Functional programming in Java 8

In my previous post, we discussed about why we should consider to use functional programming. Now, let's delve into what functional programming in Java is.

What is pure functional programming?

Shortly, functional programming is programming using functions. A function corresponds to a mathematical function such as log, sin. Basically, it takes zero or more arguments, give one or more result, and has no side effects.

We can't completely program in pure functional style in Java

Why? For example, calling Scanner.nextLine twice typically gives different result.

So, it's just called "functional-style programming". How is that?
- There is no mutating structures visible to callers. That means your side effect may not be visible to a program, but it's visible to the programmer in terms of slower execution.
- A function or method shouldn't throw any exceptions (follows the concept "pass arguments, return result"). We can use types like Optional<T> instead.
- For pragmatic reasons, we can accept some code that can't be strictly described as functional, e.g: some form of log file.

We call it "no visible side effects" or referential transparency

With functional-style programming, we follow the rule "no visible side-effects" includes no mutating structures visible to callers, no I/O and no exceptions. This concept is called "referential transparency". A function is referentially transparent if it always returns the same result value when called with the same argument value. For example, "huong".replace("h", "H") but Random.nextInt

Will function-style programming replace object-oriented programming?

Why? And, why not? These are just programming styles. It depends!
In practice, Java programmers have always mixed object-oriented and functional-style programming styles.
- Object-oriented programming style: everything is an object and programs operate by updating fields and calling methods that update their associated object.
- Functional-style programming style: referential transparency. It helps enable you to write programs that are more modular and more suitable for multicore processors. These ideas are as additional weapons in your programming armory.

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

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...

JSF, Primefaces - Invoking Application Code Even When Validation Failed

A use case I have a form which has requirements as follow: - There are some mandatory fields. - Validation is triggered when changing value on each field. - A button "Next" is enable only when all fields are entered. It turns to disabled if any field is empty. My first approach I defined a variable "isDisableNext" at a backend bean "Controller" for dynamically disabling/enabling the "Next" button by performing event "onValueChange", but, it had a problem: <h:form id="personForm"> <p:outputLabel value="First Name" for="firstName"/> <p:inputText id="firstName" value="#{person.firstName}" required="true"> <p:ajax event="change" listener="#{controller.onValueChange}" update="nextButton"/> </p:inputText> <p:outputLabel value="Last Name" for="lastName"/> <p:i...

Attribute 'for' of label component with id xxxx is not defined

I got the warning in the log file when I have used the tag <h:outputLabel> without attribute " for " in xhtml file. It was really polluting my server log files. The logged information actually makes sense anyway! We could find an answer as the following: "Having h:outputLabel without a "for" attribute is meaningless. If you are not attaching the label, you should be using h:outputText instead of h:outputLabel." However, these solutions are not possible just for my situation. Instead of using h:outputText for only displaying text, my team has used h:outputLabel too many places. We were nearly in our release time (next day) so it is quite risky and takes much efforts if we try to correct it. Because the style (with CSS) is already done with h:ouputLabel . The alternative by adding attribute " for " the existing h:outputLabel is not reasonable either. I really need to find another solution. Fortunately, I came across a way if I cha...

4 Remarkable Notes for JSF Apps Using HTML5

In the previous post , I've already shared with you how my team consults clients by using a HTML prototype. This post is about the used technologies for reusing the provided HTML template and communicating with backend. What is the issue when using HTML elements with Primefaces components? Primefaces is a great extension for developing JSF web apps. However, it is really frustrating in case we have to make it work with an existing HTML template. Why? - Primefaces has its own theme for styling. - Primefaces changes the HTML structure. Therefore, that would be a huge effort to use the Primefaces' components to replicate the elements of the HTML template; especially it is impossible for images drawing by " canvas " tag. That requires us to find a better approach. Since Java EE 7 (introducing JSF 2.2 included), it supports to use HTML5 elements . The idea is that JSF components don't effect the style and HTML structure, so we can easily reuse the provided HTM...

BIRT - Fix the size of an image

I use a dynamic image as a logo my report in pdf. At the beginning, I use table to align the logo in left or right. I meet a problem with some images with a large width or height. My customer requires that the logo should be displayed in original size. These following steps solves my problem: 1. Use Grid instead of Table 2. Set Grid "Height" is 100% and "Width" is blank 3. Set "Fit to container" for images are "true". Download the the template here .

vhandit

Search This Blog