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Unlike method overriding, in method overloading the choice of Method and constructor overloading allows declaration of methods or constructors with the same name but with different parameter lists. The compiler inspects each call to an overloaded method or constructor and uses the declared types of the method parameters to decide which method to invoke is determined at compile time. Even if the run-time type differs for each invocation, in overloading, the method invocations depend on the type of the object at compile time.

Non-Compliant Code Example

. In some cases, however, confusion may arise because of the presence of relatively new 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 when a developer fails to consult the documentation at each use of a 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, an overloaded getDistance() method in which one overloaded method returns the distance traveled from the source while another (with reordered parameters) returns the remaining distance to the destination. Implementers may fail to realize the difference unless they consult the documentation at each use.

Noncompliant Code Example (Constructor)

Constructors cannot be overridden and can only be overloaded. This noncompliant code example shows the class Con with three overloaded constructors:This non-compliant 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 run-time 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.

class Con {
  public class Overloader { Con(int i, String s) { 
    // Initialization Sequence #1
  public}
 static Stringpublic displayCon(SetString s, int i) {
    // return "Set invoked";Initialization Sequence #2 
  }
 
  public staticCon(Integer i, String display(ArrayList ls) {
    return "ArrayList invoked"; // Initialization Sequence #3 
  } 

  }

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

Compliant Solution (Constructor)

This compliant solution avoids overloading by declaring public static factory methods having distinct names in place of the public class constructors:

public static StringCon display(Collection ccreateCon1(int i, String s) { 
  /*  return "Collection is not recognized";
  }

  Initialization Sequence #1 */
}
public static Con createCon2(String s, int i) { 
  /* Initialization Sequence #2 */
}
public static voidCon maincreateCon3(Integer i, String[] argss) {
  /* Initialization Sequence Collection[] invokeAll = new Collection[] {new HashSet(), new ArrayList(), new TreeSet()};

    for (int i = 0; i < invokeAll.length; i++)
#3 */
}

Noncompliant Code Example (Method)

In this noncompliant code example, the OverLoader class holds a HashMap instance and has overloaded getData() methods. One getData() method chooses the record to return on the basis of its key value in the map; the other chooses on the basis of the actual mapped value. 

class OverLoader extends HashMap<Integer,Integer> {
  HashMap<Integer,Integer> hm;
  public OverLoader() {
    hm = new HashMap<Integer, Integer>();
    // SSN records
    hm.put(1, 111990000);
    hm.put(2, 222990000);
    hm.put(3, 333990000);	  
  }
  public String getData(Integer i) { // Overloading sequence #1
    String s = get(i).toString(); // Get a particular record
    return (s.substring(0, 3) + "-" + s.substring(3, 5) + "-" + 
            Systems.out.println(display(invokeAll[i]));
  }
}

Compliant Solution

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

public class Overloader {

  public static String display(Collection csubstring(5, 9));
	  
  }
  public Integer getData(int i) { // Overloading sequence #2
    return hm.get(i); // Get record at position 'i'
  }
  // Checks whether the ssn exists
  @Override public Integer get(Object data) {
    // SecurityManagerCheck()

    for (Map.Entry<Integer, Integer> entry : hm.entrySet()) {
      if(entry.getValue().equals(data)) {
        return (c instanceof Set ? "Set invoked" : (c instanceof ArrayList ? "ArrayList invoked" : "Collection is not recognized"))entry.getValue();  // Exists 
      }
    }
    return null;
  }

  public static void main(String[] args) {
    Collection[]OverLoader invokeAllbo = new Collection[] {new HashSetOverLoader(),;
  new ArrayList(), new TreeSet()};

    for (int i = 0; i < invokeAll.length; i++)
       // Get record at index '3'
    System.out.println(bo.getData(3));
    //Get record containing data '111990000'
    System.out.println(bo.getData(display(invokeAll[i](Integer)111990000));
  }
}

References

For purposes of overload resolution, the signatures of the getData() methods differ only in the static type of their formal parameters. The OverLoader 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 both 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.

Although the client programmer might eventually deduce such behavior, other cases, such as with the List interface, may go unnoticed, as Joshua 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 fail to realize that the wrong element has been removed from the list.

A further problem is that in the presence of autoboxing, adding a new overloaded method definition can break previously working client code. This can happen when a new overloaded method with a more specific type is added to an API whose methods used less specific types in earlier versions. For example, if an earlier version of the OverLoader class provided only the getData(Integer) method, the client could correctly invoke this method by passing a parameter of type int; the result would be selected on the basis of its value because the int parameter would be autoboxed to Integer. Subsequently, when the getData(int) method is added, the compiler resolves all calls whose parameter is of type int to invoke the new getData(int) method, thereby changing their semantics and potentially breaking previously correct code. The compiler is entirely correct in such cases; the actual problem is an incompatible change to the API.

Compliant Solution (Method)

Naming the two related methods differently eliminates both the overloading and the confusion.

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

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

Applicability

Ambiguous or confusing uses of overloading can lead to unexpected results.

Automated Detection

Bibliography

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Image Added Image Added Image AddedJEPL, Item 26, Use overloading judiciously
Java Documentation http://java.sun.com/j2se/1.4.2/docs/api/java/util/Collection.html^{Image Removed}