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. \[[Bloch 08|AA. Java References#Bloch 08]\]

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));
    }
  }
}

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. \[[Bloch 08|AA. Java References#Bloch 08]\]

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));
    }    
  }
}

\[[API 06|AA. Java References#API 06]\] [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