Skip to main content

Java 8 - Using Functions as Values


The following is a series of posts about "functional programming in Java" which is the result of my understanding by reading the book "Java 8 in Action: Lambdas, Streams, and Functional-style Programming, by Alan Mycroft and Mario Fusco".
1. Why functional programming?
2. Functional programming in Java 8
3. Java 8 - Using Functions as Values
4. Java 8 - Persistent data structure

In general, the phrase "functional-style programming" means the behavior of functions should be like that of mathematical-style functions - no side effects.

In programmers' point of views, functions may be used like other values: passed as arguments, returned as result, and stored in data structures. That means we can use the :: operator to create a method reference, and lambda expressions to directly express function values. For example:
//method reference
Function<String, Integer> strToInt = Integer::parseInt;

//lambda expression
Comparator<Integer> comparator = (a, b) -> a.compareTo(b);

There are two practical techniques to apply in our codebase.

Higher-order functions

For example:
 Comparator<Apple> c = Comparator.comparing(Apple::getWeight);
Higher-order functions (like Comparator.comparing) that can do at least one of following:
  • Take one or more functions as parameter
  • Return a function as result
Foo: If so, for what? 
Bar: Good question! To compose functions to create a pipeline of operations. It's close to declarative programming.

Currying

A function of two arguments (x and y, say) is seen instead as a function g of one argument that returns a function also one argument. The value returned by the latter function is the same as the value of the original function, that is, f(x,y) = (g(x))(y).

Brain overload! For example, you want to convert Celsius to Fahrenheit, the formula is f(x) = x *9/5 + 32. Not only this case but we can see the basic pattern of all unit conversion is as follow:
static double converter(double x, double f, double b) {
 return x * f + b;
}
This method is a bit too general. Obviously, we can use it to apply for all unit conversion but it would be tedious and you might accidentally mistyped them. There is an better approach:
static DoubleUnaryOperator curriedConverter(double f, double b){
 return (double x) -> x * f + b;
}
Instead of passing all the arguments x, f and b all at once to the converter method, you only ask for the arguments f and b and return another function, which given an argument x return x*f + b.

Why is it the better approach? This enables you to reuse the convention logic and create different functions with different convention factors:
DoubleUnaryOperator convertCtoF = curriedConverter(9.0/5, 32);
DoubleUnaryOperator convertUSDtoGBP = curriedConverter(0.6, 0);
DoubleUnaryOperator convertKmtoMi = curriedConverter(0.6214, 0);
Instead of three different method for each conversion factor:
static double convertCtoF(double x, double f, double b) {
 return x * f + b;
}

static double convertUSDtoGBP(double x, double f, double b) {
 return x * f + b;
}

static double convertUSDtoGBP(double x, double f, double b) {
 return x * f + b;
}
Foo: If so, for what?
Bar: It helps you modularize functions and reuse code!

Reference

Image result for java 8 in action

[1]. Alan Mycroft and Mario Fusco, Java 8 in Action: Lambdas, Streams, and Functional-style Programming. Part 4. Beyond Java 8.

Comments

Popular posts from this blog

[Snippet] CSS - Child element overlap parent

I searched from somewhere and found that a lot of people says a basic concept for implementing this feature looks like below: HTML code: <div id="parent">  <div id="child">  </div> </div> And, CSS: #parent{   position: relative;   overflow:hidden; } #child{   position: absolute;   top: -1;   right: -1px; } However, I had a lot of grand-parents in my case and the above code didn't work. Therefore, I needed an alternative. I presumed that my app uses Boostrap and AngularJs, maybe some CSS from them affects mine. I didn't know exactly the problem, but I believed when all CSS is loaded into my browser, I could completely handle it. www.tom-collinson.com I tried to create an example to investigated this problem by Fiddle . Accidentally, I just changed: position: parent; to position: static; for one of parents -> the problem is solved. Look at my code: <div class="modal-body dn-placeholder-parent-positi...

Coding Exercise, Episode 1

I have received the following exercise from an interviewer, he didn't give the name of the problem. Honestly, I have no idea how to solve this problem even I have tried to read it three times before. Since I used to be a person who always tells myself "I am not the one good at algorithms", but giving up something too soon which I feel that I didn't spend enough effort to overcome is not my way. Then, I have sticked on it for 24 hours. According to the given image on the problem, I tried to get more clues by searching. Thanks to Google, I found a similar problem on Hackerrank (attached link below). My target here was trying my best to just understand the problem and was trying to solve it accordingly by the Editorial on Hackerrank. Due to this circumstance, it turns me to love solving algorithms from now on (laugh). Check it out! Problem You are given a very organized square of size N (1-based index) and a list of S commands The i th command will follow t...

Installing NGINX on macOS

I have heard of a lot of NGINX recently. One of them was it can help for security issues; for sure, it much be more. It so happens that our team has got a ton of user stories from a security audit. It's time to delve into it. What is NGINX? In order to get a basic idea and have some fun , I've just picked some available posts from my favorite Vietnamese blogger communities as below: https://kipalog.com/posts/Cau-hinh-nginx-co-ban---Phan-1 https://viblo.asia/hoang.thi.tuan.dung/posts/ZabG912QGzY6 NGINX (pronounce: Engine-X) is a web server (comparing to IIS, Apache). It can be used as a reverse proxy ( this is what I need for security issues with configuration ), load balancer and more. How to get started? I found the below path for learning NGINX by googling "learn nginx": https://www.quora.com/What-are-some-good-online-resources-to-learn-Nginx In this post, I only went first step. This is installing NGINX on macOS and taking a first look at the confi...

Junit - Test fails on French or German string assertion

In my previous post about building a regex to check a text without special characters but allow German and French . I met a problem that the unit test works fine on my machine using Eclipse, but it was fail when running on Jenkins' build job. Here is my test: @Test public void shouldAllowFrenchAndGermanCharacters(){ String source = "ÄäÖöÜüß áÁàÀâÂéÉèÈêÊîÎçÇ"; assertFalse(SpecialCharactersUtils.isExistSpecialCharater(source)); } Production code: public static boolean isExistNotAllowedCharacters(String source){ Pattern regex = Pattern.compile("^[a-zA-Z_0-9_ÄäÖöÜüß áÁàÀâÂéÉèÈêÊîÎçÇ]*$"); Matcher matcher = regex.matcher(source); return !matcher.matches(); } The result likes the following: Failed tests: SpecialCharactersUtilsTest.shouldAllowFrenchAndGermanCharacters:32 null A guy from stackoverflow.com says: "This is probably due to the default encoding used for your Java source files. The ö in the string literal in the J...

Retrospective with Sailboat

Have you ever got bored with the Retrospective meeting? I have, sometime. Most of the times, this meeting just goes traditionally by answering three questions: "What good things have we done? What bad things have we done? And, what actions should we improve?" Ever and ever again! My team found a way to make it a little bit more exciting. The idea is that each member - not only our Scrum Master - will become a host. If a meeting is hosted by a memeber, the next meeting will be hold by another one. Yeah, I used "Sailboat" pattern in my turn. So, I just want to share with you guys how it was. I started the meeting by telling a short story that I hoped everyone in my team could recall the meaning behind of Retrospective meetings: There is a group of people trying pick up trash in a park. At the first look, the park seem not to have a lot of trash because they are spread out all over the place. However, these people continuously found trash when they sta...