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

Post a Comment

Popular posts from this blog

AngularJS - Build a custom validation directive for using multiple emails in textarea

AngularJS already supports the built-in validation with text input with type email. Something simple likes the following:
<input name="input" ng-model="email.text" required="" type="email" /> <span class="error" ng-show="myForm.input.$error.email"> Not valid email!</span>
However, I used a text area and I wanted to enter some email addresses that's saparated by a comma (,). I had a short research and it looked like AngualarJS has not supported this functionality so far. Therefore, I needed to build a custom directive that I could add my own validation functions. My validation was done only on client side, so I used the $validators object.

Note that, there is the $asyncValidators object which handles asynchronous validation, such as making an $http request to the backend.

This is just my implementation on my project. In order to understand that, I supposed you already had experiences with Angular…

Creating a Chatbot with RiveScript in Java

Motivation"Artificial Intelligence (AI) is considered a major innovation that could disrupt many things. Some people even compare it to the Internet. A large investor firm predicted that some AI startups could become the next Apple, Google or Amazon within five years"
- Prof. John Vu, Carnegie Mellon University.

Using chatbots to support our daily tasks is super useful and interesting. In fact, "Jenkins CI, Jira Cloud, and Bitbucket" have been becoming must-have apps in Slack of my team these days.

There are some existing approaches for chatbots including pattern matching, algorithms, and neutral networks. RiveScript is a scripting language using "pattern matching" as a simple and powerful approach for building up a Chabot.
Architecture Actually, it was flexible to choose a programming language for the used Rivescript interpreter like Java, Go, Javascript, Python, and Perl. I went with Java.


Used Technologies and ToolsOracle JDK 1.8.0_151Apache Maven 3.5…

Styling Sort Icons Using Font Awesome for Primefaces' Data Table

So far, Primefaces has used image sprites for displaying the sort icons. This leads to a problem if we want to make a different style for these icons; for example, I would make the icon "arrow up" more blurry at the first time the table loading because I want to highlight the icon "arrow down". I found a way that I can replace these icons with Font Awesome icons.


We will use "CSS Pseudo-classes" to achieve it. The hardest thing here is that we should handle displaying icons in different cases. There is a case both "arrow up" and "arrow down" showing and other case is only one of these icons is shown.

.ui-sortable-column-icon.ui-icon.ui-icon-carat-2-n-s { background-image: none; margin-left: 5px; font-size: 1.1666em; position: relative; } .ui-sortable-column-icon.ui-icon.ui-icon-carat-2-n-s:not(.ui-icon-triangle-1-s)::before { content: "\f106"; font-family: "FontAwesome"; position: absolut…

Math fundamentals and Katex

It was really tough for me to understand many articles about data science due to the requirements of understanding mathematics (especially linear algebra). I’ve started to gain some basic knowledges about Math by reading a book first.The great tool Typora and stackedit with supporting Katex syntax simply helps me to display Math-related symbols.Let’s start!The fundamental ideas of mathematics: “doing math” with numbers and functions. Linear algebra: “doing math” with vectors and linear transformations.1. Solving equationsSolving equations means finding the value of the unknown in the equation. To find the solution, we must break the problem down into simpler steps. E.g:x2−4=45x2−4+4=45+4x2=49x=49∣x∣=7x=7 or x=−7\begin{aligned} x^2 - 4 &= 45\\ x^2 - 4 + 4 &= 45 + 4\\ x^2 &= 49\\ \sqrt{x}&=\sqrt{49}\\ |x| &= 7\\ x=7 &\text{ or } x=-7 \end{aligned}x2−4x2−4+4x2x​∣x∣x=7​=45=45+4=49=49​=7 or x=−7​2. NumbersDefinitions
Mathematicians like to classify the d

Applying pipeline “tensorflow_embedding” of Rasa NLU

According to this nice article, there was a new pipeline released using a different approach from the standard one (spacy_sklearn). I wanted to give it a try to see whether it can help with improving bot’s accuracy.

After applying done, I gave an evaluation of “tensorflow_embedding”. It seemed to work better a bit. For example, I defined intents “greet” and “goodbye” with some following messages in my training data.
## intent:greet- Hey! How are you? - Hi! How can I help you? - Good to see you! - Nice to see you! - Hi - Hello - Hi there ## intent:goodbye- Bye - Bye Bye - See you later - Take care - Peace In order to play around with Rasa NLU, I created a project here. You can have a look at this change from this pull request. Yay!

When I entered message “hi bot”, then bot with “tensorflow_embedding” could detect intent “greet” with better confidence scores rather than bot with “spacy_sklearn”. The following are responses after executing curl -X POST localhost:5000/parse -d '{&qu…