Home > Articles > Programming > Java

This chapter is from the book

This chapter is from the book

3.3 Examples of Interfaces

At first glance, interfaces don’t seem to do very much. An interface is just a set of methods that a class promises to implement. To make the importance of interfaces more tangible, the following sections show you four examples of commonly used interfaces from the standard Java library.

3.3.1 The Comparable Interface

Suppose you want to sort an array of objects. A sorting algorithm repeatedly compares elements and rearranges them if they are out of order. Of course, the rules for doing the comparison are different for each class, and the sorting algorithm should just call a method supplied by the class. As long as all classes can agree on what that method is called, the sorting algorithm can do its job. That is where interfaces come in.

If a class wants to enable sorting for its objects, it should implement the Comparable interface. There is a technical point about this interface. We want to compare strings against strings, employees against employees, and so on. For that reason, the Comparable interface has a type parameter.

public interface Comparable<T> {
    int compareTo(T other);
}

For example, the String class implements Comparable<String> so that its compareTo method has the signature

int compareTo(String other)

When calling x.compareTo(y), the compareTo method returns an integer value to indicate whether x or y should come first. A positive return value (not necessarily 1) indicates that x should come after y. A negative integer (not necessarily -1) is returned when x should come before y. If x and y are considered equal, the returned value is 0.

Note that the return value can be any integer. That flexibility is useful because it allows you to return a difference of non-negative integers.

public class Employee implements Comparable<Employee> {
    ...
    public int compareTo(Employee other) {
        return getId() - other.getId(); // Ok if IDs always ≥ 0
    }
}

When comparing floating-point values, you cannot just return the difference. Instead, use the static Double.compare method. It does the right thing, even for ±∞ and NaN.

Here is how the Employee class can implement the Comparable interface, ordering employees by salary:

public class Employee implements Comparable<Employee> {
    ...
    public int compareTo(Employee other) {
        return Double.compare(salary, other.salary);
    }
}

The String class, as well as over a hundred other classes in the Java library, implements the Comparable interface. You can use the Arrays.sort method to sort an array of Comparable objects:

String[] friends = { "Peter", "Paul", "Mary" };
Arrays.sort(friends); // friends is now ["Mary", "Paul", "Peter"]

3.3.2 The Comparator Interface

Now suppose we want to sort strings by increasing length, not in dictionary order. We can’t have the String class implement the compareTo method in two ways—and at any rate, the String class isn’t ours to modify.

To deal with this situation, there is a second version of the Arrays.sort method whose parameters are an array and a comparator—an instance of a class that implements the Comparator interface.

public interface Comparator<T> {
    int compare(T first, T second);
}

To compare strings by length, define a class that implements Comparator<String>:

class LengthComparator implements Comparator<String> {
    public int compare(String first, String second) {
        return first.length() - second.length();
    }
}

To actually do the comparison, you need to make an instance:

Comparator<String> comp = new LengthComparator();
if (comp.compare(words[i], words[j]) > 0) ...

Contrast this call with words[i].compareTo(words[j]). The compare method is called on the comparator object, not the string itself.

To sort an array, pass a LengthComparator object to the Arrays.sort method:

String[] friends = { "Peter", "Paul", "Mary" };
Arrays.sort(friends, new LengthComparator());

Now the array is either ["Paul", "Mary", "Peter"] or ["Mary", "Paul", "Peter"].

You will see in Section 3.4.2, “Functional Interfaces,” on p. 109 how to use a Comparator much more easily, using a lambda expression.

3.3.3 The Runnable Interface

At a time when just about every processor has multiple cores, you want to keep those cores busy. You may want to run certain tasks in a separate thread, or give them to a thread pool for execution. To define the task, you implement the Runnable interface. This interface has just one method.

class HelloTask implements Runnable {
    public void run() {
        for (int i = 0; i < 1000; i++) {
            System.out.println("Hello, World!");
        }
    }
}

If you want to execute this task in a new thread, create the thread from the Runnable and start it.

Runnable task = new HelloTask();
Thread thread = new Thread(task);
thread.start();

Now the run method executes in a separate thread, and the current thread can proceed with other work.

3.3.4 User Interface Callbacks

In a graphical user interface, you have to specify actions to be carried out when the user clicks a button, selects a menu option, drags a slider, and so on. These actions are often called callbacks because some code gets called back when a user action occurs.

In Java-based GUI libraries, interfaces are used for callbacks. For example, in JavaFX, the following interface is used for reporting events:

public interface EventHandler<T> {
    void handle(T event);
}

This too is a generic interface where T is the type of event that is being reported, such as an ActionEvent for a button click.

To specify the action, implement the interface:

class CancelAction implements EventHandler<ActionEvent> {
    public void handle(ActionEvent event) {
        System.out.println("Oh noes!");
    }
}

Then, make an object of that class and add it to the button:

Button cancelButton = new Button("Cancel");
cancelButton.setOnAction(new CancelAction());

Of course, this way of defining a button action is rather tedious. In other languages, you just give the button a function to execute, without going through the detour of making a class and instantiating it. The next section shows how you can do the same in Java.

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