I would recommend a BufferedReader as shown here: http://www.coderslexicon.com/search-a-text-file-in-java/

I think this cousera course would provide a good background: https://www.coursera.org/learn/text-retrieval

Are those 5 words decided previously? Or these words are what you need to find in the text (that is, you do not know yet what words will appear). If the case is the former, I think you deal with regular expression (regex). I am not sure if there is any Java regex library, but I think it indeed exists in Java.

You surely can extract the keywords, but normally, the concepts you look for in the text can be represented in various forms (both semantically and morphologically). So the best choice is to do some investigation of the language processing and machine learning techniques. You can definitely can code that in Java. But consider collaboration with an experienced team for that project. We have one, so feel free to contact me, if interested.

Search for the first character of the word in the text (call LOC). If there is a match, then compare the next character in the word with the next one in the text (LOC + x: x is worth from 1 to word length), until all match or fail. If it fails check the first character of the word with LOC + 1 locas on the text. etc.

String object supports some methods to extract keywords from text.