RAKE is an algorithm for extracting keywords (technically phrases, but I don't question scientific literature) from a document that have a high relevance or importance to the contents of the document. For example, the top five significant keywords in the text:

Compatibility of systems of linear constraints over the set of natural numbers. Criteria of compatibility of a system of linear Diophantine equations, strict inequations, and nonstrict inequations are considered. Upper bounds for components of a minimal set of solutions and algorithms of construction of minimal generating sets of solutions for all types of systems are given. These criteria and the corresponding algorithms for constructing a minimal supporting set of solutions can be used in solving all the considered types of systems and systems of mixed types.

are calculated to be:

This library isn't on a central repo yet, so nab the JAR URL from the releases page and toss it into whatever dependency manager you're using. That, or just download the JAR.

Using the library is super straightforward. The main class only exports one public method, getKeywordsFromText(), and needs a language code in order to run. Any constant found in RakeLanguages can be used without issue. So for example:

public class Main {
    public static void main(String[] args) {
        String languageCode = RakeLanguages.ENGLISH;
        String rake = new Rake(languageCode);
        LinkedHashMap<String, Double> results = rake.getKeywordsFromText("Your text would go here."));
    	System.out.println(results);
    }
}

If you wish to contribute to this library, you'll need Buck to build it. Once you have Buck installed, it's as simple as:

$ buck build :rake

Algorithm taken from here.

Implementation taken from here.