Transcript Object-Oriented Programming in Java Topic : Objects and Classes

Air Force Institute of Technology
Electrical and Computer Engineering
Object-Oriented Programming in Java
Topic :
Objects and Classes (cont)
Object–Oriented Design
Maj Joel Young
[email protected]
4-Apr-16
Maj Joel Young
Object-Oriented
Programming
Design
Using Class Instances
• Accessing an instance method:
newGuy.raiseSalary( 10.0 );
• Accessing an instance attribute:
double hisSalary = newGuy._salary;
newSalary = this._salary * ( 1 + percent/100);
//”this” refers to current instance
• Note: direct access to other object’s attributes is discouraged
– (and in fact it is not allowed in above case due to private attribute)
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Object-Oriented
Programming
Design
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Constructors
Automatically called when object is created
Same name as class -- e.g. Date();
No return value
May be more than one constructor per class
Default constructor with no arguments should be specified
– initialize to reasonable default value
public Date()
{
_day = 1;
_month = 1;
_year = 1;
}
• Constructor with arguments -- initialize to user specified values
public Date( int d, int m, int y)
{
_day = d;
_month = m;
_year = y;
}
• Cannot be invoked on existing objects
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Object-Oriented
Programming
Design
Constructors
• A quick quiz on constructors– can you spot the error?
class Employee
{
public Employee( String n, double s )
{
String _name = n;
double _salary = s;
}
…
private String _name;
private double _salary;
}
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Object-Oriented
Programming
Design
Java Example
class Person
{
// Attributes
int m_age;
double m_gpa;
String m_name;
static public void
main(String args[])
{
// Create a Person instance
Person p1 =
new Person("Pyle",27,1.9);
Person p2;
// Operations
public Person(String name,
int age, double gpa)
{
m_age = age;
m_name = name;
m_gpa = gpa;
}
private void printName()
{
System.out.print(m_name);
}
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// Print the name of
// the person
System.out.print("Name: ");
p1.printName();
p2 = p1;
// Print the same name again
System.out.print(" Name: ");
p2.printName();
}
} // End class Person
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Electrical and Computer Engineering
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Object-Oriented
Programming
Design
•
Class Attributes/Methods
Class methods/attributes
–
Use static keyword in front of declaration
Class MyMath
{…
static void add( int val1, int val2 );
}
– May be accessed by using ClassName.method() or ClassName.attribute notation
MyMath.add( 1, 3 );
– An instance of the class is not required for access
• Class-wide Attributes
– An attribute shared by all instances of a class
– If one instance changes it … all others see the change
– Analog: Global Variable … use SPARINGLY
•
Class-wide Operation
– A “meta operation” … operates on the class, not instances of the class
– Typical application:
– Creating new class members, assigning class ID numbers, etc.
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Electrical and Computer Engineering
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Object-Oriented
Programming
Design
Packages
• Java code is distributed in separate packages (you can also
create your own packages)
• To access code in a package
– refer to class by full name
java.util.Vector v = new java.util.Vector();
– or import class into your program
import java.util.Vector
Vector v = new Vector();
– or import entire package
import java.util.*
• Package paths map to directory structure
java.util.Vector 
<CLASSPATH>\java\util\<vector package files>
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Electrical and Computer Engineering
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Air Force Institute of Technology
Electrical and Computer Engineering
Object-Oriented Programming in Java
Topic :
Object–Oriented Design
Maj Joel Young
[email protected]
4-Apr-16
Maj Joel Young
Object-Oriented
Programming
Design
Development Process
• Analysis
– Transform vague understanding of a problem into a precise description of
the tasks to be solved
– Concerned with description of what must be done, not how it should be
done
• Design
– Structure programming tasks into classes and packages (logically grouped
clusters of objects)
– Specify operations and attributes of each class
– Specify relationships with other classes in the system
• Implementation
– Classes and operations are coded, tested, and integrated
– Object-orientation encourages evolutionary development since behavior
and state is encapsulated into objects with predefined interfaces – thus
objects or packages can be incrementally added and tested more easily
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Object-Oriented
Programming
Design
Object-Oriented Design
• Goal: decompose a programming task into data types or
class and define the functionality of these classes
• Sub-goals
– Identify classes
– Identify functionality of classes
– Identify relationships among classes
• A good design greatly reduces time required for
implementation and testing
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Object-Oriented
Programming
Design
Object-Oriented Design
• Finding classes
– Look for nouns in problem analysis
– Many are good choices for classes
– Other classes may be necessary
• Finding operations
– Look for verbs in problem analysis
– Each operation must have exactly one class responsible for carrying it out
• Finding class relationships
– Association  uses; a class uses another class if manipulates objects of
that class in any way; should be minimized
– Aggregation  “has-a”, contains;
– Inheritance  “is-a”, specialization; useful in select places
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Object-Oriented
Programming
Design
Object-Oriented vs. Traditional Design
Traditional
Object-Oriented
• Decomposition into sub-tasks
– Often results in numerous
procedures or functions
– Difficult to manage and
understand
• Task modules can simulate
objects by organizing related
tasks and attributes
– Only one instance (e.g. queue)
• Encapsulation achieved with
opaque types (e.g. Unix file
interface)
– No inheritance
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• Decomposition into objects with
associated tasks
– Convenient clustering
– Data encapsulation helps
debugging
– Multiple instances with similar
behavior
– Inheritance captures
commonalities among related
classes of objects
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Object-Oriented
Programming
Design
Design Hints
• Always keep data private
– If user of the class has a need to both read and change a field (attribute),
the following 3 items must be implemented in the class
– A private data field
– A public field accessor method
– A public field mutator method
– Benefits
– Internal implementation can be changed without affecting any code
outside of the class
– Mutator methods can perform error-checking to ensure field is set
properly
– Accessors to fields with mutable objects
– Should return clone of object so that calling object can not modify the
field object contained in the class instance
• Always initialize data
– Avoids unexpected results
– Simplifies debugging
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Electrical and Computer Engineering
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Object-Oriented
Programming
Design
Design Hints
• Don't use too many basic types in a class
– Group related types into a separate class and use that class instead
– Good
– Bad
class
{
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}
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Employee
private
private
private
private
private
private
private
String _lname;
String _fname;
char _initial;
String _street;
String _city;
String _state;
long _zip;
class Employee
{
private Name _name;
private Address _address;
private Date _hiredate;
}
Not all fields need individual field accessors and mutators
Use a standard format for class definitions
Break up classes with too many responsibilities
Make the names of your classes and methods reflect their responsibilities
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Object-Oriented
Programming
Design
Homework Discussion
• Compile and run the Person example code
• Write and test a swap method
– Create a simple class with one int data member
– Write a swap method for that class that swaps values:
– : static public void swap(mytype a, mytype b);
– Such that after swap(a,b) is called a looks like b looked and b looks
like a looked
• Analyze the requirements for a Calculator program
– Identify the classes, the operations, and the relationships
between classes
– Make stub code for all your classes and operations with
definitions
for the has-a relationships
– Remember each class goes in a separate file
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Object-Oriented
Programming
Design
Homework Assignment Due Monday
• COMACC Calculator Question and Answer Session
• Analyze the requirements for a Calculator program
– Provide a narrative discussion of the requirements and behavior
COMACC expects from the Calculator
– Prepare clarification questions. Proposing alternatives is a strong plus
– Discuss what is feasible in the next 3 weeks. Remember that you will
know more in the next few weeks
– Provide me this in hard copy with your email address on it
• Be prepared to:
– Identify the classes, the operations, and the relationships
between classes
– Make stub code for all your classes and operations with
definitions for the has-a relationships
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