Most efficient way of searching very large strings of text in java

I have a question that I have been thinking about recently but I am still not sure of the answer(s) so I would like a broader opinion.

Imagine the scenario where text strings of up to lets say 500,000,000 characters are processed in Java. But each string of text is searched for the words "the", "and" and "yes".

What is the most efficient way of completing this task?