of confusion now. I'm trying to use the String java class to replace all occurences of word in sub to word in put. The problem is, this doesn't work:

String sub = "Hello";
String put = "Hi";

line.replaceAll(sub, put);

I've been searching through google and other stuff, but I still couldn't understand what went wrong, please help!