Object oriented programming 3 object oriented concepts

Object oriented programming 3 object oriented concepts

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  Object Oriented Programming: ProfNeeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
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  1-2 Object-Oriented Concepts • ADTsare usually called classes • Class instances are called objects • A class that inherits is a derived class or a subclass • The class from which another class inherits is a parent class or superclass • Subprograms that define operations on objects are called methods
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  1-3 Object-Oriented Concepts (continued) •Calls to methods are called messages • The entire collection of methods of an object is called its message protocol or message interface • Messages have two parts--a method name and the destination object • In the simplest case, a class inherits all of the entities of its parent
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  1-4 Object-Oriented Concepts (continued) •Inheritance can be complicated by access controls to encapsulated entities – A class can hide entities from its subclasses – A class can hide entities from its clients – A class can also hide entities for its clients while allowing its subclasses to see them • Besides inheriting methods as is, a class can modify an inherited method – The new one overrides the inherited one – The method in the parent is overriden
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  Object-Oriented Concepts (continued) •Three ways a class can differ from its parent: 1. The subclass can add variables and/or methods to those inherited from the parent 2. The subclass can modify the behavior of one or more of its inherited methods. 3. The parent class can define some of its variables or methods to have private access, which means they will not be visible in the subclass 1-5
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  1-6 Object-Oriented Concepts (continued) •There are two kinds of variables in a class: – Class variables - one/class – Instance variables - one/object • There are two kinds of methods in a class: – Class methods – accept messages to the class – Instance methods – accept messages to objects • Single vs. Multiple Inheritance • One disadvantage of inheritance for reuse: – Creates interdependencies among classes that complicate maintenance
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  1-7 Dynamic Binding • Apolymorphic variable can be defined in a class that is able to reference (or point to) objects of the class and objects of any of its descendants • When a class hierarchy includes classes that override methods and such methods are called through a polymorphic variable, the binding to the correct method will be dynamic • Allows software systems to be more easily extended during both development and maintenance
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  1-8 Dynamic Binding Concepts •An abstract method is one that does not include a definition (it only defines a protocol) • An abstract class is one that includes at least one virtual method • An abstract class cannot be instantiated
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  1-9 Design Issues forOOP Languages • The Exclusivity of Objects • Are Subclasses Subtypes? • Single and Multiple Inheritance • Object Allocation and Deallocation • Dynamic and Static Binding • Nested Classes • Initialization of Objects
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  1-10 The Exclusivity ofObjects • Everything is an object – Advantage - elegance and purity – Disadvantage - slow operations on simple objects • Add objects to a complete typing system – Advantage - fast operations on simple objects – Disadvantage - results in a confusing type system (two kinds of entities) • Include an imperative-style typing system for primitives but make everything else objects – Advantage - fast operations on simple objects and a relatively small typing system – Disadvantage - still some confusion because of the two type systems
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  1-11 Are Subclasses Subtypes? •Does an “is-a” relationship hold between a parent class object and an object of the subclass? – If a derived class is-a parent class, then objects of the derived class must behave the same as the parent class object • A derived class is a subtype if it has an is-a relationship with its parent class – Subclass can only add variables and methods and override inherited methods in “compatible” ways • Subclasses inherit implementation; subtypes inherit interface and behavior
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  1-12 Single and MultipleInheritance • Multiple inheritance allows a new class to inherit from two or more classes • Disadvantages of multiple inheritance: – Language and implementation complexity (in part due to name collisions) – Potential inefficiency - dynamic binding costs more with multiple inheritance (but not much) • Advantage: – Sometimes it is quite convenient and valuable
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  1-13 Allocation and DeAllocationof Objects • From where are objects allocated? – If they behave line the ADTs, they can be allocated from anywhere • Allocated from the run-time stack • Explicitly create on the heap (via new) – If they are all heap-dynamic, references can be uniform thru a pointer or reference variable • Simplifies assignment - dereferencing can be implicit – If objects are stack dynamic, there is a problem with regard to subtypes – object slicing • Is deallocation explicit or implicit?
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  1-14 Dynamic and StaticBinding • Should all binding of messages to methods be dynamic? – If none are, you lose the advantages of dynamic binding – If all are, it is inefficient • Maybe the design should allow the user to specify
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  1-15 Nested Classes • Ifa new class is needed by only one class, there is no reason to define so it can be seen by other classes – Can the new class be nested inside the class that uses it? – In some cases, the new class is nested inside a subprogram rather than directly in another class • Other issues: – Which facilities of the nesting class should be visible to the nested class and vice versa
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  Initialization of Objects •Are objects initialized to values when they are created? – Implicit or explicit initialization • How are parent class members initialized when a subclass object is created? 1-16
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  Assignment • Explain theConcept of Inheritance in Object Oriented Programming • Discuss in detail the Design Issues for OOP Languages