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 List. The output that one would typically expect 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 gets 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 08]]

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) {
    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. The output is ArrayList, LinkedList, List is not recognized, as expected. As a general rule, do not introduce ambiguity while using overloading so that the code is clean and easy to understand. [[Bloch 08]]

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));
    }
  }
}

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 since two overloadings can contain the same number of similarly typed parameters. This noncompliant 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 different types. This 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 in order 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.

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
    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 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 situation.

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.

Other Languages

This rule appears in the C++ Secure Coding Standard as OBJ07-CPP. Do not inherit from multiple classes that have distinct objects with the same name.

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