parsergenerator | functional parser generator for deterministic context | Parser library

Your code indicates that you tested the lexer. However, the test indicates that the lexer is not producing the correct tokens:

If you split up the functions so that each production has its own function -- which is, indeed, best practice -- then thete is absolutely no point checking the token type of the operator. You know what it is because the logic of the parser means that the function will only be called with a match to the production.

So you can write reasonably compact code:

ParserFactory is the most common nomenclature used for the example you have given since it very closely matches the Factory Pattern. As mentioned in the comments, the more common way of expressing this pattern is to return an object of type Parser rather than simply Runnable and calling the method that creates the Parser createParser (or even just create) rather than get. Conventionally, get refers to obtaining an existing object (i.e. a private field holds a value and it is returned such that two subsequent calls to get return the same object, until the field is reassigned) rather than creating a new object for each call.

For example, the common way of expressing this pattern would resemble the following: