Java Stream API

Nov 11th 2020

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Java and Functional Programming

Java is one of the godfathers of object-oriented programming. Java got its start with print servers, and is still very popular to this day. With applications run on the Java Virtual Machine(JVM), Developers use Java for applications across the spectrum, from Android to enterprise.

Functional programming, in contrast to object-oriented programming, an application application is composed of mathematically pure functions. You can read more about object-oriented vs functional programming here. Functions are first class members in functional programming, they are the equivalent of objects that can be passed around a common variable.

Lambda Expressions and Anonymous Functions

Anonymous functions are essentially functions without names. Lambda expressions are short hand for a anonymous function declaration. They can usually be noted for some kind of arrow syntax.

Java 8, along with other things(including the Stream API), brought us lambda expressions and somewhat easier ways to run operations asynchronously.

Lambda Syntax

In Java, the lambda expression for an anonymous function of return type void that takes no arguments, would like something like this

() -> {
  //do something
  //return nothing

This is the equivalent to

void anonFunction() {
  //do something
  //return nothing

This is the oneAdder method

int oneAdder(int addOneToMe){
  return addOneToMe + 1;

The lambda syntax for this is

(int addOneToMe) -> {
  return addOneToMe + 1;

Lambda and Asynchronous Processing

You might notice that our first lambda example

() -> {
  //do something
  //return nothing

Has something in common to the Runnable interface, it has the same signature as the method

public void run();

We can use the Runnable interface as a functional interface(a topic for another post) to pass these lambda syntax anonymous functions around as first class members.

Back to asynchronous processing, if we look at the execute method of the Executor interface

  public synchronized void execute(Runnable r);

Now we can pass the execute method a lambda expression to run in another thread

Executor executor = Executors.newSingleThreadExecutor();

executor.execute(() -> {
  //do something in another thread
  //return nothing

Java Stream API

Java 8 brought us the Stream API. The Stream API allows us to process collections in a more streamlined, syntactically cleaner fashion. The stream package provides us with a tool belt full of methods for processing collections.

A collection can be turned into a stream by calling the new stream() method.

Stream Methods

The Stream API provides quite a few handy methods, but we will concentrate on the most ones.


The map methods is used when you want to return a new object(can be different type) for each element in the stream.

List<String> phrases = new ArrayList<>(aPopulatedList);

List<Integer> charLengths = phrase -> {
  return new Integer(phrase.toCharArray().length); 

This is equal to

List<Integer> charLengths = new ArrayList<>();
for(String phrase : phrases){


Filter allows us to remove elements from the stream, by returning a boolean value. length -> {
  return length > 5;

This is equal to

List<Integer> filteredLengths = new ArrayList<>();

for(Integer length : charLengths){
  if(length > 5){


Last but not least, forEach is the simplest case length -> {

Which equals

for(Integer length : charLengths){
  length+= 1;


We can chain stream operations together to do more complex transformations of collections
  .filter(element -> {
    //remove elements
  .map(element -> {
    //transform elements


So this is a micro breakdown of the Stream API to get you going. The Stream API allows us to replace our ugly for loops with much easier to read lambda expressions.


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