MET05-J. Do not subject overloaded methods to polymorphic invocations

Unlike method overriding, in method overloading the choice of which method to invoke is determined at compile time. Even if the runtime type differs for each invocation, in overloading, the method invocations depend on the type of the object at compile time.

Noncompliant Code Example

This noncompliant code 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 is 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 2008]]

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) {
    // 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));
    }
  }
}

Compliant Solution

This compliant solution uses a single display method and instanceof to distinguish between different types. As expected, the output is ArrayList, LinkedList, List is not recognized. As a general rule, do not introduce ambiguity while overloading so that the code is clean and easy to understand. [[Bloch 2008]]

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) {
    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.

Automated Detection

TODO

Related Vulnerabilities

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

Bibliography

[[API 2006]] Interface Collection
[[Bloch 2008]] Item 41: Use overloading judiciously

MET04-J. Ensure that constructors do not call overridable methods      16. Methods (MET)      MET06-J. Do not call overridable methods from a privileged block