Pure_function sometimes are modeled as static methods in Java should not be present in classes which also have state. Static methods are appropriate for things that don't have associated state. Some factory methods, "purely functional" methods like Math.min, etc are all perfectly acceptable static methods. Purely functional methods have no need for dependency injection, or to interact with the enclosing object and hence should be refactored into their own utility classes.
At the meager cost of a few more files and some more code we will have simpler objects and a better set of interfaces. Such a version might only be a couple of lines shorter than the other implementation but it knows much less about its constituent parts. This ensures, the functionality of helper functions is no longer locked in the context of another object. The Unix philosophy illustrates that small components that work together through an interface can be extraordinarily powerful. Nesting an aspect of your domain as an implementation detail of a specific model conflates responsibilities, bloats code, makes tests less isolated and slower, and hides concepts that should be first-class in your system. In "The Art of Unix Programming" Eric Raymond states Rule of Modularity as “write simple parts connected by clean interfaces.” This philosophy is a powerful strategy to manage complexity. Like Unix, systems/libraries should consist of many small classes each of which are focused on a specific task and work with each other to accomplish a larger task. Finally to quote Kent Beck from the Smalltalk Best Practice Patterns "Good code invariably has small methods and small objects. I get lots of resistance to this idea, especially from experienced developers, but no one thing I do to systems provides as much help as breaking it into more pieces.”