DCL09-J. Enable compile-time type checking of varargs types

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]]. 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 method(Object... args) { }

Noncompliant Code Example (Generic Type)

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

<T> ReturnType method(T... args) { }

Compliant Solution

Be as specific as possible when declaring parameter types; avoid Object and imprecise generic types in varargs.

ReturnType method(SpecificObjectType... 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]].

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

Exceptions

DCL09-EX0: Varargs signatures using Object and imprecise generic types are acceptable when the body of the method does not use casts or auto-boxing and compiles without error. Consider the following example, which operates correctly for all object types and type checks successfully.

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

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]] Item 42: "Use Varargs Judiciously"
[[Steinberg 2005]] "Using the Varargs Language Feature"
[[Sun 2006]] varargs

DCL08-J. Avoid ambiguous overloading of varargs methods      01. Declarations and Initialization (DCL)      DCL10-J. Ensure proper initialization by declaring class and instance variables final