Chapter 3 Implementing Classes

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Chapter 3 Implementing Classes
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Syntax 3.5 The return Statement

• return expression or
• return
• Example
• return balance
• Purpose
• To specify the value that a method returns, and
  exit the method immediately. The return value
  becomes the value of the method call expression.

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Wus Template
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Template in action

• /
• This class is used to do the currency
  conversion
• between a foreign currency and the US Dollar
• _at_author Ameet Soni
• /

Class Comment
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Defining the class
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Template in action

• /
• This class is used to do the currency
  conversion
• between a foreign currency and the US Dollar
• _at_author Ameet Soni
• /
• public class CurrencyConverter

Class Name
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Defining the class
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Template in action

• /
• This class is used to do the currency
  conversion
• between a foreign currency and the US Dollar
• _at_author Ameet Soni
• /
• public class CurrencyConverter
• /
• how much 1 USD is worth in foreign currency
• /
• private double exchangeRate

Declaration (Data Members/Instance Fields
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Defining the class
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Template in action

• /
• This class is used to do the currency
  conversion
• between a foreign currency and the US Dollar
• _at_author Ameet Soni
• /
• public class CurrencyConverter
• /
• how much 1 USD is worth in foreign currency
• /
• private double exchangeRate
• public CurrencyConverter()
• exchangeRate 0
• public CurrencyConverter(double inRate)
• exchangeRate inRate

Constructors
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Template in action

• /
• This class is used to do the currency
  conversion
• between a foreign currency and the US Dollar
• _at_author Dr. Caffeine
• /
• class CurrencyConverter
• /
• how much 1 USD is worth in foreign currency
• /
• private double exchangeRate
• public CurrencyConverter()
• exchangeRate 0
• public CurrencyConverter(double inRate)
• exchangeRate inRate
• public void setExchangeRate (double rate )

Methods
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Not in Book

• In this course, there are three types of classes
  you will write

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What's a Class? Defn 1

• Source code that defines a new data type by
  encapsulating
• data members
• constructors
• method.
• Ex. BankAccount class we just created
• AKA Instantiable class designed to be created
  as objects

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• / Stores information about one pet. /
• public class Pet
• / Name of the pet. /
• private String name, kind
• private int age
• / Initializes a new instance.
• _at_param n The pet's name.
• /
• public Pet( String n, String k, int a )
• name n kind k age a
• / Returns the name of this pet. /
• public String getName() return name
• / Changes the name of this pet.
• _at_param newName The new name of the pet.

Data member
Constructor
Methods
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What's a Class? Defn 2

• Code that is written to test another class.
• Has a main method that
• Creates instances of the class
• Executes methods
• Compares the actual with expected.
• Classes should be tested before being used in
  Java Applications.
• Ex. Part of Assignment 1

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• / A Test Program. /
• public class TestPet
• /
• Test Program starts here.
• _at_param args Unused in this program.
• /
• public static void main( String args )
• // Declare and create a pet
• Pet pet1 new Pet( "George", "bird", 14 )
• // Test the pet's information
• testGetName( pet1, George )
• / Tests Pet getName method. /
• public static void testGetName( Pet pet,
• String expectedName )

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What's a Class? Defn 3

• Java Application
• Source code that uses instances of other classes
  (objects) to solve problems.
• Must have a main method.
• Ex. Programs you completed in A0

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• / A Java Application. /
• public class PetApplication
• /
• Program starts here.
• _at_param args Unused in this program.
• /
• public static void main( String args )
• // Declare and create a pet
• Pet pet1 new Pet( "George", "bird", 14 )
• // Output the pet's information
• display( pet1 )
• / Prints Pet information to screen. /
• public static void display( Pet pet )
• System.out.println(

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3.6 Testing Classes

• Previous section was designing a class
• By itself, we cannot actually run a program
• No main() method, like most classes
• We create instances of it in other classes
• Idea use a test class to make sure it works
  properly before dispensing the product

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3.6 Testing A Class

• Test class a class with a main method that
  contains statements to test another class.
• Typically carries out the following steps
• Construct one or more objects of the class that
  is being tested
• Invoke one or more methods
• Print out one or more results
• Verify output is correct and program behaves as
  expected

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Youve done this

• A0 asked you to make test classes
• Perimeter.java tested getHeight() and getWidth()
  methods
• MoveTester tested the translate() method

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• Details for building the program vary. In most
  environments, you need to carry out these steps
• Write a test program for the methods
• Include the class being tested in the project
• Run the test program

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• /
• A class to test the BankAccount class.
• /
• public class BankAccountTester
• /
• Tests the methods of the BankAccount class.
• _at_param args not used
• /
• public static void main(String args)
• BankAccount harrysChecking new
  BankAccount()
• harrysChecking.deposit(2000)
• harrysChecking.withdraw(500)
• System.out.println(harrysChecking.getBalance())
  

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3.7 Categories of Variables

• Categories of variables
• Instance fields (balance in BankAccount)
• Local variables (newBalance in deposit method)
• Parameter variables (amount in deposit method)
• Share the same properties of declaring and
  creating
• Differ in their lifetime (AKA scope)

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Instance Field

• An instance field belongs to an object
• AKA Instance variable
• Each object has its own copy of the instance
  field
• The fields stay alive until no method uses the
  object any longer
• More specifically, until the object no longer
  exists

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Garbage

• In Java, the garbage collector periodically
  reclaims objects when they are no longer used
• If no variables reference to the object anymore,
  it can no longer be used and is collected

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Local Variables

• Local and parameter variables belong to a method
• You declare them and create within the method
• When method is done executing, variable is thrown
  out
• EVERYTIME the method is executed, a new copy of
  any variables is created, values do not carry over

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Initialization with scope

• Instance fields are initialized to a default
  value, but you must initialize local variables
• Default values for numbers is 0
• Objects is null
• COMMON ERROR forgetting to create objects for
  instance fields, declaration leaves it at null
• Not an issue with parameter variables

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Parameter variables

• Parameter variables are initialized to the values
  that are passed to them
• Maintain same lifetime as local variables
• Only difference is that they receive values
  automatically

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Lifetime Of Variables

• harrysChecking.deposit(500)
• double newBalance balance amount
• balance newBalance

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3.8 Implicit and Explicit Method Parameters

• The implicit parameter of a method is the object
  on which the method is invoked
• Why is this important?
• When a method is invoked, and an instance field
  is used, how does it know which object it belongs
  to?

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• Use of an instance field name in a method denotes
  the instance field of the implicit parameter
• public void withdraw(double amount) double
  newBalance balance - amount balance
  newBalance

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• balance is the balance of the object to the left
  of the dot
• momsSavings.withdraw(500)
• means
• double newBalance momsSavings.balance -
  amountmomsSavings.balance newBalance

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Implicit Parameters and this

• Every method has one implicit parameter
• The implicit parameter is always called this
• The method knows what object called it based on a
  reference, but does not know/care about the
  identifier name
• E.g. you cannot say momsSavings.balance in the
  class, because momsSavings is a local variable of
  another class
• Exception Static methods do not have an implicit
  parameter (more on Chapter 9)

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Implicit Parameters and this

• double newBalance balance amount// actually
  meansdouble newBalance this.balance amount
• When you refer to an instance field in a method,
  the compiler automatically applies it to the this
  parameter
• momsSavings.deposit(500)

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Implicit Parameters and this
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Advanced Info

• this operator very powerful
• Can call one constructor from another to avoid
  redundancy
• Important goal in programming, never have
  duplicate code

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Example without this

• public class BankAccount
• double balance
• public BankAccount(double initialBalance)
• balance initialBalance
• public BankAccount
• balance 0

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Example with this

• public class BankAccount
• double balance
• public BankAccount(double initialBalance)
• balance initialBalance
• public BankAccount
• this(0)

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HOW TO 3.1 Designing and Implementing a Class

• Step 1 Find out what you are asked to do with an
  object of the class
• What do you want to be able to do with the
  object?
• Step 2 Specify the Public Interface
• Convert the list from step 1 into methods, and
  the parameters (think input) they need
• Constructors how do I want to initialize this
  object

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HOW-TO 3.1 (cont)

• Step 3 Document the public interface
• Create Javadoc comments for each method and the
  class
• Step 4 Determine instance fields
• What information needs to be maintained?
• Step 5 Implement
• One at a time, from easiest to most difficult

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HOW-TO 3.1 (cont)

• Go back to Step 2 if implementation doesnt work
• Step 6 Test your class

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Note on Style 1

• A couple ways to use curly braces
• Option 1
• public class SomeClass
• Option 2
• public class SomeClass

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Note on Style 2

• Book says to place instance fields at the bottom
  of a class
• public class SomeClass
• //constructors
• //methods
• //instance fields
• Most conventions have it at the top of a class
• public class SomeClass
• //instance fields
• //constructors
• //methods

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Note on Style 3

• Already discussed variations on import
• Dont use import (fully qualify all classes)
• Import just one class (java.awt.Rectangle)
• Import the entire package (java.awt.)

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What is the right way?

• You can use any of the options
• You MUST(will grade on this) be consistent though
• Dont use both styles of curly braces in the same
  program