MET02-J. Avoid ambiguous uses of overloading

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

Furthermore, methods with the same parameter types that just differ in their declaration order, are typically not flagged by Java compilers. This can be potentially error prone if a developer does not consult the documentation at every use of the method. A related pitfall is to associate differing semantics with each of the overloaded methods. 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.

This recommendation also applies to constructors as they can also be overloaded.

Noncompliant Code Example

Notably, constructors cannot be overridden and can only be overloaded. Failure to exercise caution while passing arguments to them can create confusion as two overloadings can contain the same number of similarly typed parameters. This noncompliant code example shows the constructor Con with three overloadings Con(int, String), Con(String, int) and Con(Integer, String). Overloading should also be avoided when the overloaded methods provide inconsistent functionality for arguments of the same types, differing just in their declaration order. This noncompliant code example violates both these conditions.

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 */ } 
}

Compliant Solution

To be compliant, prefer the use of public static factory methods over public class constructors.

public static void ConName1(int i, String s) { /* Initialization Sequence #1 */ }
public static void ConName2(String s, int i) { /* Initialization Sequence #2 */ }
public static void ConName3(Integer i, String s) { /* Initialization Sequence #3 */ }

Noncompliant Code Example

This program provides another concrete example of noncompliance. The InformationLeak class holds a HashMap instance and returns a particular record to the caller based on either its key value in the map or the actual mapped value. The getData() method has been overloaded to return the contained data indexed by the key value in one case, whereas in the other, it checks whether a particular record exists before formatting and returning it as a String object. The InformationLeak 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 will expect the getData() methods to behave identically and not variably, depending on whether an index of the record is specified or the retrievable value.

Worse, in the presence of autoboxing, an innocent int argument may end up invoking 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 based on 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 {
  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().compareTo((Integer)data) == 0)
        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 will deduce such behavior sooner or later, other cases such as with the List interface may go unnoticed as Bloch [[Bloch 08]] 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.

Compliant Solution

Naming the two related methods differently disqualifies the overloading and is highly recommended in this case.

public Integer getDataByIndex(int i) { /* overloading  sequence #1 */ }
public String getDataByValue(Integer i) { /* overloading sequence #2 */ }

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

MET00-J. Follow good design principles while defining methods      12. Methods (MET)      MET03-J. For methods that return an array or collection prefer returning an empty array or collection over a null value