Air Force Institute of Technology

1
Object-Oriented Programming in JavaTopic
Objects and Classes (cont)ObjectOriented Design

• Maj Joel Young
• Joel.Young_at_afit.edu

Maj Joel Young
2
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 objects attributes
  is discouraged
• (and in fact it is not allowed in above case due
  to private attribute)

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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

4
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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Java Example

• class Person
• // Attributes
• int m_age
• double m_gpa
• String m_name
• // 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)

• static public void main(String args)
• // Create a Person instance
• Person p1 new Person("Pyle",27,1.9)
• Person p2
• // 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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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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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 ?
• ltCLASSPATHgt\java\util\ltvector package filesgt

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Object-Oriented Programming in JavaTopic
ObjectOriented Design

• Maj Joel Young
• Joel.Young_at_afit.edu

Maj Joel Young
9
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 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 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 vs. Traditional Design

• Traditional
• 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

• Object-Oriented
• 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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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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Design Hints

• Don't use too many basic types in a class
• Group related types into a separate class and use
  that class instead
• Bad
• class Employee
• private String _lname
• private String _fname
• private char _initial
• private String _street
• private String _city
• private String _state
• private long _zip
• 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

• Good
• class Employee
• private Name _name
• private Address _address
• private Date _hiredate

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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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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