Home > Articles > Programming > Java

This chapter is from the book

This chapter is from the book

3.6 Processing Lambda Expressions

Up to now, you have seen how to produce lambda expressions and pass them to a method that expects a functional interface. In the following sections, you will see how to write your own methods that can consume lambda expressions.

3.6.1 Implementing Deferred Execution

The point of using lambdas is deferred execution. After all, if you wanted to execute some code right now, you’d do that, without wrapping it inside a lambda. There are many reasons for executing code later, such as:

  • Running the code in a separate thread
  • Running the code multiple times
  • Running the code at the right point in an algorithm (for example, the comparison operation in sorting)
  • Running the code when something happens (a button was clicked, data has arrived, and so on)
  • Running the code only when necessary

Let’s look at a simple example. Suppose you want to repeat an action n times. The action and the count are passed to a repeat method:

repeat(10, () -> System.out.println("Hello, World!"));

To accept the lambda, we need to pick (or, in rare cases, provide) a functional interface. In this case, we can just use Runnable:

public static void repeat(int n, Runnable action) {
    for (int i = 0; i < n; i++) action.run();
}

Note that the body of the lambda expression is executed when action.run() is called.

Now let’s make this example a bit more sophisticated. We want to tell the action in which iteration it occurs. For that, we need to pick a functional interface that has a method with an int parameter and a void return. Instead of rolling your own, I strongly recommend that you use one of the standard ones described in the next section. The standard interface for processing int values is

public interface IntConsumer {
    void accept(int value);
}

Here is the improved version of the repeat method:

public static void repeat(int n, IntConsumer action) {
    for (int i = 0; i < n; i++) action.accept(i);
}

And here is how you call it:

repeat(10, i -> System.out.println("Countdown: " + (9 - i)));

3.6.2 Choosing a Functional Interface

In most functional programming languages, function types are structural. To specify a function that maps two strings to an integer, you use a type that looks something like Function2<String, String, Integer> or (String, String) -> int. In Java, you instead declare the intent of the function using a functional interface such as Comparator<String>. In the theory of programming languages this is called nominal typing.

Of course, there are many situations where you want to accept “any function” without particular semantics. There are a number of generic function types for that purpose (see Table 3–1), and it’s a very good idea to use one of them when you can.

Table 3–1 Common Functional Interfaces

Functional Interface

Parameter types

Return type

Abstract method name

Description

Other methods

Runnable

none

void

run

Runs an action without arguments or return value

Supplier<T>

none

T

get

Supplies a value of type T

Consumer<T>

T

void

accept

Consumes a value of type T

andThen

BiConsumer<T, U>

T, U

void

accept

Consumes a value of type T and U

andThen

Function<T, R>

T

R

apply

A function with argument of type T

compose, andThen, identity

BiFunction<T, U, R>

T, U

R

apply

A function with argument of type T and U

andThen

UnaryOperator<T>

T

T

apply

A unary operator on the type T

compose, andThen, identity

BinaryOperator<T>

T, T

T

apply

A binary operator on the type T

andThen, maxBy, minBy

Predicate<T>

T

boolean

test

A boolean-valued function

and, or, negate, isEqual

BiPredicate<T, U>

T, U

boolean

test

A boolean-valued function with two arguments

and, or, negate

For example, suppose you write a method to process files that match a certain criterion. Should you use the descriptive java.io.FileFilter class or a Predicate<File>? I strongly recommend that you use the standard Predicate<File>. The only reason not to do so would be if you already have many useful methods producing FileFilter instances.

Table 3–2 lists the 34 available specializations for primitive types int, long, and double. It is a good idea to use these specializations to reduce autoboxing. For that reason, I used an IntConsumer instead of a Consumer<Integer> in the example of the preceding section.

Table 3–2 Functional Interfaces for Primitive Types p, q is int, long, double; P, Q is Int, Long, Double

Functional Interface

Parameter types

Return type

Abstract method name

BooleanSupplier

none

boolean

getAsBoolean

PSupplier

none

p

getAsp

PConsumer

p

void

accept

ObjPConsumer<T>

T, p

void

accept

PFunction<T>

p

T

apply

PToQFunction

p

q

applyAsQ

ToPFunction<T>

T

p

applyAsP

ToPBiFunction<T, U>

T, U

p

applyAsP

PUnaryOperator

p

p

applyAsP

PBinaryOperator

p, p

p

applyAsP

PPredicate

p

boolean

test

3.6.3 Implementing Your Own Functional Interfaces

Ever so often, you will be in a situation where none of the standard functional interfaces work for you. Then you need to roll your own.

Suppose you want to fill an image with color patterns, where the user supplies a function yielding the color for each pixel. There is no standard type for a mapping (int, int) -> Color. You could use BiFunction<Integer, Integer, Color>, but that involves autoboxing.

In this case, it makes sense to define a new interface

@FunctionalInterface
public interface PixelFunction {
    Color apply(int x, int y);
}

Now you are ready to implement a method:

BufferedImage createImage(int width, int height, PixelFunction f) {
    BufferedImage image = new BufferedImage(width, height,
        BufferedImage.TYPE_INT_RGB);

    for (int x = 0; x < width; x++)
        for (int y = 0; y < height; y++) {
            Color color = f.apply(x, y);
            image.setRGB(x, y, color.getRGB());
        }
    return image;
}

To call it, supply a lambda expression that yields a color value for two integers:

BufferedImage frenchFlag = createImage(150, 100,
    (x, y) -> x < 50 ? Color.BLUE : x < 100 ? Color.WHITE : Color.RED);

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020