- Java Basic
- Java Object Oriented
- Java Advanced
- Java Polymorphism
Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.
Any java object that can pass more than one IS-A test is considered to be polymorphic. In Java, all java objects are polymorphic since any object will pass the IS-A test for their own type and for the class Object. It is important to know that the only possible way to access an object is through a reference variable. A reference variable can be of only one type. Once declared the type of a reference variable cannot be changed.
The reference variable can be reassigned to other objects provided that it is not declared final. The type of the reference variable would determine the methods that it can invoke on the object. A reference variable can refer to any object of its declared type or any subtype of its declared type. A reference variable can be declared as a class or interface type.
Example:
Now the Deer class is considered to be polymorphic since this has multiple inheritance. Following are true for the above example:
- A Deer IS-A Animal
- A Deer IS-A Vegetarian
- A Deer IS-A Deer
- A Deer IS-A Object
When we apply the reference variable facts to a Deer object reference, the following declarations are legal:
All the reference variables d,a,v,o refer to the same Deer object in the heap.
Virtual Methods:
In this section, I will show you how the behavior of overridden methods in Java allows you to take advantage of polymorphism when designing your classes.
We already have discussed method overriding, where a child class can override a method in its parent. An overridden method is essentially hidden in the parent class, and is not invoked unless the child class uses the super keyword within the overriding method.
Now suppose we extend Employee class as follows:
Now you study the following program carefully and try to determine its output:
This would produce following result:
Here we instantiate two Salary objects . one using a Salary reference s, and the other using an Employee reference.
While invoking s.mailCheck() the compiler sees mailCheck() in the Salary class at compile time, and the JVM invokes mailCheck() in the Salary class at run time. Invoking mailCheck() on e is quite different because e is an Employee reference. When the compiler seese.mailCheck(), the compiler sees the mailCheck() method in the Employee class.
Here, at compile time, the compiler used mailCheck() in Employee to validate this statement. At run time, however, the JVM invokes mailCheck() in the Salary class. This behavior is referred to as virtual method invocation, and the methods are referred to as virtual methods. All methods in Java behave in this manner, whereby an overridden method is invoked at run time, no matter what data type the reference is that was used in the source code at compile time.