Chapter 6 More About Problem Domain Classes Dr. James Jiang

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Chapter 6More About Problem Domain ClassesDr.
James Jiang
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Chapter 6 Topics

• More about writing problem domain classes
• Writing and invoking custom methods
• Formatting numerical data for display
• Using static variables and methods
• Writing overloaded methods
• Working with exceptions

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Writing a Definition for the Slip Class

• Process
• Write class header
• Write attribute definition statements
• Write a parameterized constructor
• Argument data types must be compatible with
  parameter data types

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Writing a Definition for the Slip Class

• Process (cont.)
• Write standard accessors to populate the
  attributes
• Write a tellAboutSelf method
• Polymorphic method
• Two methods with the same name residing in
  different classes that behave differently
• Write a tester class
• Sequence diagram

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Writing Custom Methods

• Standard Methods
• Written to store and retrieve values
• Accessor methods
• Custom Methods
• Written to do some processing

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Writing Custom Methods

• Custom Methods
• Process
• Write method header
• Public accessibility (if required)
• Appropriate return type to match data type of
  value returned (if any)
• Write code to implement required process
• Write tester class to demonstrate proper
  operation

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Slip class definition with leaseSlip method (I)
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Slip class definition with leaseSlip method (II)
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TesterTwo.java
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Formatting Output

• NumberFormat DecimalFormat Classes
• NumberFormat Class
• Member of java.text package
• Provides methods to format numerical data as
  currency with commas, dollar signs, and decimal
  points
• Also provides for formatting currency for various
  countries

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Formatting Output

• NumberFormat DecimalFormat Classes
• NumberFormat Class
• Two steps to format data
• Invoke getCurrencyInstance method to obtain a
  NumberFormat instance
• NumberFormat currencyFormat
• NumberFormat.getCurrencyInstance()
• Invoke the format method for the instance
  obtained
• System.out.println(Currency
  currencyFormat.format(fee)

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Formatting Output

• NumberFormat DecimalFormat Classes
• DecimalFormat Class
• Member of java.text package
• Provides methods to format numerical data with
  commas and a decimal point

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Formatting Output

• NumberFormat DecimalFormat Classes
• DecimalFormat Class
• Two steps to format data
• Create an instance of DecimalFormat using the new
  operator and pass the format mask
• DecimalFormat decimalFormat new
• DecimalFormat(,0.00)
• Invoke the format method for the instance
  obtained
• System.out.println(Decimal
  decimalFormat.format(fee)

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Formatting Output

• Using Escape Sequences
• Escape sequence
• The backslash character (\) followed by the
  escape character
• Used to display
• Characters that do not appear on the keyboard
• Characters that have special meanings within
  certain contexts
• Example
• Tab ? \t
• Double quote in an output String ? the \real\
  deal

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TesterThree.java
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TesterThree.java Output
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Using Static Variables and Methods

• Instance Variables and Methods
• A new instance receives its own copy of all
  instance variables and methods
• Methods not actually copied - to avoid redundancy
• Class Variables and Methods
• A new instance shares a copy of all class
  variables and methods
• Keyword
• Static ? used to declare class variables and
  methods as static

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Slip class with Static method and variable (I)
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Slip class with Static method and variable (II)
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Slip class with Static method and variable (III)
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TesterFour.java
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Overloading Methods

• Method Signature
• Consists of
• Method name
• Its parameter list
• Java identifies a method by its name and
  signature
• Overloaded method
• Methods within the same class having the same
  name but a different signature

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Overloading Methods

• Overridden Methods (polymorphism)
• A method with the same signature as an inherited
  method
• Replaces inherited method
• Polymorphic Method
• A method in one class has the same signature as a
  method in another class

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Overloading Methods

• Overloading a Constructor
• Multiple constructors with the same name and
  different signatures
• Should have a constructor for each way it makes
  sense to instantiate objects of the class

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Overloading Methods

• Overloading a Custom Method
• Any method can be overloaded
• Should have a method for each way it makes sense
  to input data to perform the required process

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Slip class with Overloaded methods (I)
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Slip class with Overloaded methods (II)
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TesterFive.java
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Working with Exceptions

• Exception
• An object instance that notifies of errors,
  problems, and other unusual conditions that may
  occur when the system is running
• Keywords
• try
• catch
• finally
• throw
• throws

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Client-Server exception handling
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Working with Exceptions

• Exception Process
• When a client invokes a method that may create
  and throw an exception, the invoking code must be
  placed in a try block
• Server method indicates it may throw an exception
  by including throws keyword in header
• If exception is detected, server sends exception
  instance to invoking client using throw keyword
• Client catches exception in a catch block
• finally block executes regardless of whether an
  exception is caught

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Working with Exceptions

• Data Validation for slipId
• If a method is to create and throw an exception,
  its header must contain the throws keyword
  followed by the exception class it throws
• public void setSlipId(int anId) throws Exception

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Working with Exceptions

• Data Validation for Width
• If a method is to create and throw an exception,
  its header must contain the throws keyword
  followed by the exception class it throws
• public void setWidth(int aWidth) throws Exception

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Working with Exceptions

• Catching Exceptions
• the invoking code must be prepared to catch the
  exception
• Otherwise JVM will terminate processing
• try
• method that throws exception
• catch (Exception e)
• code that handles it

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Slip class with Data Validation (I)
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Slip class with Data Validation (II)
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Slip class with Data Validation (III)
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TesterSix.java
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TesterSix.java Output