Home > Articles

Declarations

  • Print
  • + Share This
This chapter is from the book

3.6 Variable Arity Methods

A fixed arity method must be called with the same number of actual parameters (also called arguments) as the number of formal parameters specified in its declaration. If the method declaration specifies two formal parameters, every call of this method must specify exactly two arguments. We say that the arity of this method is 2. In other words, the arity of such a method is fixed, and it is equal to the number of formal parameters specified in the method declaration.

Java also allows declaration of variable arity methods, meaning that the number of arguments in its call can be varied. As we shall see, invocations of such a method may contain more actual parameters than formal parameters. Variable arity methods are heavily employed in formatting text representation of values, as demonstrated by the variable arity method System.out.printf() that is used in many examples for this purpose.

The last formal parameter in a variable arity method declaration is declared as follows:

type... formal_parameter_name

The ellipsis (...) is specified between the type and the formal_parameter_name. The type can be a primitive type, a reference type, or a type parameter. Whitespace can be specified on both sides of the ellipsis. Such a parameter is usually called a variable arity parameter (also known as varargs).

Apart from the variable arity parameter, a variable arity method is identical to a fixed arity method. The method publish() is a variable arity method:

public static void publish(int n, String... data) {      // (int, String[])
  System.out.println("n: " + n + ", data size: " + data.length);
}

The variable arity parameter in a variable arity method is always interpreted as having an array type:

type[]

In the body of the publish() method, the variable arity parameter data has the type String[], so it is a simple array of Strings.

Only one variable arity parameter is permitted in the formal parameter list, and it is always the last parameter in the formal parameter list. Given that the method declaration has n formal parameters, and the method call has k actual parameters, k must be equal to or greater than n – 1. The last kn + 1 actual parameters are evaluated and stored in an array whose reference value is passed as the value of the actual parameter. In the case of the publish() method, n is equal to 2, so k can be 1, 2, 3, and so on. The following invocations of the publish() method show which arguments are passed in each method call:

publish(1);                  // (1, new String[] {})
publish(2, "two");           // (2, new String[] {"two"})
publish(3, "two", "three");  // (3, new String[] {"two", "three"})

Each method call results in an implicit array being created and passed as an argument. This array can contain zero or more argument values that do not correspond to the formal parameters preceding the variable arity parameter. This array is referenced by the variable arity parameter data in the method declaration. The preceding calls would result in the publish() method printing the following output:

n: 1, data size: 0
n: 2, data size: 1
n: 3, data size: 2

To overload a variable arity method, it is not enough to change the type of the variable arity parameter to an explicit array type. The compiler will complain if an attempt is made to overload the method transmit(), as shown in the following code:

public static void transmit(String... data) {  }  // Compile-time error!
public static void transmit(String[] data)  {  }  // Compile-time error!

These declarations would result in two methods with equivalent signatures in the same class, which is not permitted.

Overloading and overriding of methods with variable arity are discussed in §7.10, p. 316.

Calling a Variable Arity Method

Example 3.11 illustrates various aspects of calling a variable arity method. The method flexiPrint() in the VarargsDemo class has a variable arity parameter:

public static void flexiPrint(Object... data) { // Object[]
  //...
}

The variable arity method prints the name of the Class object representing the actual array that is passed at runtime. It prints the number of elements in this array as well as the text representation of each element in the array.

The method flexiPrint() is called in the main() method. First with the values of primitive types and Strings ((1) to (8)), then it is called with the program arguments (p. 85) supplied in the command line, ((9) to (11)).

Compiling the program results in a warning at (9), which we ignore for the time being. The program can still be run, as shown in Example 3.11. The numbers at the end of the lines in the output relate to numbers in the code, and are not printed by the program.

Example 3.11 Calling a Variable Arity Method

public class VarargsDemo {
  public static void flexiPrint(Object... data) { // Object[]
    // Print the name of the Class object for the varargs parameter.
    System.out.print("Type: " + data.getClass().getName());

    System.out.println("  No. of elements: " + data.length);

    System.out.print("Element values: ");
    for(Object element : data)
      System.out.print(element + " ");
    System.out.println();
  }

  public static void main(String... args) {
    int    day       = 13;
    String monthName = "August";
    int    year      = 2009;

    // Passing primitives and non-array types:
    flexiPrint();                      // (1) new Object[] {}
    flexiPrint(day);                   // (2) new Object[] {Integer.valueOf(day)}
    flexiPrint(day, monthName);        // (3) new Object[] {Integer.valueOf(day),
                                       //                   monthName}
    flexiPrint(day, monthName, year);  // (4) new Object[] {Integer.valueOf(day),
                                       //                   monthName,
                                       //                   Integer.valueOf(year)}
    System.out.println();

    // Passing an array type:
    Object[] dateInfo = {day,          // (5) new Object[] {Integer.valueOf(day),
                         monthName,    //                   monthName,
                         year};        //                   Integer.valueOf(year)}
    flexiPrint(dateInfo);              // (6) Non-varargs call
    flexiPrint((Object) dateInfo);     // (7) new Object[] {(Object) dateInfo}
    flexiPrint(new Object[]{dateInfo});// (8) Non-varargs call
    System.out.println();

    // Explicit varargs or non-varargs call:
    flexiPrint(args);                  // (9) Warning!
    flexiPrint((Object) args);         // (10) Explicit varargs call
    flexiPrint((Object[]) args);       // (11) Explicit non-varargs call
  }
}

Compiling the program:

>javac VarargsDemo.java
VarargsDemo.java:41: warning: non-varargs call of varargs method with inexact
argument type for last parameter;
    flexiPrint(args);                  // (9) Warning!
               ^
  cast to Object for a varargs call
  cast to Object[] for a non-varargs call and to suppress this warning
1 warning

Running the program:

>java VarargsDemo To arg or not to arg
Type: [Ljava.lang.Object;  No. of elements: 0                (1)
Element values:
Type: [Ljava.lang.Object;  No. of elements: 1                (2)
Element values: 13
Type: [Ljava.lang.Object;  No. of elements: 2                (3)
Element values: 13 August
Type: [Ljava.lang.Object;  No. of elements: 3                (4)
Element values: 13 August 2009

Type: [Ljava.lang.Object;  No. of elements: 3                (6)
Element values: 13 August 2009
Type: [Ljava.lang.Object;  No. of elements: 1                (7)
Element values: [Ljava.lang.Object;@1eed786
Type: [Ljava.lang.Object;  No. of elements: 1                (8)
Element values: [Ljava.lang.Object;@1eed786

Type: [Ljava.lang.String;  No. of elements: 6                (9)
Element values: To arg or not to arg
Type: [Ljava.lang.Object;  No. of elements: 1                (10)
Element values: [Ljava.lang.String;@187aeca
Type: [Ljava.lang.String;  No. of elements: 6                (11)
Element values: To arg or not to arg

Variable Arity and Fixed Arity Method Calls

The calls in (1) to (4) in Example 3.11 are all variable arity calls, as an implicit Object array is created, in which the values of the actual parameters are stored. The reference value of this array is passed to the method. The printout shows that the type of the parameter is actually an array of Objects ([Ljava.lang.Object;).

The call at (6) differs from the previous calls, in that the actual parameter is an array that has the same type (Object[]) as the variable arity parameter, without having to create an implicit array. In such a case, no implicit array is created, and the reference value of the array dateInfo is passed to the method. See also the result from this call at (6) in the output. The call at (6) is a fixed arity call (also called a non-varargs call), where no implicit array is created:

flexiPrint(dateInfo);              // (6) Non-varargs call

However, if the actual parameter is cast to the type Object as in (7), a variable arity call is executed:

flexiPrint((Object) dateInfo);     // (7) new Object[] {(Object) dateInfo}

The type of the actual argument is now not the same as that of the variable arity parameter, resulting in an array of the type Object[] being created, in which the array dateInfo is stored as an element. The printout at (7) shows that only the text representation of the dateInfo array is printed, and not its elements, as it is the sole element of the implicit array.

The call at (8) is a fixed arity call, for the same reason as the call in (6). Now, however, the array dateInfo is explicitly stored as an element in an array of the type Object[] that matches the type of the variable arity parameter:

flexiPrint(new Object[]{dateInfo});// (8) Non-varargs call

The output from (8) is the same as the output from (7), where the array dateInfo was passed as an element in an implicitly created array of type Object[].

The compiler issues a warning for the call at (9):

flexiPrint(args);                  // (9) Warning!

The actual parameter args is an array of the type String[], which is a subtype of Object[]—the type of the variable arity parameter. The array args can be passed in a fixed arity call as an array of the type String[], or in a variable arity call as an element in an implicitly created array of the type Object[]. Both calls are feasible and valid in this case. Note that the compiler chooses a fixed arity call rather than a variable arity call, but also issues a warning. The result at (9) confirms this course of action.

The array args of the type String[] is explicitly passed as an Object in a variable arity call at (10), similar to the call at (7):

flexiPrint((Object) args);         // (10) Explicit varargs call

The array args of type String[] is explicitly passed as an array of the type Object[] in a fixed arity call at (11). This call is equivalent to the call at (9), where the widening reference conversion is implicit, but now without a warning at compile time. The two calls print the same information, as is evident from the output at (9) and (11):

flexiPrint((Object[]) args);       // (11) Explicit non-varargs call
  • + Share This
  • 🔖 Save To Your Account

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