Chapter 2b

Implementing Classes

Chapter Goals

• To become familiar with the process of implementing classes
• To be able to implement simple methods
• To understand the purpose and use of constructors
• To understand how to access instance fields and local variables
• To appreciate the importance of documentation comments

Black Boxes

• A black box magically does its thing
• Hides its inner workings
• Encapsulation: the hiding of unimportant details
• What is the right concept for each particular black box?
• Concepts are discovered through abstraction
• Abstraction: taking away inessential features, until only the essence of the concept remains
• In object-oriented programming the black boxes from which a program is manufactured are called objects

Levels of abstraction: A Real Life Example

• Black boxes in a car: transmission, electronic control module, etc.

Levels of abstraction: A Real Life Example

• Users of a car do not need to understand how black boxes work
  
• Interaction of a black box with outside world is well-defined
  • Drivers interact with car using pedals, buttons, etc.
  • Mechanic can test that engine control module sends the right firing signals to the spark plugs
  • For engine control module manufacturers, transistors and capacitors are black boxes magically produced by an electronics component manufacturer
• Encapsulation leads to efficiency:
  • Mechanic deals only with car components (e.g. electronic control module), not with sensors and transistors
  • Driver worries only about interaction with car (e.g. putting gas in the tank), not about motor or electronic control module

Levels of abstraction: Software Design

Levels of abstraction: Software Design

• Old times: computer programs manipulated primitive types such as numbers and characters
• Manipulating too many of these primitive quantities is too much for programmers and leads to errors
• Solution: Encapsulate routine computations to software black boxes
• Abstraction used to invent higher-level data types
• In object-oriented programming, objects are black boxes
• Encapsulation: Programmer using an object knows about its behavior, but not about its internal structure
• In software design, you can design good and bad abstractions with equal facility; understanding what makes good design is an important part of the education of a software engineer
• First, define behavior of a class; then, implement it

Self Check

1. In Chapters 1 and 2, you used System.out as a black box to cause output to appear on the screen. Who designed and implemented System.out?
2. Suppose you are working in a company that produces personal finance software. You are asked to design and implement a class for representing bank accounts. Who will be the users of your class?

Answers

1. The programmers who designed and implemented the Java library
2. Other programmers who work on the personal finance application

Designing the Public Interface of a Class

• deposit money
• withdraw money
• get balance

Designing the Public Interface of a Class: Methods

• deposit
• withdraw
• getBalance

   harrysChecking.deposit(2000);
   harrysChecking.withdraw(500);
   System.out.println(harrysChecking.getBalance());

Designing the Public Interface of a Class: Method Definition

• access specifier (such as public)
• return type (such as String or void)
• method name (such as deposit)
• list of parameters (double amount for deposit)
• method body in { }

• public void deposit(double amount) { . . . }
• public void withdraw(double amount) { . . . }
• public double getBalance() { . . . }

Syntax 3.1: Method Definition

accessSpecifier returnType methodName(parameterType parameterName, . . .)
{
   method body
}

public void deposit(double amount)
{
   . . .
}

Designing the Public Interface of a Class: Constructor Definition

• A constructor initializes the instance variables
• Constructor name = class name
  

  public BankAccount()
  {
     // body--filled in later
  }

• Constructor body is executed when new object is created
• Statements in constructor body will set the internal data of the object that is being constructed
• All constructors of a class have the same name
• Compiler can tell constructors apart because they take different parameters

Syntax 3.2 : Constructor Definition

accessSpecifier ClassName(parameterType parameterName, . . .)
{
   constructor body 
}

public BankAccount(double initialBalance)
{ 
   . . . 
}

BankAccount Public Interface

public class BankAccount
{
   // Constructors
   public BankAccount()
   {
      // body--filled in later
   }
   public BankAccount(double initialBalance)
   {
      // body--filled in later
   }

   // Methods
   public void deposit(double amount)
   {
      // body--filled in later
   }
   public void withdraw(double amount)
   {
      // body--filled in later
   }
   public double getBalance()
   {
      // body--filled in later
   }
   // private fields--filled in later
}

Syntax 3.3 : Class Definition

accessSpecifier class ClassName
{
   constructors
   methods
   fields
}

public class BankAccount
{
   public BankAccount(double initialBalance) { . . . }
   public void deposit(double amount) { . . . }
   . . . 
}

Self Check

3. How can you use the methods of the public interface to empty the harrysChecking bank account?
4. Suppose you want a more powerful bank account abstraction that keeps track of an account number in addition to the balance. How would you change the public interface to accommodate this enhancement?

Answers

3. harrysChecking.withdraw(harrysChecking.getBalance())
4. Add an accountNumber parameter to the constructors, and add a getAccountNumber method. There is no need for a setAccountNumber method–the account number never changes after construction.

Commenting the Public Interface

/**
   Withdraws money from the bank account.
   @param the amount to withdraw
*/
public void withdraw(double amount)
{
   // implementation filled in later 
}

/**
   Gets the current balance of the bank account.
   @return the current balance
*/
public double getBalance()
{
   // implementation filled in later
}

Class Comment

/**
   A bank account has a balance that can
   be changed by deposits and withdrawals.
*/
public class BankAccount
{
   . . .
}

• Provide documentation comments for
  
• every class
• every method
•  every parameter
• every return value.

Javadoc Method Summary

Javadoc Method Detail

Self Check

5. Suppose we enhance the BankAccount class so that each account has an account number. Supply a documentation comment for the constructor BankAccount(int accountNumber, double initialBalance)
6. Why is the following documentation comment questionable?

   /**
      Each account has an account number.
      @return the account number of this account.
   */
   int getAccountNumber()

Answers

5. /**
      Constructs a new bank account with a given initial balance.
      @param accountNumber the account number for this account
      @param initialBalance the initial balance for this account
   */

6. The first sentence of the method description should describe the method–it is displayed in isolation in the summary table

Instance Fields

• An object stores its data in instance fields
• Field: a technical term for a storage location inside a block of memory
• Instance of a class: an object of the class
• The class declaration specifies the instance fields:
  public class BankAccount
  { 
     . . .
     private double balance;
  }

Instance Fields

• An instance field declaration consists of the following parts:
  • access specifier (such as private)
  • type of variable (such as double)
  • name of variable (such as balance)
• Each object of a class has its own set of instance fields
• You should declare all instance fields as private

Instance Fields

 Syntax 3.4 : Instance Field Declaration

accessSpecifier class ClassName
{
   . . . 
   accessSpecifier fieldType fieldName;
   . . . 
}

public class BankAccount
{
   . . .
   private double balance;
   . . .
}

Accessing Instance Fields

• The deposit method of the BankAccount class can access the private instance field:
  public void deposit(double amount)
  {
     double newBalance = balance + amount;
     balance = newBalance;
  }
• Other methods cannot:
  public class BankRobber
  {
     public static void main(String[] args)
     {
       BankAccount momsSavings = new BankAccount(1000);
       . . .
       momsSavings.balance = -1000; // ERROR
     }
  }
• Encapsulation = Hiding data and providing access through methods

Self Check

7. Suppose we modify the BankAccount class so that each bank account has an account number. How does this change affect the instance fields?
8. What are the instance fields of the Rectangle class?

Answers

7. An instance field
   private int accountNumber;
   needs to be added to the class

8. private int x;
   private int y;
   private int width;
   private int height;

Implementing Constructors

• Constructors contain instructions to initialize the instance fields of an object

  public BankAccount()
  {
     balance = 0;
  }
  
  public BankAccount(double initialBalance)
  {
     balance = initialBalance;
  }

Constructor Call Example

• BankAccount harrysChecking = new BankAccount(1000);
  • Create a new object of type BankAccount
  • Call the second constructor (since a construction parameter is supplied)
  • Set the parameter variable initialBalance to 1000
  • Set the balance instance field of the newly created object to initialBalance
  • Return an object reference, that is, the memory location of the object, as the value of the new expression
  • Store that object reference in the harrysChecking variable

Implementing Methods

• Some methods do not return a value

  public void withdraw(double amount)
  {
     double newBalance = balance - amount;
     balance = newBalance;
  }
  

• Some methods return an output value

  public double getBalance()
  {
     return balance;
  }

Method Call Example

• harrysChecking.deposit(500);
  • Set the parameter variable amount to 500
  • Fetch the balance field of the object whose location is stored in harrysChecking
  • Add the value of amount to balance and store the result in the variable newBalance
  • Store the value of newBalance in the balance instance field, overwriting the old value

Syntax 3.5: The return Statement

File BankAccount.java

Self Check

9. How is the getWidth method of the Rectangle class implemented?
10. How is the translate method of the Rectangle class implemented?

Answers

9. public int getWidth()
   {
      return width;
   }

10. There is more than one correct answer. One possible implementation is as follows:

    public void translate(int dx, int dy)
    {
       int newx = x + dx;
       x = newx;
       int newy = y + dy;
       y = newy;
    }

Testing a Class

• Test class: a class with a main method that contains statements to test another class.
• Typically carries out the following steps:
  1. Construct one or more objects of the class that is being tested
  2. Invoke one or more methods
  3. Print out one or more results
• Details for building the program vary. In most environments, you need to carry out these steps:
  1. Make a new subfolder for your program
  2. Make two files, one for each class
  3. Compile both files
  4. Run the test program

File BankAccountTester.java

Output:

Testing With BlueJ

Self Check

11. When you run the BankAccountTester program, how many objects of class BankAccount are constructed? How many objects of type BankAccountTester?
12. Why is the BankAccountTester class unnecessary in development environments that allow interactive testing, such as BlueJ?

Answers

11. One BankAccount object, no BankAccountTester object. The purpose of the BankAccountTester class is merely to hold the main method
12. In those environments, you can issue interactive commands to construct BankAccount objects, invoke methods, and display their return values

Categories of Variables

• Categories of variables
  1. Instance fields (balance in BankAccount)
  2. Local variables (newBalance in deposit method)
  3. Parameter variables (amount in deposit method)
• An instance field belongs to an object
• The fields stay alive until no method uses the object any longer
• In Java, the garbage collector periodically reclaims objects when they are no longer used
• Local and parameter variables belong to a method
• Instance fields are initialized to a default value, but you must initialize local variables

Lifetime of Variables

harrysChecking.deposit(500);
double newBalance = balance + amount;
balance = newBalance;

Self Check

13. What do local variables and parameter variables have in common? In which essential aspect do they differ?
14. During execution of the BankAccountTester program in the preceding section, how many instance fields, local variables, and parameter variables were created, and what were their names?

Answers

13. Variables of both categories belong to methods–they come alive when the method is called, and they die when the method exits. They differ in their initialization. Parameter variables are initialized with the call values; local variables must be explicitly initialized.
14. One instance field, named balance. Three local variables, one named harrysChecking and two named newBalance (in the deposit and withdraw methods); two parameter variables, both named amount (in the deposit and withdraw methods).

Implicit and Explicit Method Parameters

• The implicit parameter of a method is the object on which the method is invoked
• The this reference denotes the implicit parameter
• 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;
  }

• balance is the balance of the object to the left of the dot:
  

  momsSavings.withdraw(500)

  means
  

     double newBalance = momsSavings.balance - amount;
     momsSavings.balance = newBalance;

Implicit Parameters and this

• Every method has one implicit parameter
• The implicit parameter is always called this
• Exception: Static methods do not have an implicit parameter (more on Chapter 9)

• double newBalance = balance + amount;
  // actually means
  double 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);

Implicit Parameters and this

Self Check

15. How many implicit and explicit parameters does the withdraw method of the BankAccount class have, and what are their names and types?
16. In the deposit method, what is the meaning of this.amount? Or, if the expression has no meaning, why not?
17. How many implicit and explicit parameters does the main method of the BankAccountTester class have, and what are they called?

Answers

15. One implicit parameter, called this, of type BankAccount, and one explicit parameter, called amount, of type double.
16. It is not a legal expression. this is of type BankAccount and the BankAccount class has no field named amount.
17. No implicit parameter–the method is static–and one explicit parameter, called args.