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A variable arity (aka varargs) method is a method that can take a variable number of arguments. The method must contain at least one fixed argument. When processing a variable arity method call, the Java compiler checks the types of all arguments, and all of the variable actual arguments must match the variable formal argument type. However, compile-time type checking is ineffective when Object or generic parameter types are used [Bloch 2008]. The presence of initial parameters of specific types is irrelevant; the compiler will remain unable to check Object or generic variable parameter types. Enable strong compile-time type checking of variable arity methods by using the most specific type possible for the method parameter.

Noncompliant Code Example (Object)

This noncompliant code example sums a set of numbers using a variable arity method that uses Object as the variable arity type. Consequently, this method accepts an arbitrary mix of parameters of any object type. Legitimate uses of such declarations are rare (but see the "Applicability" section of this guideline).

double sum(Object... args) {
   double result = 0.0;
   for (Object arg : args) {
     if (arg instanceof Byte) {
       result += ((Byte) arg).byteValue();
     } else if (arg instanceof Short) {
       result += ((Short) arg).shortValue();
     } else if (arg instanceof Integer) {
       result += ((Integer) arg).intValue();
     } else if (arg instanceof Long) {
       result += ((Long) arg).longValue();
     } else if (arg instanceof Float) {
       result += ((Float) arg).floatValue();
     } else if (arg instanceof Double) {
       result += ((Double) arg).doubleValue();
     } else {
       throw new ClassCastException();
     }
   }
   return result;
}

Compliant Solution (Number)

This compliant solution defines the same method but uses the Number type. This abstract class is general enough to encompass all numeric types, yet specific enough to exclude nonnumeric types.

double sum(Number

The Java compiler type checks the arguments to each varargs method to ensure that the arguments are of the same type or object reference. However, the compile-time checking is ineffective when {{Object}} or generic {{T}} parameter types are used \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. Another requirement for providing strong compile-time type checking of variable argument methods is to be as specific as possible when declaring the type of the method parameter.

Noncompliant Code Example (Object)

This noncompliant code example declares a vararg method using Object. It accepts an arbitrary mix of parameters of any object type. Legitimate uses of such declarations are rare (see exception below).

ReturnType function(Object... args) {
  // ...
}

Noncompliant Code Example (Generic Type)

This noncompliant code example declares a vararg the same variable arity method using a generic type parameter. It accepts a variable number of parameters that are all of the same object type; however, it may be any object type. Again, legitimate uses of such declarations are rare.

<T> ReturnTypedouble functionsum(T... args) {
  // ...
}

Compliant Solution

...

(Generic Type)

This compliant solution defines the same generic method using the Number typeBe as specific as possible when declaring parameter types; avoid Object and imprecise generic types in varargs.

ReturnType function(SpecificObjectType<T extends Number> double sum(T... args) {
  // ...
}

Retrofitting old methods containing final array parameters with generically-typed varargs is not always a good idea. For example, given a method that does not accept an argument of a particular type, it could be possible to override the compile-time checking --- through the use of generic varargs parameters --- so that the method would compile cleanly rather than correctly, causing a compile-time error \[[Bloch 2008|AA. Bibliography#Bloch 08]\]. 

Be as specific as possible when declaring parameter types; avoid Object and imprecise generic types in variable arity methods. Retrofitting old methods containing final array parameters with generically typed variable arity parameters is not always a good idea. For example, given a method that does not accept an argument of a particular type, it could be possible to override the compile-time checking—through the use of generic variable arity parameters—so that the method would compile cleanly rather than correctly, causing a runtime error [Bloch 2008].

Also, note that autoboxing prevents Also, note that autoboxing does not allow strong compile-time type checking of primitive types and their corresponding wrapper classes.

Exceptions

For instance, this compliant solution produces the following warning but works as expected:

Java.java:10: warning: [unchecked] Possible heap pollution from parameterized vararg type T
 <T extends Number> double sum(T... args) {

Applicability

Injudicious use of variable arity parameter types prevents strong compile-time type checking, creates ambiguity, and diminishes code readability.

Variable arity DCL09-EX1: Varargs signatures using Object and imprecise generic types are acceptable when the body of the method does not use casts or auto-boxing and lacks both casts and autoboxing and it also compiles without error. Consider the following example, which operates correctly for all object types and type-checks successfully. :

<T> Collection<T> assembleCollection(T... args) {
  Collection<T> resultreturn = new HashSet<T>(Arrays.asList();
  // add each argument to the result collection
  return result args));
}

Risk Assessment

Unmindful use of the varargs feature prevents strong compile-time type checking, creates ambiguity, and diminishes code readability.

Automated Detection

Automated detection appears to be straightforward.

Bibliography

\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 42: "Use Varargs Judiciously"
\[[Steinberg 2005|AA. Bibliography#Steinberg 05]\] "Using the Varargs Language Feature"
\[[Sun 2006|AA. Bibliography#Sun 06]\] [varargs|http://java.sun.com/j2se/1.5.0/docs/guide/language/varargs.html] 

In some circumstances, it is necessary to use a variable arity parameter of type Object.  A good example is the method java.util.Formatter.format(String format, Object... args), which can format objects of any type.

Automated detection is straightforward.

Bibliography

...

Image Added Image Added Image AddedDCL08-J. Avoid ambiguous overloading of varargs methods      01. Declarations and Initialization (DCL)      DCL10-J. Ensure proper initialization by declaring class and instance variables final