Advanced Programming in Java

Salman MarvastiSharif University of Technology

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AgendainterfaceMultiple Inheritance

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Review : AbstractAbstract Methods

No ImplementationSub-classes may implement abstract methods

Abstract ClassesCan not be instantiated(Usually) A class with one or more abstract

methodsA class which extends an abstract class

Is abstract unless it implements all the abstract methods

Concrete class Not abstract classFall 2014

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Abstract Example

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Abstract MethodAll subclasses have the methodBut we can not implement the method in

super-classSo why we define it in super-class?

Polymorphism

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InterfaceSometimes we have an abstract class, with no

concrete methodinterface : pure abstract classno implemented method

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Interface

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InterfaceAll the methods are implicitly abstract

No need to abstract specifier

All the methods are implicitly publicNo need to public specifier

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Interface ImplementationSome classes may inherit abstract classesSome classes may implement interfaces

Is-a relation existsIf a class implements an interfaceBut does not implement all the methodsWhat will happen?

The class becomes an abstract class

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Multiple Inheritance in JavaA class can inherit one and only one classA class may implement zero or more

interfaces

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A QuestionWhy multiple inheritance is not supported for

classes?Why multiple inheritance is supported for

interfaces?

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What About Name Collision?

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The return types are incompatible for the inherited methods B.f(), A.f()

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Interface ExtensionInterfaces may inherit other interfacesCode reuseIs-a relation

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Interface propertiesNo member variable

Variables : implicitly final and staticUsually interfaces declare no variable

Only operations are declaredNo constructor

Why?

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Example

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Interfaces ApplicationsPure Abstract classesDescribe the design

The architect designs interfaces, the programmer implements them

Only interfaces are delivered to code consumers

More powerful inheritance and polymorphism

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