Let’s see how we can paralellize java application. Suppose we have asked to develop application where given the libraries list we need to calculate the number of books in each library. Suppose calculation is an expensive operation. The first thing that comes to our mind is to do this by streams.
Default methods are introduced in Java 8. The main purpose of default method is to make possible evolving an existing API in compatible way. Interfaces can contain method implementation for which implementing class doesn’t need to provide implementation.