Lec 1.8 Object Oriented Programming

Lec 1.8 Object Oriented Programming

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  CMP-2123-Object Oriented Programming Lecture 8 By AbrarAhmad Abrar.ahmad14@ce.ceme.edu.pk
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  Java – Polymorphism BadarWaseer arbabwaseergmail.com 2
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  • Polymorphism isthe 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 Badar Waseer arbabwaseergmail.com 3
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  • The referencevariable 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. Badar Waseer arbabwaseergmail.com 4
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  • Let uslook at an example. Badar Waseer arbabwaseergmail.com 5
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  • Now, theDeer class is considered to be polymorphic since this has multiple inheritance. Following are true for the above examples − • A Deer IS-A Animal • A Deer IS-A Vegetarian • A Deer IS-A Deer • A Deer IS-A Object Badar Waseer arbabwaseergmail.com 6
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  • When weapply 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. Badar Waseer arbabwaseergmail.com 7
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  Virtual Methods • Inthis 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. Badar Waseer arbabwaseergmail.com 8
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  Badar Waseer arbabwaseergmail.com9
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  • Now supposewe extend Employee class as follows − Badar Waseer arbabwaseergmail.com 10
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  Badar Waseer arbabwaseergmail.com11
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  • Now, youstudy the following program carefully and try to determine its output − Badar Waseer arbabwaseergmail.com 12
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  Badar Waseer arbabwaseergmail.com13
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  • This willproduce the following result − Badar Waseer arbabwaseergmail.com 14
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  Badar Waseer arbabwaseergmail.com15
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  • Here, weinstantiate two Salary objects. One using a Salary reference s, and the other using an Employee reference e. • 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. • mailCheck() on e is quite different because e is an Employee reference. When the compiler sees e.mailCheck(), the compiler sees the mailCheck() method in the Employee class. Badar Waseer arbabwaseergmail.com 16
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  • Here, atcompile 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 these methods are referred to as virtual methods. 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. Badar Waseer arbabwaseergmail.com 17
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  Any Questions ? BadarWaseer arbabwaseergmail.com 18
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  Thank You Badar Waseerarbabwaseergmail.com 19