Inheritance And Polymorphism

Inheritance and Polymorphism • Class Hierarchy • Restrictions on Overriding • Method Lookup • Inheritance and Constructor • The Superclass in Object Construction • this and super • Polymorphism Prepared by: Dexie C. Gaspar

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Class Hierarchies

Object Equals() Number

Object extends

byteValue() doubleValue() intValue() :

Number extends Integer

Integer

extends Integer zero = new Integer(0);

If(zero.equals(x)) byte = zero.byteValue();

Prepared by: Dexie C. Gaspar

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Restrictions on Method Overriding –The parameter list and return type must match the inherited method exactly –The access modifier must not be more restrictive than that of the inherited method For example, if overriding a protected method, the new method can be protected or public, but not private Prepared by: Dexie C. Gaspar

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Method Lookup •When a method is invoked, the lookup begins in the object's class definition •If the method is not found in the object's own class, the search continues in the superclass and up the hierarchy until it is found •When the method is found, it will be invoked on the object to which the method was passed •If the method was never implemented in any of the classes in the hierarchy, an error is issued at compile time Prepared by: Dexie C. Gaspar 4

Inheritance and Constructors •Only constructors within the class being instantiated and within the immediate superclass can be invoked •Constructors in the superclass are invoked with the keyword super and the parameter list –the parameter list must match that of an existing constructor in the superclass

•Constructors in the same class are invoked with the keyword this and the parameter list •The first line of your constructor can be one of: – super(…); – this(…);

Prepared by: Dexie C. Gaspar

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The Superclass in Object Construction •Superclass objects are built before the subclass

–The compiler supplies an implicit super() call for all constructors

•super(…) initializes superclass members •If the first line of your constructor is not a call to another constructor, super() is called automatically

–Zero argument constructor in the superclass is called as a result –This can cause an error if the superclass does not have a zero-argument constructor Prepared by: Dexie C. Gaspar

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More on this and super •this and super can be used in instance methods and constructors in Java –They cannot be used in class (static) methods

•The keyword super lets you use code from a superclass when you are overriding a method •this and super affect method look-up –this: method look-up starts in the current class –super: method look-up starts in the immediate superclass Prepared by: Dexie C. Gaspar

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Polymorphism – The ability of a reference variable to change behavior according to what object it is holding. – This allows multiple objects of different subclasses to be treated as objects of a single superclass, while automatically selecting the proper methods to apply to a particular object based on the subclass it belongs to. Prepared by: Dexie C. Gaspar

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Polymorphism Given the parent class Person and the subclass Student and another subclass of Person which is Employee. Below is the class hierarchy for that Person

Student

getName()

Employee

overrides: getName()

overrides: Prepared by: Dexie C. Gaspar

getName()

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Polymorphism In Java, we can create a reference that is of type superclass to an object of its subclass. public static main( String[] args ) { Person ref; Student studentObject = new Student(); Employee employeeObject = new Employee(); ref = studentObject;  ref.getName(); ref = employeeObject; ref.getName(); } Prepared by: Dexie C. Gaspar 10