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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
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