Functional Programming with Java - Overview

In functional programming paradigm, an application is written mostly using pure functions. Here pure function is a function having no side effects. An example of side effect is modification of instance level variable while returning a value from the function.

Following are the key aspects of functional programming.

  • Functions − A function is a block of statements that performs a specific task. Functions accept data, process it, and return a result. Functions are written primarily to support the concept of re usability. Once a function is written, it can be called easily, without having to write the same code again and again.

    Functional Programming revolves around first class functions, pure functions and high order functions.

    • A First Class Function is the one that uses first class entities like String, numbers which can be passed as arguments, can be returned or assigned to a variable.

    • A High Order Function is the one which can either take a function as an argument and/or can return a function.

    • A Pure Function is the one which has no side effect while its execution.

  • Functional Composition − In imperative programming, functions are used to organize an executable code and emphasis is on organization of code. But in functional programming, emphasis is on how functions are organized and combined. Often data and functions are passed together as arguments and returned. This makes programming more capable and expressive.

  • Fluent Interfaces − Fluent interfaces helps in composing expressions which are easy to write and understand. These interfaces helps in chaining the method call when each method return type is again reused. For example −

LocalDate futureDate =;
  • Eager vs Lazy Evaluation − Eager evaluation means expressions are evaluated as soon as they are encountered whereas lazy evaluation refers to delaying the execution till certain condition is met. For example, stream methods in Java 8 are evaluated when a terminal method is encountered.

  • Persistent Data Structures

    − A persistent data structure maintains its previous version. Whenever data structure state is changed, a new copy of structure is created so data structure remains effectively immutable. Such immutable collections are thread safe.
  • Recursion − A repeated calculation can be done by making a loop or using recursion more elegantly. A function is called recursive function if it calls itself.

  • Parallelism − Functions with no side effects can be called in any order and thus are candidate of lazy evaluation. Functional programming in Java supports parallelism using streams where parallel processing is provided.

  • Optionals − Optional is a special class which enforces that a function should never return null. It should return value using Optional class object. This returned object has method isPresent which can be checked to get the value only if present.