MET02-J. Avoid ambiguous uses of overloading

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 example shows how the programmer can confuse overloading with overriding. At compile time, the type of the object array is Collection. The messages that one would typically expect are Set invoked, ArrayList invoked and Collection is not recognized. However, in all three instances Collection is not recognized gets displayed. This is because in overloading, the method invocations are not affected by the runtime types but only the compile time type (Collection). It is dangerous to implement overloading to tally with overriding, more so, because the latter is characterized by inheritance unlike the former. [[Bloch 08]]

public class Overloader {
  private static String display(Set<Integer> s) {
    return "Set invoked";
  }

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

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

  public static void main(String[] args) {
    Collection<?>[] invokeAll = new Collection<?>[] {new HashSet<Integer>(), 
    new ArrayList<String>(), new TreeSet<Integer>()};

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

Compliant Solution

This compliant solution uses a single display method and instanceof to distinguish between different types. The output is Set invoked, ArrayList invoked, Set invoked which is expected. Do not introduce ambiguity while using overloading so that the code is clean and easy to understand. [[Bloch 08]]

class Overloader {
  private static String display(Collection<?> c) {
    return (c instanceof Set ? "Set invoked" : (c instanceof ArrayList ? "ArrayList invoked"
    : "Collection is not recognized"));
  }

  public static void main(String[] args) {
    Collection<?>[] invokeAll = new Collection<?>[] {new HashSet<Integer>(), new ArrayList<String>(), new TreeSet<Integer>()};

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

Notably, constructors cannot be overridden and can only be overloaded. Exercise caution while passing arguments to them.

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.

References

[[API 06]] Interface Collection
[[Bloch 08]] Item 41: Use overloading judiciously

MET01-J. Follow good design principles while defining methods      09. Methods (MET)      MET03-J. For methods that return an array or collection prefer returning an empty array or collection over a null value