Develop Instantiable classes

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Develop Instantiable classes
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Lesson plan

• What does that mean?
• Why do we have to create instantiable classes?
• How do we go about creating them?
• Practice lab

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Why do we have to create instantiable classes?

• What does that mean by instantiable classes?
• We can create instances of these classes
• Example

(create objects from these classes)
String DecimalFormat
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Why do we have to create instantiable classes?

• Steps in LoanCalculator.java or AnnuityFund.java
  contains

Describe the program Get inputs from
users Computation Display output to users
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Why do we have to create instantiable classes?

• Is there any problems with this design? What if
  we need to solve a more complicated problem?

Large method, impossible to manage If dealing
with more complicated problem
Pre defined classes (provided by Java library,
Third party, etc..) dont provide exactly what
you need
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How do we create instantiable classes?

• Understand the structure of a class
• Specification for the class
• How the class (that we are going to create)
  interact with other classes?
• Specify the behaviors and fields that this class
  supports
• Implementation

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Structure of a class

• A class definition provides a description of a
  typical object within that class.
• A class has its behavior (methods) and attributes
  (fields)
• Example
• class String
• attributes and behavior/method

length returns the length of a string substring
returns a substring from the original
string charAt returns a character at a specific
position
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How a class interacts with other classes?

• When you are creating a class, we should think of
  the objects that can be created from this class.
• Example Class Student, each object should be an
  individual student (e.g studentA, studentB)
• What would be a natural and logical way to create
  an object?
• Example we can create a student object if we
  know his/her studentID, and name

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How a class interacts with other classes?

• What would be a natural and logical way for us to
  interact with it (or to use it)?
• Harry Hi! How are you? How is your Java midterm?
• Sally Not too bad. I was working hard for the
  test though
• Harry So what are you doing this week-end? Any
  plan in particular? We have a tailgate party
  before the football game. Wanna join?
• Sally Sound great! See you there

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How a class interacts with other classes?

• Analyze the conversation
• How are you?
• Attribute Health
• Value well, good, tired
• Java midterm
• Attribute Course
• Value Java, Database, GenEd, French
• Working hard
• Attribute Work Ethic
• Value hard working, lazy, moderate
• Tailgate party and football game
• Attribute Hobby
• Value party, football, basketball

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How a class interacts with other classes?

• Attribute Health
• Value well, good, tired
• How two students (objects) ask each other about
  this attribute
• How are you? (One way of saying how is your
  health?)
• Therefore, we need a method in the Student class
  so that one student can provide an answer to
  another student if he/she is asked about his/her
  health
• Lets name this method howIsYourHealth
• Similarly whatAreYourCourse?
• whatAreYourHobby?

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How a class interacts with other classes?

• Step 1 Understand the purpose of the class that
  you are creating
• Step 2 Use your imagination to identify
  attributes and method for this class
• What would be a natural and logical way to create
  an object?
• What would be a natural and logical way for an
  object of this class to interact with another
  object of the same class?
• Step 3 List them out on a paper

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Implementation

• Class header and class body.
• Member definitions in the body.
• Methods and attributes.

Class Header
// class called ClassName. class
ClassName // Attribute definitions go in the
class body // Method definitions go in the class
body ...
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Implementation

• For each attribute, determine the followings
• What does it represent?
• Example
• attribute health represents a students
  well-being.
• Which datatype does we use to represent this
  attribute?
• This can be determined from the possible values
  that this attribute will be assigned.
• Example
• attribute health.
• possible values tired, excellent, good,etc
• possible datatype String
• Any default value for this attribute
• Example good

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Implementation

• Declare an attribute in a class in the same way
  you declare a variable
• ltdata typegt ltattribute namegt
• Example String health
• Then add modifier (private or public) in front of
  the attribute declaration
• Example private String health
• You can initialize an attribute with its default
  value as
• ltdata typegt ltattribute namegt ltvaluegt
• Example private String healthgood

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Implementation

• For each method, determine the followings
• What does it do?
• E.g howIsYourHealth answers the question about
  your health. It will return the value
  representing your health (whether you are tired,
  feeling well or not..)
• What does it return?
• E.g howIsYourHealth returns a string (tired, or
  well)
• What are the parameters it requires? (parameter
  additional information)
• E.g howIsYourHealth did not require any
  parametter
• howIsYouCourse require a course number