Java Functions

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This note is about using functions in a java

intro: functional programming

there is a formal definition for functional programing

In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions that map values to other values, rather than a sequence of imperative statements which update the running state of the program.

but I am taking the liberty of making a personal interpretation that I already mentioned in this article about functional reactive pipelines

thou the above is ok, I understand functional programming like this: it is programing by delegation

a simple Function

in BMC_Anvil most of the code is already functional-like given that I am using the reactive paradigm, but I created a small utility on the Pageable class to illustrate how the Function interface can be used by ourselves.

this example is a simple example, and I’ll build a few more complex ones to illustrate chaining and how Functions can be used on Streams or methods

let’s examine a Function:

public class Pageable {
    private final ToIntFunction<Integer> checkPageSize = (pageSize) -> max(min(pageSize, MAX_PAGE_SIZE), MIN_PAGE_SIZE);


a Function in java is itself a type that represents a function, and it comprises one argument and one result. In our case, the ToIntFunction is a specialization that is telling us that the return type is an int, so we don’t need to specify the return type.

in this simple example the Function checkPageSize receives a page size as an Integer and returns a page size between the minimum or maximum configured. If the page size is between boundaries, it returns the page size or the corresponding max / min when over / underflowing.

let’s examine how it is used:

public class Pageable {
    public Pageable(final String sortBy, final String sortDir, final Integer pageIx, final Integer pageSize) {
        // omitted code here = Optional.ofNullable(pageIx)
                .map(index -> Page.of(max(index, 0), checkPageSize.applyAsInt(pageSize)))
                .orElseGet(() -> Page.ofSize(pageSize));

all Function have an apply() method and in this case an applyAsInt(), just like a method, you pass the corresponding argument and that’s it.


in the above case using a Function instead of a method achieves little. I’m going to add more complex and useful examples of function shortly.