Currently, WPCleaner is still compatible with Java 7, preventing the usage of cool new features that would be beneficial to clean the code.

First, there's a need to check that there are no usages preventing from going to Java 8 (for example, is Toolforge OK ?). After that, sub tasks should be created for the various improvements (switching to Java 8, using Optional<>, using Nullable and Nonnnull annotations, streams, lambda...)