I forgot to add the interfaces versus classes, inner classes, and lambda notation, with the tons of trivial FunctionalInterface interfaces to deal with a smattering of the combinations of number of inputs, void or primitive or class output, which inputs or inputs can be null, and which exceptions can be thrown by the evaluation method (having many different names, etc).

Almost all of these monstrosities are caused by the “object purity” mindset that kept the designers from including first-class functions in the language. Avail attempts to include a large set of paradigms instead of trying to “axiomatize” the exposed surface (although we do axiomatize the construction of these constructs).