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Overloading polymorphism refers to using a single identifier for different operations. For example, the + identifier -- I use the term identifier in a general sense -- signifies any one of several operations based on its operands' types. If both operands have integer
types, the integer addition operation occurs. Similarly, if both operands have floating-point types, a floating-point operation
occurs. Finally, if those operands are strings, string concatenation is guaranteed to be the operation.
Along with its language-defined operator overloading, Java also permits method names to overload -- as long as the number and/or types of each method's parameters differ. That way, the same method-name identifier can apply to different operations.
Many developers do not regard the coercion and overloading polymorphism categories as true forms of polymorphism. At close inspection, coercion and overloading are seen as convenient type conversion aids and syntactic sugar. In contrast, parametric and inclusion polymorphism are considered genuine polymorphism.
Parametric polymorphism refers to a class declaration that allows the same field names and method signatures to associate with a different
type in each instance of that class. For example, you might create a LinkedList class with a data field that holds any type of data. To allow for proper type checking at compile time, you do not want to give that field
Object type (as in Object data;) as the compiler cannot inform you if the code attempts to perform invalid operations on data, because only the JVM knows the actual type of data at runtime. However, you do not want to tie data to a specific type in your source code, because you lose the benefit of being able to store different object types in your
LinkedList objects. To achieve the best of both worlds, parametric polymorphism gives you the benefits of static type checking -- which
alerts you to attempts that perform invalid operations on data -- and allows data to hold different object types. Probably the best-known example of parametric polymorphism is the C++ template language feature.
Although Java does not officially support parametric polymorphism, developers expect Sun to add that support to version 1.5
of the SDK, which is scheduled for a 2003 release. For more information on parametric polymorphism, I refer you to Eric Allen's
well-written article "Behold the Power of Parametric Polymorphism."
Inclusion polymorphism refers to a situation in which a type can be another type's subtype. Every subtype value can appear in a supertype context, where the execution of the supertype's operations (on that value) results in the execution of the subtype's equivalent operations. For that reason, inclusion polymorphism is also known as subtype polymorphism. To understand inclusion (or subtype) polymorphism, you must understand how Java binds method calls to classes and objects.
Recall from an earlier article in this series that Java supports two categories of methods: class (also known as static) and instance (also known as nonstatic). Class methods associate with classes, and instance methods associate with objects. When you call a method, Java binds that method call to either a class or an object, depending on that method's category.
switch statements start reappearing in your code in "Java Tip 30Polymorphism and Java," Philip Bishop (JavaWorld): http://www.javaworld.com/javaworld/javatips/jw-javatip30.html