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Java supports overloading methods and can distinguish between methods with different method signatures. This means that, with some qualifications, methods within a class can have the same name if they have different parameter lists. In method overloading, the determination of the method to be invoked at runtime is determined at compile time. Consequently, the overloaded method associated with the static type of the object is invoked, even when the runtime type differs for each invocation.

Do not introduce ambiguity while overloading (see [MET00-J. Avoid ambiguous uses of overloading]) and use overloaded methods sparingly \[[Tutorials 2010|AA. Bibliography#Tutorials 10]\] as they can make code much less readable.

Noncompliant Code Example

This noncompliant code example attempts to use the overloaded display() method to perform different actions depending on whether the method is passed an ArrayList<Integer> or a LinkedList<String>.

public class Overloader {
  private static String display(ArrayList<Integer> a) {
    return "ArrayList";
  }

  private static String display(LinkedList<String> l) {
    return "LinkedList";
  }

  private static String display(List<?> l) {
    return "List is not recognized";
  }

  public static void main(String[] args) {
    // Single ArrayList
    System.out.println(display(new ArrayList<Integer>()));
    // Array of lists
    List<?>[] invokeAll = new List<?>[] {new ArrayList<Integer>(), 
    new LinkedList<String>(), new Vector<Integer>()};

    for (List<?> i : invokeAll) {
      System.out.println(display(i));
    }
  }
}

At compile time, the type of the object array is {{List}}. The expected output is {{ArrayList}}, {{ArrayList}}, {{LinkedList}} and {{List is not recognized}} ( because {{java.util.Vector}} does not inherit from {{java.util.List}}). The actual output is {{ArrayList}} followed by three instances of {{List is not recognized}}. The cause of this unexpected behavior is that overloaded method invocations are affected _only_ by the compile time type of their arguments: {{ArrayList}} for the first invocation and {{List}} for the others. Do not use overloading where overriding would be natural \[[Bloch 2008|AA. Bibliography#Bloch 08]\].

Compliant Solution

This compliant solution uses a single display method and instanceof to distinguish between different types. As expected, the output is ArrayList, ArrayList, LinkedList, List is not recognized.

class Overloader {
public class Overloader {
  private static String display(List<?> l) {
    return (
      l instanceof ArrayList ? "Arraylist" : 
      (l instanceof LinkedList ? "LinkedList" : 
      "List is not recognized")
    );
  }

  public static void main(String[] args) {
    // Single ArrayList
    System.out.println(display(new ArrayList<Integer>()));

    List<?>[] invokeAll = new List<?>[] {new ArrayList<Integer>(), 
    new LinkedList<String>(), new Vector<Integer>()};

    for (List<?> i : invokeAll) {
      System.out.println(display(i));
    }
  }
}

Risk Assessment

Ambiguous uses of overloading can lead to unexpected results.

Guideline	Severity	Likelihood	Remediation Cost	Priority	Level
MET05-J	low	unlikely	high	P1	L3

Automated Detection

Sound automated detection of violations is infeasible, because it would require determination of programmer intent. Heuristic techniques may be useful.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] [Interface Collection|http://java.sun.com/j2se/1.4.2/docs/api/java/util/Collection.html]
\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 41: Use overloading judiciously
\[[Tutorials 2010|AA. Bibliography#Tutorials 10]\] [Defining Methods|http://download.oracle.com/javase/tutorial/java/javaOO/methods.html]

MET04-J. Ensure that constructors do not call overridable methods 05. Methods (MET)