Transcript Programming Languages and Paradigms

Programming Languages
and Paradigms
Object-Oriented Programming
Object-Oriented Programming
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Objects, classes, fields, methods
Encapsulation
Inheritance
Abstraction and reuse
Polymorphism and dynamic binding
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Program units: Classes
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6/15/2005
Copyright 2005, by the authors of these slides, and Ateneo de
Manila University. All rights reserved.
L6: OOP
Slide 2
Object
Definition: a thing that has identity, state,
and behavior
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identity: a distinguished instance of a class
state: collection of values for its variables
behavior: capability to execute methods
* variables and methods are defined in a class
6/15/2005
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Manila University. All rights reserved.
L6: OOP
Slide 3
Class
Definition: a collection of data (fields/
variables) and methods that operate on
that data
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data/methods define the contents/capabilities
of the instances (objects) of the class
a class can be viewed as a factory for objects
a class defines a recipe for its objects
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L6: OOP
Slide 4
Instantiation
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Object creation
Memory is allocated for the object’s fields
as defined in the class
Initialization is specified through a
constructor
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6/15/2005
a special method invoked when objects are
created
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Manila University. All rights reserved.
L6: OOP
Slide 5
Encapsulation
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A key OO concept: “Information Hiding”
Key points
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6/15/2005
The user of an object should have access only
to those methods (or data) that are essential
Unnecessary implementation details should be
hidden from the user
In Java/C++, use classes and access modifiers
(public, private, protected)
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L6: OOP
Slide 6
Inheritance
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Inheritance:
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Subclass relationship
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programming language feature that allows for the
implicit definition of variables/methods for a class
through an existing class
B is a subclass of A
B inherits all definitions (variables/methods) in A
Superclass variables, subclass objects
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6/15/2005
Polymorphism and dynamic binding
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L6: OOP
Slide 7
Abstraction
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OOP is about abstraction
Encapsulation and Inheritance are
examples of abstraction
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6/15/2005
What does the verb “abstract” mean?
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L6: OOP
Slide 8
Reuse
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Inheritance encourages software reuse
Existing code need not be rewritten
Successful reuse occurs only through
careful planning and design
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6/15/2005
when defining classes, anticipate future
modifications and extensions
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Manila University. All rights reserved.
L6: OOP
Slide 9
Polymorphism
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“Many forms”
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Example:
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allow several definitions under a single method name
“move” means something for a person object but
means something else for a car object
Dynamic binding:
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capability of an implementation to distinguish
between the different forms during run-time
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L6: OOP
Slide 10
Java and C++ Comparison
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Program Structure and Execution
Encapsulation and Inheritance
Objects, Variables, and Arrays
Constructors
Methods, Operators, and Binding
Containers and Reuse
GUI Programming
6/15/2005
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L6: OOP
Slide 11
Program Structure
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Class definition similar in Java and C++
Java: two types of programs
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application (with main() function)
applet (typically embedded in a web page)
C++
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6/15/2005
a program is (still) a collection of functions
that may use objects and classes
main() function serves as driver
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L6: OOP
Slide 12
Program Execution
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Java: Virtual Machine (VM)
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programs: both compiled and interpreted
compiler produces .class from .java
VM loads .class file(s) as needed
C++: compiled, linked, and loaded
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6/15/2005
modules separately compiled
linked to produce executable
static vs dynamic libraries
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L6: OOP
Slide 13
Encapsulation
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Enforced through access keywords
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In Java
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public: for interface
private: to make implementation inaccessible
protected: access for subclasses only
each member is prefixed with a keyword
In C++
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6/15/2005
public, private, and protected sections
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L6: OOP
Slide 14
Breaking Encapsulation
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Possible in C++ through the friend keyword
A method or class may be declared as a friend of
an existing class
Allows access to private members
Java: package access-level (no specifier) allow
classes under the same package to access each
other’s attributes
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6/15/2005
But private still means accessible to that class only
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L6: OOP
Slide 15
Inheritance
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Feature that allows a class to be defined based
on another class
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Java and C++ difference
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methods and attributes are inherited
Java: public class A extends B { … }
C++: class A: public B { … }
(different types of inheritance)
Multiple inheritance possible in C++, not in Java
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6/15/2005
But in Java, one may implement several interfaces
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L6: OOP
Slide 16
Objects and Identity
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Questions:
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How/when are objects created?
What is the relationship between a variable
and an object?
Difference between Java and C++
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distinction between primitive (built-in) type
variables and variables for objects
reference relationship between variable and
actual object
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L6: OOP
Slide 17
Variables for Built-in Types
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Variables for built-in types (C++ and Java)
int x;
…
x = 5;
6/15/2005
X
X
5
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Slide 18
Reference Variables
(in Java)
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Reference type variables
X
Button x;
…
x = new Button(“click”);
6/15/2005
Button Object
X
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“click”
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Slide 19
Variables That “hold” Objects
(in C++)
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Declaration of an object variable allocates
space for the object
X
Button x(“click”);
“click”
6/15/2005
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L6: OOP
Slide 20
Pointers (in C++)
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Variables can be explicitly declared as
pointers to objects
X
Button *x;
…
x = new Button(“click”);
6/15/2005
Button Object
X
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“click”
L6: OOP
Slide 21
Disposing of
Allocated Memory
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In Java, garbage collection is automatic
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Memory allocated objects are reclaimed when
no variables refer to them
Need to set reference variables to null when
the object is no longer needed
In C++, object destruction is the
programmer’s responsibility using the
delete keyword
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L6: OOP
Slide 22
delete in C++
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There should be a delete for every new
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SomeClass *x = new SomeClass(…);
// … use object pointed to by x
delete x; // done using object
Memory leak
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Occurs when you forget to delete
Wasted memory
Can this occur in Java?
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L6: OOP
Slide 23
Object Construction
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Constructor
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Default Constructor
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place where you include code that initializes
the object
no additional info required
User-defined Constructor
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with parameters that specify values or sizes
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L6: OOP
Slide 24
Arrays
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int x[20]; Button b[20];
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Valid declarations in C++, not in Java
Creates 20 ints and 20 Button objects
In Java,
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6/15/2005
Declaration and array creation separate
For object arrays, individual object creation
necessary
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L6: OOP
Slide 25
Pointers and Arrays
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In C++, there is a close relationship
between pointers and arrays
Instead of int x[20]; can issue
int *x; x = new int[20];
to allow for dynamic allocation
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Usage of the array (e.g., x[3] = 5;) identical in
both cases
To deallocate, use delete [] x;
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L6: OOP
Slide 26
Constructors in Java and C++
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In Java,
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a constructor is invoked only through the new
keyword
recall that all object variables are references
In C++,
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6/15/2005
a constructor is called upon variable declaration, or
explicitly through new with pointers, or in other
situations
other types of constructors
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L6: OOP
Slide 27
C++ Destructor
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Special method whose signature is a ~
followed by the name of the class
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e.g., ~SomeClass();
Particularly if the class contains pointers
and the constructor contains calls to new,
a destructor needs to be defined
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e.g., SomeClass() { A = new int[20]; }
~SomeClass() { delete [] A; }
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L6: OOP
Slide 28
C++ Control Over Copy and
Assignment
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In C++, the semantics of “a = b”
(assignment) can be specified
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In C++, there is a copy constructor
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by defining the copy-assignment operator
specifies what happens during object
copying, e.g., when function parameters are
passed
There is more low-level control
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shallow copy vs deep copy
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L6: OOP
Slide 29
Methods
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Defines object behavior
Static methods vs instance methods
Method overloading
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within class, two methods with the same name
but different signatures
Method overriding
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6/15/2005
same signatures across different classes
(subclass and superclass)
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Slide 30
Operators
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In C++, operators like =, +, *, ==, etc. can
be defined, just like methods
Example:
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6/15/2005
class Matrix {
// ...
Matrix operator+(Matrix m) { … } // …
}
c = a + b; // equiv to c = a.operator+(b);
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L6: OOP
Slide 31
Method Binding
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Let Teacher be a subclass of Employee
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Employee variables can refer to Teachers
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Also, suppose promote() is a method defined in
both classes
In Java, Employee e; … e = new Teacher();
In C++, Employee *e; … e = new Teacher;
e.promote() (or e->promote() ) calls which
promote() method?
6/15/2005
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L6: OOP
Slide 32
Static vs Dynamic Binding
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In C++, Employee’s promote() is called
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Determined at compile time and deduced from
the type of the variable (static binding)
In Java, Teacher’s promote is called
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Determined at run-time because the actual
type of the referred object is checked then
(dynamic binding)
* C++ uses virtual functions for dynamic binding
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L6: OOP
Slide 33
Static Binding
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Pointer is typed to know the base class and is
ignorant of the structure or existence of the
derived classes
If the virtual keyword is NOT used, if a derived
class has its own variation on the
implementation of a base class member
function, it will NOT cause the derived class
version to be selected when a function is
invoked on an object of than class through a
variable declared in terms of the base class.
6/15/2005
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L6: OOP
Slide 34
Another example
class Employee{
class Manager: public
Employee
public:
{
double salary()
public:
{return sal;}
double computeRaise()
double computeRaise()
{return 100;}
{return 25;}
Manager(double Salary) :
Employee(double salary)
Employee (salary)
{sal = salary;}
{}
private:
};
double sal;
};
L6: OOP
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Manila University. All rights reserved.
Slide 35
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
6/15/2005
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Manila University. All rights reserved.
L6: OOP
Slide 36
C++ Run Time Binding
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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:
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6/15/2005
Declare some base class members to be virtual
If virtual, the compiler will deposit the “type
field” of the class in the object
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L6: OOP
Slide 37
Virtual Functions
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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
6/15/2005
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L6: OOP
Slide 38
Containers
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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
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6/15/2005
unified container code for a stack of integers,
a stack of webpages, a stack of strings, ...
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L6: OOP
Slide 39
Java and the Object
Hierarchy
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All classes extend the Object class:
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A variable of class Object can refer to any Java
object
Example:
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6/15/2005
public class Stack {
Object A[]; int top; // …
void push(Object elt) // ...
}
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L6: OOP
Slide 40
C++ and Templates
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Templates allow for a generic definition
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parameterized definition, where the element
type is the parameter
Example:
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6/15/2005
template<class T>
class Stack<T> {
T A[MAX]; int top;
public: void push(T element) // …
}
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L6: OOP
Slide 41
C++ Templates
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<class T> indicates that a template is being
declared
T is the type name (can be a class)
Usage example:
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Stack <int> iStack;
Stack <Cards> cStack;
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where Cards is a user defined class
A type used as a template argument must provide
the interface expected by the template
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L6: OOP
Slide 42
Defining a Template
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When defining a template member outside
of its class, it must be explicitly declared a
template
Example
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6/15/2005
template <class T> Stack<T>::Stack()
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Slide 43
C++ Standard Containers
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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
6/15/2005
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L6: OOP
Slide 44
Java Generics
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Recent addition to Java
Similar to C++ templates
Why are templates/generics better than
using the Object class as a parameter?
6/15/2005
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L6: OOP
Slide 45
GUI Programming
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In Java, GUI is part of its development kit
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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++
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6/15/2005
not part of the language
libraries dependent on platform (e.g., MFCs
and Motif)
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L6: OOP
Slide 46