JAVA HACKERRANK SOLUTIONS app is all you need to learn or to go through the solutions of the JAVA section on the hackerrank website.
You can copy the solution by just one tap and can run in your compiler.
Some important notes are there in this app so before going through the solutions for the first time I will suggest you to go through them once.
This app is for educational purpose. Get the solutions practice them.
Good luck for your 5 stars on Hackerrank.