A default method added to maintain the backward compatibility which allows older classes (without modifications) to access new version of an interface.

Java 9 interfaces can have private methods and private staic methods. These methods support code reusabilit in the interface level.

Classes can be defined inside other classes to encapsulate logic and constrain the context of use. For example:

E. F. Codd proposed three normal forms 1NF, 2NF and 3NF (1970). Revised definition (1974) was given by F. Boyce and Codd which is known as Boyce-Codd Normal Form (BCNF which is 3.5NF) to distinguish it from the old definition of third normal form. R. Faign introduced 4NF(1977) and 5NF(1979) and DKNF(1981). All the normal forms depend on the functional dependency, but 4NF and 5NF have been proposed which are based on the concept of multivalued dependency and joining dependency, respectively.

Hadoop MapReduce well explains the pain is writing too much code for simple MapReduce in Java. This organic blog explains how to use MRJob package in Python to write and execute Movie ratings.

After install the sandbox from the Hortonworks, you can visit the http://localhost:50070 page to find the information about the HDFS cluster. YARN job manager can be access via http://localhost:8088.