Programming Languages and Paradigms

1. Programming Languagesand Paradigms Object-Oriented Programming (Part II)

2. Java and C++ Comparison(continued) • Arrays • Constructors and destructors • Operators • Static vs Dynamic Binding • Containers and Reuse • GUI Programming

3. Arrays • int x[20]; Button b[20]; • Valid declarations in C++, not in Java • Creates 20 ints and 20 Button objects • In Java, • Declaration and array creation separate • For object arrays, individual object creation necessary

4. Pointers and Arrays • In C++, there is a close relationship between pointers and arrays • Instead of int x[20]; can issueint *x; x = new int[20];to allow for dynamic allocation • Usage of the array (e.g., x[3] = 5;) identical in both cases • To deallocate, use delete [] x;

5. Constructors in Java and C++ • In Java, • a constructor is invoked only through the new keyword • recall that all object variables are references • In C++, • a constructor is called upon variable declaration, or explicitly through new with pointers, or in other situations • other types of constructors

6. C++ Destructor • Special method whose signature is a ~ followed by the name of the class • e.g., ~SomeClass(); • Particularly if the class contains pointers and the constructor contains calls to new, a destructor needs to be defined • e.g., SomeClass() { A = new int[20]; } ~SomeClass() { delete [] A; }

7. C++ Copy and Assignment • In C++, the semantics of “a = b” (assignment) can be specified • by defining the copy-assignment operator • In C++, there is a copy constructor • specifies what happens during object copying, e.g., when function parameters are passed • There is more low-level control • shallow copy vs deep copy

8. Operators • In C++, operators like =, +, *, ==, etc. can be defined, just like methods • Example: • class Matrix { // ... Matrix operator+(Matrix m) { … } // …} • c = a + b; // equiv to c = a.operator+(b);

9. Method Binding • Let Teacher be a subclass of Employee • Also, suppose promote() is a method defined in both classes • Employee variables can refer to Teachers • In Java, Employee e; … e = new Teacher(); • In C++, Employee *e; … e = new Teacher; • e.promote() (or e->promote() ) calls which promote() method?

10. Static vs Dynamic Binding • In C++, Employee’s promote() is called • Determined at compile time and deduced from the type of the variable (static binding) • In Java, Teacher’s promote is called • Determined at run-time because the actual type of the referred object is checked then (dynamic binding) * C++ uses virtual functions for dynamic binding

11. class Employee{ public: double salary() {return sal;} double computeRaise() {return 25;} Employee(double salary) {sal = salary;} private: double sal; }; class Manager: public Employee { public: double computeRaise() {return 100;} Manager(double Salary) : Employee (salary) {} }; Another example

12. Sample continued Driver Code: Manager * boss1 = new Manager(2000); double boss1Salary = boss1->salary(); // 2000 Employee *boss2 = new Manager(2300); double *boss2Salary = boss2->salary(); //2300 double boss1Raise = boss1->computeRaise(); // 100 double boss2Raise = boss2->computeRaise(); // 25

13. C++ Run Time Binding • If the intent is for the selection of the function to be determined by the object’s class, not by the declaration of the pointer used to address it: • Declare some base class members to be virtual • If virtual, the compiler will deposit the “type field” of the class in the object

14. Virtual Functions • Base class usually defines a body for a virtual function. • Inherited by derived class as default if it chooses not to override the implementation • Virtual keyword in function declaration, not in definition

15. Containers • Examples: Lists, Stacks, Files, etc. • Structures that “contain” elements • Often, the element’s type has little or nothing to do with the containers’ operations • Possible room for re-use • unified container code for a stack of integers, a stack of webpages, a stack of strings, ...

16. Java and the Object Hierarchy • All classes extend the Object class: • A variable of class Object can refer to any Java object • Example: • public class Stack { Object A[]; int top; // … void push(Object elt) // ...}

17. C++ and Templates • Templates allow for a generic definition • parameterized definition, where the element type is the parameter • Example: • template<class T>class Stack<T> { T A[MAX]; int top; public: void push(T element) // …}

18. C++ Templates • <class T> indicates that a template is being declared • T is the type name (can be a class) • Usage example: • Stack <int> iStack; • Stack <Cards> cStack; • where Cards is a user defined class • A type used as a template argument must provide the interface expected by the template

19. C++ Standard Containers • vector : 1-D array of T • list : double linked list of T • dequeue : double-ended queue of T • queue : queue of T • stack : stack of T • map : associative array of T • set : set of T • bitset : set of booleans

20. Java Generics • Recent addition to Java • Similar to C++ templates • Why are templates/generics better than using the Object class as a parameter?

21. GUI Programming • In Java, GUI is part of its development kit • java.awt.* is a collection of classes that support visual programming and graphics • visual objects (buttons, text fields, etc), layout managers, events, etc. • In C++ • not part of the language • libraries dependent on platform (e.g., MFCs and Motif)