Home > Articles > Programming > Java

Practical Java Praxis 65: Use Inheritance or Delegation to Define Immutable Classes

You can use a number of techniques to define immutable classes in Java. In this article, Java expert Peter Haggar explains the immutable interface, the common interface, and immutable delegation class.
Like this article? We recommend

The first article in this series outlined some advantages of immutable objects and how to design and implement your own. This article discusses three additional techniques that you can use to define immutable classes. Each has its own advantages and disadvantages. The techniques discussed are listed here:

  • Immutable interface

  • Common interface or base class

  • Immutable delegation class

Immutable Interface

Assume that you have an existing mutable class, MutableCircle, that represents a circle. Because of the thread-safety advantages of an immutable object, you want to let other code access an object of this class as an immutable object. The original MutableCircle class looks like this:

class MutableCircle
{
  private double radius;

  public MutableCircle(double r)
  {
    radius = r;
  }

  public void setRadius(double r)
  {
    radius = r;
  }

  public double radius()
  {
    return radius;
  }
  //...
}

To provide this class as an immutable class, you can declare an immutable interface that the mutable class implements, as in this example:

interface ImmutableCircle
{
  public double radius();
}

class MutableCircle implements ImmutableCircle
{
  private double radius;

  public MutableCircle(double r)
  {
    radius = r;
  }

  public void setRadius(double r)
  {
    radius = r;
  }

  public double radius()
  {
    return radius;
  }
  //...
}

Because the immutable interface exposes only the nonmutating methods of the underlying class, access to the object through the interface type preserves immutability. This allows you to use the immutable interface to prevent mutation. For example, the following code returns a reference to the MutableCircle object through the ImmutableCircle interface type, thereby properly preventing this code from compiling:

public class Test
{
  public ImmutableCircle createWheel(double r)
  {
    return new MutableCircle(r);
  }

  public static void main(String args[])
  {
    Test t = new Test();
    ImmutableCircle iWheel = t.createWheel(5.0);
    iWheel.setRadius(7.4);
  }
}

Note that the createWheel method returns a reference to an ImmutableCircle object. Objects of type ImmutableCircle can access only methods defined in the ImmutableCircle interface. In this case, the only method available is the nonmutating radius method. Attempts to access the methods of MutableCircle from an ImmutableCircle object reference are flagged by the compiler. Compiling the previous code results in the following error message:

Test.java:12: Method setRadius(double) not found in interface ImmutableCircle.
    iWheel.setRadius(7.4);
                    ^
1 error

This is what you want to happen with code written in this way. This design, however, has a flaw. It works until the users of this class realize how to get around the immutability constraints you have established with the interface. Consider the following code, which breaks these immutability constraints:

public class Test
{
  public ImmutableCircle createWheel(double r)
  {
    return new MutableCircle(r);
  }

  public static void main(String args[])
  {
    Test t = new Test();
    ImmutableCircle iWheel = t.createWheel(5.0);
    System.out.println("Radius of wheel is " +
                       iWheel.radius());
    ((MutableCircle)iWheel).setRadius(7.4);
    System.out.println("Radius of wheel is now " +
                       iWheel.radius());
  }
}

This code not only compiles cleanly, but it also generates the following output:

Radius of wheel is 5.0
Radius of wheel is now 7.4

The output shows that the supposedly immutable ImmutableCircle object has been altered. With this approach, however, users of the ImmutableCircle class can easily expunge its immutability with a simple cast. Remember, an interface declares a reference type. Therefore, an object reference of type ImmutableCircle can be cast to its derived type of MutableCircle. An object reference cast to a MutableCircle then can access the methods of this class and break immutability.

Because the programmer must extend the effort to code the cast, you might think that this serves as enough of a deterrent. Nevertheless, the mutability constraints can be breached.

Common Interface or Base Class

Preventing breaches of immutability requires another approach. One is to use one common interface or base class and two derived classes. These are organized as follows:

  • An interface or abstract base class that contains the immutable methods that are common for its derived classes

  • A derived class that provides a mutable implementation

  • A derived class that provides an immutable implementation

For example, you might design an interface and two derived classes like this:

interface PinNumbers
{
  public String accountOwner();
  public int checkingPin();
  public int savingsPin();
}

class MutablePinNumbers implements PinNumbers
{
  private String acctOwner;
  private int checkingAcctPin;
  private int savingsAcctPin;

  MutablePinNumbers(String owner, int cPin, int sPin)
  {
    acctOwner = owner;
    checkingAcctPin = cPin;
    savingsAcctPin = sPin;
  }
  public void setAccountOwner(String str)
  {
    acctOwner = str;
  }


  public String accountOwner()
  {
    return acctOwner;
  }
  public void setCheckingPin(int pin)
  {
    checkingAcctPin = pin;
  }
  public int checkingPin()
  {
    return checkingAcctPin;
  }
  public void setSavingsPin(int pin)
  {
    savingsAcctPin = pin;
  }
  public int savingsPin()
  {
    return savingsAcctPin;
  }
}

final class ImmutablePinNumbers implements PinNumbers
{
  private String acctOwner;
  private int checkingAcctPin;
  private int savingsAcctPin;

  ImmutablePinNumbers(String owner, int cPin, int sPin)
  {
    acctOwner = owner;
    checkingAcctPin = cPin;
    savingsAcctPin = sPin;
  }
  public String accountOwner()
  {
    return acctOwner;
  }
  public int checkingPin()
  {
    return checkingAcctPin;
  }
  public int savingsPin()
  {
    return savingsAcctPin;
  }
}

This technique allows a method to specify the following in its signature:

  • The mutable class, if it requires a mutable object

  • The immutable class, if it wants to preserve immutability

  • The neutral interface or base class, if it does not care about immutability

This solution also prevents the casting problem exposed with the immutable interface class. The immutability of these classes cannot be cast away. For example, consider the following code:

public void foo(MutablePinNumbers p)
{}
public void bar(ImmutablePinNumbers p)
{}
MutablePinNumbers m = new MutablePinNumbers("person1", 101,
                                            201);
ImmutablePinNumbers im = new ImmutablePinNumbers("person2", 102,
                                                  202);
foo((MutablePinNumbers)im);   //Compiler error
bar((ImmutablePinNumbers)m);  //Compiler error

Method foo takes an object reference of MutablePinNumbers as a parameter. Therefore, it can access the mutating methods of the MutablePinNumbers class. By contrast, method bar takes an object reference of type ImmutablePinNumbers as a parameter. Therefore, it cannot change the object referred to by parameter p. The object remains immutable for the duration of this method. If code tries to cast between these two types, the compiler generates an error.

This implementation ensures that the immutability constraints cannot be breached by a simple cast.

Immutable Delegation Class

Another approach uses an immutable delegation class. This class contains only immutable methods and delegates these calls to the mutable object that it contains. For example, returning to the circle classes, the delegation technique looks like this:

class MutableCircle
{
  private double radius;
  public MutableCircle(double r)
  {
    radius = r;
  }

  public void setRadius(double r)
  {
    radius = r;
  }

  public double radius()
  {
    return radius;
  }
}

final class ImmutableCircle
{
  private MutableCircle mCircle;
  public ImmutableCircle(double r)
  {
    mCircle = new MutableCircle(r);
  }

  public double radius()
  {
    return mCircle.radius();
  }
}

The ImmutableCircle class uses layering, or the "has-a" relationship, with the MutableCircle class. When you create an ImmutableCircle object, you also create a MutableCircle object. Users of the ImmutableCircle object, however, cannot access the underlying MutableCircle object. They can access only the immutable methods provided in the ImmutableCircle class. Unlike the earlier immutable interface example, the user of these classes cannot cast between them.

This solution is particularly useful when you are unable to modify an existing mutable class. For example, the class might be part of a library you are using, and you do not have access to the source code to use the other techniques. In this case, you can use the layering approach.

However, this solution has a downside. Coding the delegation model requires more work to implement and more effort to understand and maintain. In addition, a performance penalty is associated with each delegated method call. Consider these factors before deciding which technique to use.

Table 1 lists the advantages and disadvantages of the techniques to provide immutable objects.

Table 1

Immutability Techniques

Technique Advantages Disadvantages
Immutable interface Easy and straightforward.
No performance penalty.
Can be breached
Common interface or base class Cannot be breached.
Clean way to separate mutable objects from immutable objects.
Extra classes to implement.
Deeper class hierarchy.
Immutable delegation class Cannot be breached.
Useful when you cannot change the source of an existing mutable class.
Performance penalty.

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