IMPORTANT : The notes that I took for myself. I hope they will help you too.. Each resource that I used is added as reference at the end of the page.

Programming Style Series

  1. Programming Style Part 1 - Imperative and Declarative Style Programming
  2. Programming Style Part 2 - Higher Order Functions

First things first, we have to know some definitions to understand higher order functions. I would like to start with the term of “first-class citizen

First-class citizen

According to the wiki definition, in programming language design, a first-class citizen (also type, object, entity, or value) in a given programming language is an entity which supports all the operations generally available to other entities. These operations typically include being passed as an argument, returned from a function, and assigned to a variable.

According to this definition, anything can be first-class citizen if it supports the highlighted operations in the above. I want to put a comma here and move on to the next definition which is first-class function

First-class function

We have a little bit idea what the first-class citizen is. We can say that a programming language is said to have first-class functions if it treats functions as first-class citizens.

Higher Order Functions

In normal functions in general;

  • We can pass objects to functions
  • We can create objects in functions
  • We can also return objects from functions

In higher order functions;

  • We can pass functions to functions
  • We can create functions in functions
  • We can also return functions from functions

I would like to share two code examples to clarify.

For instance;

Thread t1 = new Thread(new Runnable() {
    public void run() {
        System.out.println("This is a thread");


In the above example, we pass a Runnable object to the Thread constructor. In here, an object treats as first-class citizen.

Thread t2 = new Thread( () -> System.out.println("Another thread"));


But, we can treat functions or codes as first class citizens as well. In this example we pass a function which is lambda expression(anonymous) as an argument to the Thread constructor. This makes it higher order function because it receives another function as its parameter.