MET50-JG. Avoid ambiguous uses of overloading

Method and constructor overloading allows one of two or more methods or constructors with the same name to be invoked, depending on the types of their arguments. The compiler inspects a call to an overloaded method or constructor and, using the declared types of the method parameters, decides which method to use. In some cases, however, ambiguity may arise because of the presence of relatively newer language features such as autoboxing and generics.

Furthermore, methods or constructors with the same parameter types that differ only in their declaration order are typically not flagged by Java compilers. Errors can result if a developer does not consult the documentation at every use of the method or constructor. A related pitfall is to associate different semantics with each of the overloaded methods or constructors. Defining different semantics sometimes necessitates different orderings of the same method parameters, creating a vicious circle. Consider, for example, a getDistance() method whose one overloading returns the distance traveled from the source while another (with rearranged parameters) returns the uncovered distance to the destination. An implementer may not realize the difference unless the documentation is consulted at every use.

Noncompliant Code Example (Constructor)

Constructors cannot be overridden and can only be overloaded. This noncompliant code example shows the constructor Con with three overloadings: Con(int, String), Con(String, int), and Con(Integer, String).

class Con {
  public Con(int i, String s) { /* Initialization Sequence #1 */ }
  public Con(String s, int i) { /* Initialization Sequence #2 */ } 
  public Con(Integer i, String s) { /* Initialization Sequence #3 */ } 
}

Failure to exercise caution while passing arguments to these constructors can create confusion because these overloadings can contain the same number of similarly typed parameters. Overloading must also be avoided when the overloaded constructors or methods provide inconsistent functionality for arguments of the same types, differing just in their declaration order.

Compliant Solution (Constructor)

This compliant solution uses public static factory methods instead of public class constructors:

public static Con conName1(int i, String s) { /* Initialization Sequence #1 */ }
public static Con conName2(String s, int i) { /* Initialization Sequence #2 */ }
public static Con conName3(Integer i, String s) { /* Initialization Sequence #3 */ }

Noncompliant Code Example (Method)

In this noncompliant code example, the BadOverloading class holds a HashMap instance and returns a particular record to the caller on the basis of either its key value in the map or the actual mapped value. The getData() method is overloaded to return the contained data indexed by the key value in the former case. In the latter case, it checks whether a particular record exists before formatting and returning it as a String object. The BadOverloading class inherits from java.util.HashMap and overrides its get() method to provide the checking functionality. This implementation can be extremely confusing to the client who expects the getData() methods to behave in a similar fashion and not depend on whether an index of the record or the value to be retrieved is specified.

A further problem is that in the presence of autoboxing, an int argument may invoke the undesired overloading for Integer. This can happen if the overloading with the primitive int type is added to a class at a later date. Clients who expect the getData() method to fetch data on the basis of its value will suddenly start invoking the new overloading whenever an int argument is passed and proceed to return the record by index. This happens because a primitive argument becomes more specific in the new version, whereas in the old one, autoboxing allows the selection of the method with the Integer type parameter when an int is passed.

class BadOverloading extends HashMap<Integer,Integer> {
  HashMap<Integer,Integer> hm;
  public BadOverloading() {
    hm = new HashMap<Integer, Integer>();
    hm.put(1, 111990000); hm.put(2, 222990000); hm.put(3, 333990000);  // ssn records	  
  }
  public Integer getData(int i) { // Overloading sequence #1
    return hm.get(i); // Get record at position 'i'
  }
  public String getData(Integer i) { // Overloading sequence #2
    String s = get(i).toString(); // Get a particular record
    return (s.substring(0, 3) + "-" + s.substring(3, 5) + "-" + s.substring(5, 9));
	  
  }
  @Override public Integer get(Object data) {  // Checks whether the ssn exists
    // SecurityManagerCheck()

    for (Map.Entry<Integer, Integer> entry : hm.entrySet()) {
      if(entry.getValue().equals(data)) {
        return entry.getValue();  // Exists 
      }
    }
    return null;
  }
  public static void main(String[] args) {
    BadOverloading bo = new BadOverloading();
    System.out.println(bo.getData(3)); // Get record at index '3'
    System.out.println(bo.getData((Integer)111990000)); // Get record containing data '111990000'
  }
}

Although the client programmer might eventually deduce such behavior, other cases, such as with the List interface, may go unnoticed, as Bloch [Bloch 2008] describes:

The List<E> interface has two overloadings of the remove method: remove(E) and remove(int). Prior to release 1.5 when it was "generified," the List interface had a remove(Object) method in place of remove(E), and the corresponding parameter types, Object and int, were radically different. But in the presence of generics and autoboxing, the two parameter types are no longer radically different.

Consequently, a client programmer may not realize that the wrong element has been removed from the list.

Compliant Solution (Method)

Naming the two related methods differently eliminates overloading:

public Integer getDataByIndex(int i) { /* No longer overloaded */ }

public String getDataByValue(Integer i) { /* No longer overloaded */ }

Applicability

Ambiguous uses of overloading can lead to unexpected results.

Bibliography