Programming Language ConceptsObject-Oriented Programming

Janyl Jumadinova28 February, 2017

Three Properties of Object-Oriented Languages:

I Encapsulation

I Inheritance

I Dynamic method binding (polymorphism)

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Encapsulation

I Data and functions bound together into a single object.

I “Data hiding” – hide implementation details from user.

I More accurately, control access to data using public and privatevariables and methods.

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Inheritance

I Hierarchy of classes and objects.

I Shared behaviors and data – code re-use.

I Static polymorphism: one interface for many kinds of object.

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Multiple Inheritance

I In Java, classes and subclasses form a tree – a class may havemany subclasses, but each subclass extends exactly one parentclass. This is called the class hierarchy.

I Java does not permit “multiple inheritance.”

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Multiple Inheritance

I On the other hand, the C++ language allows classes to inheritfrom several different parent classes: multiple inheritance.

I For example, consider the following set of classes:

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Multiple Inheritance in C++

class Person { ... };

class Student : public Person { ... };

class Employee : public Person { ... };

class StudentEmployee : public Student, public Employee

{...};

In C++, constructors for subclasses can invoke the constructors oftheir parent classes, e.g.,Student(string name, int year, double gpa): Person(name)...

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Multiple Inheritance in C++

In our example, StudentEmployee can invoke the constructors ofboth parents:

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Multiple Inheritance in C++

I In our example, assume name is an instance variable in the classPerson, with accessor method getName().

I Then both Student and Employee will inherit this variable aswell as the getName method.

I Now consider the class StudentEmployee. It inherits name andgetName from both Student and Employee.

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Multiple Inheritance in C++

I In our example, assume name is an instance variable in the classPerson, with accessor method getName().

I Then both Student and Employee will inherit this variable aswell as the getName method.

I Now consider the class StudentEmployee. It inherits name andgetName from both Student and Employee.

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Multiple Inheritance in C++

I In our example, assume name is an instance variable in the classPerson, with accessor method getName().

I Then both Student and Employee will inherit this variable aswell as the getName method.

I Now consider the class StudentEmployee. It inherits name andgetName from both Student and Employee.

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Multiple Inheritance

I This is called the “diamond problem” (a.k.a. “Diamond ofDeath”).

I In C++, one way to avoid the error on the previous page is tosimply choose one of the “getName()” methods and ignore theother one.

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Multiple Inheritance

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Multiple Inheritance

Can we gain the benefits of multiple inheritance in Java?

I Sort of ... in Java we can create “interfaces”.

I They are similar to classes, but an interface has no instancevariables and contains only abstract methods.

I A class can implement more than one interface.

I It is not quite the same as multiple inheritance, but yields manyof the same benefits.

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Dynamic Method Binding

An object’s methods are determined at runtime rather thancompilation time, since subclasses can override methods and can beused wherever the superclass is allowed.

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Dynamic Binding

DynamicDemo.java in the shared repo.

At runtime, Java determined the correct class of the parameter andinvoked ys talk method. This is dynamic method binding.

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Dynamic Binding

DynamicDemo.java in the shared repo.

At runtime, Java determined the correct class of the parameter andinvoked ys talk method. This is dynamic method binding.

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Dynamic Binding

DynamicDemo.java in the shared repo.

At runtime, Java determined the correct class of the parameter andinvoked ys talk method. This is dynamic method binding.

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Static vs. Dynamic Binding

I See program staticbind.cpp in the shared repo.

I There is a parent class Super and a child class Sub, each with aget() method.

Sub a(10,20);

Super b = a;

cout << a.get() << endl; // Which "get"?

cout << b.get() << endl; // Which "get"?

Both will use Super’s “get()” method!

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Static vs. Dynamic Binding

I By default, C++ uses static binding.

I However, you can still obtain the same behavior as dynamicbinding by using virtual methods and pointers as shown inprogram dynamicbind.cpp

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Overloading

I When two methods in a class have the same name but differentparameters, we say that the method name is “overloaded.”

I This is familiar from Java (where, for instance, we have twodifferent “substring” methods for the String class or multipleconstructor methods).

I In C++ we can even overload symbolic operators like “+” and“*” (really, any operator).

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Overloading

I When two methods in a class have the same name but differentparameters, we say that the method name is “overloaded.”

I This is familiar from Java (where, for instance, we have twodifferent “substring” methods for the String class or multipleconstructor methods).

I In C++ we can even overload symbolic operators like “+” and“*” (really, any operator).

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Overloading an Operator in C++Overload.cpp

class Pirate {

public:

Pirate(string name) { this->name = name;}

Pirate operator +(Pirate p) {

return Pirate(p.getName() + name);

}

... some code omitted ...

...

Pirate x("Fred");

Pirate y("Mary");

Pirate a = x + y; // Creates a Pirate named "MaryFred"

See overload.cpp and overload1.cpp in the shared repo. 18/19

Overloading an Operator in C++

I There are many aspects of operators that we must worry about:precedence, associativity, etc.

I C++ avoids these by forcing overloaded operators to have thesame precedence and associativity that the original operatorshad.

I See program overload2.cpp in the shared repo.

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