Java™ Application Development on Linux - Dator

3.4 Encapsulation, Inheritance, and Polymorphism913.4.3 PolymorphismPolymorphism (from the Greek poly meaning “many” and morph meaning“shape”) refers to the language’s ability to deal with objects of many different“shapes,” that is, classes, as if they were all the same. We have already seen thatJava does this via the extends and implements keywords. You can define aninterface and then define two classes that both implement this interface.Remember our Sample class (Example 3.26). We’ll now define anotherclass, Employee, which also implements the Identifiable interface(Example 3.27).Example 3.27 The Employee classclassEmployeeextends Personimplements Identifiable{private int empl_id;}public int getID(){return empl_id;}Notice that the same method, getID(), is implemented in the Employeeclass, but that the field from which it gets the ID value is a different field. That’simplementation-specific—the interface defines only the methods that can becalled but not their internal implementation. The Employee class not onlyimplements Identifiable, but it also extends the Person class, so we bettershow you what our example Person class looks like (Example 3.28).To make a really useful Person class would take a lot more code than weneed for our example. The important part for our example is only that it isquite different from the Sample class we saw earlier.Example 3.29 demonstrates the use of polymorphism. We only show somesmall relevant snippets of code; there would be a lot more code for this to becomean entire, complete example. Don’t be distracted by the constructors; wemade up some new ones just for this example, that aren’t in the class definitionsabove. Can you see where the polymorphism is at work?