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Unlike method overriding, in method overloading the choice of which method to invoke Java supports overloading methods and can distinguish between methods with different signatures. Consequently, with some qualifications, methods within a class can have the same name if they have different parameter lists. In method overloading, the method to be invoked at runtime is determined at compile time. Even if Consequently, the overloaded method associated with the static type of the object is invoked even when the runtime type differs for each invocation, in overloading, the method invocations depend on the type of the object at compile time.

For program understandability, do not introduce ambiguity while overloading (see MET50-J. Avoid ambiguous or confusing uses of overloading), and use overloaded methods sparingly [Tutorials 2013].

Noncompliant Code Example

Wiki MarkupThis noncompliant example shows how the programmer can confuse overloading with overriding. At compile time, the type of the object array is {{List}}. The expected output is {{ArrayList}}, {{LinkedList}} and {{List is not recognized}} ({{java.util.Vector}} does not inherit from {{java.util.List}}). However, in all three instances {{List is not recognized}} gets displayed. This happens because in overloading, the method invocations are not affected by the runtime types but only the compile time type ({{List}}). It is dangerous to implement overloading to tally with overriding, more so, because the latter is characterized by inheritance unlike the former. \[[Bloch 08|AA. Java References#Bloch 08]\]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> aarrayList) {
    return "ArrayList";
  }

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

  private static String display(List<?> llist) {
    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<?> ilist : invokeAll) {
      System.out.println(display(ilist));
    }
  }
}

Compliant Solution

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 is neither an ArrayList nor a LinkedList). The actual output is ArrayList followed by List is not recognized repeated three times. 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.

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: Wiki MarkupThis compliant solution uses a single {{display}} method and {{instanceof}} to distinguish between different types. The output is {{ArrayList, LinkedList, List is not recognized}}, as expected. As a general rule, do not introduce ambiguity while using overloading so that the code is clean and easy to understand. \[[Bloch 08|AA. Java References#Bloch 08]\]

class Overloader {
public class Overloader {
  private static String display(List<?> llist) {
    return (l
      list instanceof ArrayList ? "Arraylist" : 
      (llist 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<?> ilist : invokeAll) {
      System.out.println(display(ilist));
    }
  }
}

...

Applicability

Ambiguous uses of overloading can lead to unexpected results.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[API 06|AA. Java References#API 06]\] [Interface Collection|http://java.sun.com/j2se/1.4.2/docs/api/java/util/Collection.html]
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 41: Use overloading judiciously

Bibliography

...

Image Added Image Added Image AddedMET32-J. Ensure that constructors do not call overridable methods      12. Methods (MET)      MET34-J. Follow the general contract when implementing the compareTo method