Chapter Goals

• To become familiar with the process of implementing classes
• To be able to implement and test simple methods
• To understand the purpose and use of constructors
• To understand how to access instance variables and local variables
• To be able to write javadoc comments
• To implement classes for drawing graphical shapes

Instance Variables and Encapsulation

Figure 1 Tally counter



• Simulator statements:

  Counter tally = new Counter(); 
  tally.click(); 
  tally.click();
  int result = tally.getValue(); // Sets result to 2
  

• Each counter needs to store a variable that keeps track of the number of simulated button clicks.

Instance Variables

• Instance variables store the data of an object.
• Instance of a class: an object of the class.
• An instance variable is a storage location present in each object of the class.
• The class declaration specifies the instance variables:

  public class Counter
  {
     private int value;
     ...
  }

• An object's instance variables store the data required for executing its methods.

Instance Variables

• An instance variable declaration consists of the following parts:
  • access specifier (private)
  • type of variable (such as int)
  • name of variable (such as value)
• You should declare all instance variables as private.

Instance Variables

• Each object of a class has its own set of instance variables.

Figure 2 Instance Variables

Syntax 3.1 Instance Variable Declaration

Instance Variables

 

These clocks have common behavior, but each of them has a different state. Similarly, objects of a class can have their instance variables set to different values.

The Methods of the Counter Class

• The click method advances the counter value by 1:

  public void click() 
  {  
     value = value + 1; 
  }

  • Affects the value of the instance variable of the object on which the method is invoked
  • The method call concertCounter.click();
    
    • Advances the value variable of the concertCounter object

The Methods of the Counter Class

• The getValue method returns the current value:

  public int getValue() 
  { 
     return value; 
  }

• The return statement
  • Terminates the method call
  • Returns a result (the return value) to the method's caller
• Private instance variables can only be accessed by methods of the same class.

Encapsulation

• Encapsulation is the process of hiding implementation details and providing methods for data access.
• To encapsulate data:
  • Declare instance variables as private and
  • Declare public methods that access the variables
• Encapsulation allows a programmer to use a class without having to know its implementation.
• Information hiding makes it simpler for the implementor of a class to locate errors and change implementations.

Encapsulation

 

A thermostat functions as a "black box" whose inner workings are hidden.

• When you assemble classes, like Rectangle and String, into programs you are like a contractor installing a thermostat.
• When you implement your own classes you are like the manufacturer who puts together a thermostat out of parts.

section_1/Counter.java

Self Check 3.1

Supply the body of a method public void unclick() that undoes an unwanted button click.

• Answer:

  public void unclick()
  { 
     value = value - 1; 
  }

Self Check 3.2

Suppose you use a class Clock with private instance variables hours and minutes. How can you access these variables in your program?

• Answer: You can only access them by invoking the methods of the Clock class.

Self Check 3.3

Consider the Counter class. A counter’s value starts at 0 and is advanced by the click method, so it should never be negative. Suppose you found a negative value variable during testing. Where would you look for the error?

• Answer: In one of the methods of the Counter class.

Self Check 3.4

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?

• Answer: The programmers who designed and implemented the Java library.

Self Check 3.5

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?

• Answer: Other programmers who work on the personal finance application.

Specifying the Public Interface of a Class

• In order to implement a class, you first need to know which methods are required.
• Essential behavior of a bank account:
  • deposit money
  • withdraw money
  • get balance

Specifying the Public Interface of a Class

• We want to support method calls such as the following:

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

• Here are the method headers needed for a BankAccount class:
  public void deposit(double amount)
  public void withdraw(double amount)
  public double getBalance()

Specifying the Public Interface of a Class: Method Declaration

• A method's body consisting of statements that are executed when the method is called:

  public void deposit(double amount) 
  {  
     implementation - filled in later 
  }
  

• You can fill in the method body so it compiles:

  public double getBalance()
  {
     // TODO: fill in implementation
     return 0;
  }

Specifying the Public Interface of a Class

• BankAccount methods were declared as public.
• public methods can be called by all other methods in the program.
• Methods can also be declared private
  • private methods only be called by other methods in the same class
  • private methods are not part of the public interface

Specifying Constructors

• Initialize objects
• Set the initial data for objects
• Similar to a method with two differences:
  • The name of the constructor is always the same as the name of the class
  • Constructors have no return type

Specifying Constructors: BankAccount

• Two constructors
  public BankAccount()
  public BankAccount(double initialBalance)
• Usage
  BankAccount harrysChecking = new BankAccount();
  BankAccount momsSavings = new BankAccount(5000);

Specifying Constructors: BankAccount

• The constructor name is always the same as the class name.
• The compiler can tell them apart because they take different arguments.
• A constructor that takes no arguments is called a no-argument constructor.
• BankAccount's no-argument constructor - header and body:

  public BankAccount()
  {
     constructor body—implementation filled in later
  }

• The statements in the constructor body will set the instance variables of the object.

BankAccount Public Interface

The constructors and methods of a class go inside the class declaration:

public class BankAccount
{
   // private instance variables--filled in later

   // 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
   }
}

Specifying the Public Interface of a Class

• public constructors and methods of a class form the public interface of the class.
• These are the operations that any programmer can use

Syntax 3.2 Class Declaration

Using the Public Interface

• Example: transfer money

  // Transfer from one account to another
  double transferAmount = 500;
  momsSavings.withdraw(transferAmount);
  harrysChecking.deposit(transferAmount)

• Example: add interest

  double interestRate = 5; // 5 percent interest
  double interestAmount = momsSavings.getBalance() * interestRate / 100;
  momsSavings.deposit(interestAmount);

• Programmers use objects of the BankAccount class to carry out meaningful tasks
  • without knowing how the BankAccount objects store their data
  • without knowing how the BankAccount methods do their work

Commenting the Public Interface

• Use documentation comments to describe the classes and public methods of your programs.
• Java has a standard form for documentation comments.
• A program called javadoc can automatically generate a set of HTML pages.
• Documentation comment
  • placed before the class or method declaration that is being documented

Commenting the Public Interface - Documenting a method

• Start the comment with a /**.
• Describe the method’s purpose.
• Describe each parameter:
  • start with @param
  • name of the parameter that holds the argument
  • a short explanation of the argument
• Describe the return value:
  • start with @return
  • describe the return value
• Omit @param tag for methods that have no arguments.
• Omit the @return tag for methods whose return type is void.
• End with */

Commenting the Public Interface - Documenting a method

• Example:

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

• Example:

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

Commenting the Public Interface - Documenting a class

• Place above the class declaration.
• Supply a brief comment explaining the class's purpose.
• Example:

  /**
     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 variable
  • every return value

Method Summary

Figure 3 A Method Summary Generated by javadoc

Method Details

Figure 4 Method Detail Generated by javadoc

Self Check 3.6

How can you use the methods of the public interface to empty the harrysChecking bank account?

• Answer: harrysChecking.withdraw(harrysChecking.getBalance())

Self Check 3.7

What is wrong with this sequence of statements?

BankAccount harrysChecking = new BankAccount(10000); 
System.out.println(harrysChecking.withdraw(500));

• Answer: The withdraw method has return type void. It doesn’t return a value. Use the getBalance method to obtain the balance after the withdrawal.

Self Check 3.8

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?

• Answer: 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.

Self Check 3.9

Suppose we enhance the BankAccount class so that each account has an account number. Supply a documentation comment for the constructor

public BankAccount(int accountNumber, double initialBalance)

• Answer:

  /**
     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
  */

Self Check 3.10

Why is the following documentation comment questionable?

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

• Answer: The first sentence of the method description should describe the method—it is displayed in isolation in the summary table.

Providing the Class Implementation

• The private implementation of a class consists of:
  • instance variables
  • the bodies of constructors
  • the bodies of methods.

Providing Instance Variables

• Determine the data that each bank account object contains.
• What does the object need to remember so that it can carry out its methods?
  
• Each bank account object only needs to store the current balance.
• BankAccount instance variable declaration:

  public class BankAccount
  {
     private double balance;
     // Methods and constructors below
     . . .
  }

Providing Instance Variables

 

Like a wilderness explorer who needs to carry all items that may be needed, an object needs to store the data required for its method calls.

Providing Constructors

• Constructor's job is to initialize the instance variables of the object.
• The no-argument constructor sets the balance to zero.

  public BankAccount()
  {
     balance = 0;
  }

• The second constructor sets the balance to the value supplied as the construction argument.

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

Providing Constructors - Tracing the Statement

Steps carried out when the following statement is executed:

BankAccount harrysChecking = new BankAccount(1000);  

• Create a new object of type BankAccount.
• Call the second constructor
  • because an argument is supplied in the constructor call
• Set the parameter variable initialBalance to 1000.
• Set the balance instance variable of the newly created object to initialBalance.
• Return an object reference, that is, the memory location of the object.
• Store that object reference in the harrysChecking variable.

Providing Constructors - Tracing the Statement

Figure 5 How a Constructor Works

Providing Constructors

 

A constructor is like a set of assembly instructions for an object.

Providing Methods

• Is the method an accessor or a mutator
  • Mutator method
    • Update the instance variables in some way
  • Accessor method
    • Retrieves or computes a result
• deposit method - a mutator method
  • Updates the balance

  public void deposit(double amount) 
  {  
     balance = balance + amount; 
  }

Providing Methods - continued

• withdraw method - another mutator

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

• getBalance method - an accessor method
  • Returns a value

  public double getBalance()
  {
     return balance;
  }
      

Table 1 Implementing Classes

section_3/BankAccount.java

Self Check 3.11

Suppose we modify the BankAccount class so that each bank account has an account number. How does this change affect the instance variables?

• Answer: An instance variable needs to be added to the class:

  private int accountNumber;
  

Self Check 3.12

Why does the following code not succeed in robbing mom's bank account?

public class BankRobber 
{ 
   public static void main(String[] args) 
   { 
      BankAccount momsSavings = new BankAccount(1000); 
      momsSavings.balance = 0;  
   } 
}

• Answer: Because the balance instance variable is accessed from the main method of BankRobber. The compiler will report an error because main is not a method of the BankAccount class and main has no access to BankAccount instance variables.

Self Check 3.13

The Rectangle class has four instance variables: x, y, width, and height. Give a possible implementation of the getWidth method.

• Answer:

  public int getWidth()
  {
     return width;
  }

Self Check 3.14

Give a possible implementation of the translate method of the Rectangle class.

• Answer: 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;
  }

Unit Testing

• BankAccount.java can not be executed:
  • It has no main method
  • Most classes do not have a main method
• Before using BankAccount.java in a larger program:
  • You should test in isolation
• Unit test: verifies that a class works correctly in isolation, outside a complete program.

Unit Testing

• To test a class, either
  • use an environment for interactive testing, or
  • write a tester class to execute test instructions.
• Tester 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
  4. Print the expected results

Unit Testing

 

An engineer tests a part in isolation.
This is an example of unit testing.

Unit Testing with Bluej

Figure 6 The Return Value of the getBalance Method in BlueJ

section_4/BankAccountTester.java

Program Run:

• 1500
  Expected: 1500

Unit Testing - Building a program

• To produce a program: combine both BankAccount and BankAccountTester classes.
• 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
• BankAccount and BankAccountTest have entirely different purposes:
  • BankAccount class describes objects that compute bank balances
  • BankAccountTester class runs tests that put a BankAccount object through its paces

Self Check 3.15

When you run the BankAccountTester program, how many objects of class BankAccount are constructed? How many objects of type BankAccountTester?

• Answer: One BankAccount object, no BankAccountTester object. The purpose of the BankAccountTester class is merely to hold the main method.

Self Check 3.16

Why is the BankAccountTester class unnecessary in development environments that allow interactive testing, such as BlueJ?

• Answer: In those environments, you can issue interactive commands to construct BankAccount objects, invoke methods, and display their return values.

Problem Solving: Tracing Objects

• Important skill: the ability to simulate the actions of a program with pencil and paper.
• Use an index card or a sticky note for each object:
  • Write the methods on the front
  • Make a table for the values of the instance variables on the back
• CashRegister class
  

Problem Solving: Tracing Objects

• When an object is constructed, fill in the initial values of the instance variables.
  
  
• Update the values of the instance variables when a mutator method is called.
• After a call to cashRegister's recordPurchase method
  
  
• More than one object: create multiple cards
  
  

Problem Solving: Tracing Objects

• Useful when enhancing a class..
• Enhance CashRegister class to compute the sales tax.
• Add methods recordTaxablePurchase and getSalesTax to the front of the card.
• Don’t have enough information to compute sales tax:
  • need tax rate
  • need total of the taxable items
• Need additional instance variables for:
  • taxRate
  • taxablePurchase

Problem Solving: Tracing Objects

• Example: CashRegister class enhancement
  • The code:

  CashRegister reg3(7.5); // 7.5 percent sales tax
  reg3.recordPurchase(3.95); // Not taxable
  reg3.recordTaxablePurchase(19.95); // Taxable

  • The card:
    

Self Check 3.17

Consider a Car class that simulates fuel consumption in a car. We will assume a fixed efficiency (in miles per gallon) that is supplied in the constructor. There are methods for adding gas, driving a given distance, and checking the amount of gas left in the tank. Make a card for a Car object, choosing suitable instance variables and showing their values after the object was constructed.

• Answer:
  

Self Check 3.18

Trace the following method calls:

Car myCar(25);
myCar.addGas(20);
myCar.drive(100);
myCar.drive(200);
myCar.addGas(5); 

• Answer:
  

Self Check 3.19

Suppose you are asked to simulate the odometer of the car, by adding a method getMilesDriven.
Add an instance variable to the object’s card that is suitable for computing this method’s result.

• Answer:
  
  

Self Check 3.20

Trace the methods of Self Check 18, updating the instance variable that you added in Self Check 19.

• Answer:
  

Local Variables

• Local variables are declared in the body of a method:

  public double giveChange() 
  { 
     double change = payment - purchase; 
     purchase = 0; 
     payment = 0; 
     return change; 
  }
  

• When a method exits, its local variables are removed.
• Parameter variables are declared in the header of a method:

  public void enterPayment(double amount)

Local Variables

• Local and parameter variables belong to methods:
  • When a method runs, its local and parameter variables come to life
  • When the method exits, they are removed immediately
• Instance variables belong to objects, not methods:
  • When an object is constructed, its instance variables are created
  • The instance variables stay alive until no method uses the object any longer
• Instance variables are initialized to a default value:
  • Numbers are initialized to 0
  • Object references are set to a special value called null
    • A null object reference refers to no object at all
• You must initialize local variables:
  • The compiler complains if you do not

Self Check 3.21

What do local variables and parameter variables have in common? In which essential aspect do they differ?

• Answer: 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.

Self Check 3.22

Why was it necessary to introduce the local variable change in the giveChange method? That is, why didn’t the method simply end with the statement

return payment - purchase;

• Answer: After computing the change due, payment and purchase were set to zero. If the method returned payment - purchase, it would always return zero.

Self Check 3.23

Consider a CashRegister object reg1 whose payment instance variable has the value 20 and whose purchase instance variable has the value 19.5. Trace the call reg1.giveChange(). Include the local variable change. Draw an X in its column when the variable ceases to exist.

• Answer:
  

The this Reference

• Two types of inputs are passed when a method is called:
  • The object on which you invoke the method
  • The method arguments
• In the call momsSavings.deposit(500) the method needs to know:
  • The account object (momsSavings)
  • The amount being deposited (500)
• The implicit parameter of a method is the object on which the method is invoked.
• All other parameter variables are called explicit parameters.

The this Reference

• Look at this method:
  

  public void deposit(double amount)
  {
     balance = balance + amount;
  }

  • amount is the explicit parameter
  • The implicit parameter(momSavings) is not seen
  • balance means momSavings.balance
• When you refer to an instance variable inside a method, it means the instance variable of the implicit parameter.

The this Reference

• The this reference denotes the implicit parameter

balance = balance + amount;

actually means

this.balance = this.balance + amount;

• When you refer to an instance variable in a method, the compiler automatically applies it to the this reference.

The this Reference

• Some programmers feel that inserting the this reference before every instance variable reference makes the code clearer:

  public BankAccount(double initialBalance) 
  {  
     this.balance = initialBalance; 
  }

The this Reference

Figure 7 The Implicit Parameter of a Method Call

The this Reference

• The this reference can be used to distinguish between instance variables and local or parameter variables:

  public BankAccount(double balance) 
  {  
     this.balance = balance; 
  }
  

• A local variable shadows an instance variable with the same name.
  • You can access the instance variable name through the this reference.
• In Java, local and parameter variables are considered first when looking up variable names.
• Statement

  this.balance = balance;

  means: "Set the instance variable balance to the parameter variable balance".

The this Reference

• A method call without an implicit parameter is applied to the same object.
• Example:

  public class BankAccount 
  {  
     . . . 
     public void monthlyFee() 
     {  
        withdraw(10); // Withdraw $10 from this account 
     } 
  }
  

• The implicit parameter of the withdraw method is the (invisible) implicit parameter of the monthlyFee method
• You can use the this reference to make the method easier to read:

  public class BankAccount 
  {  
     . . . 
     public void monthlyFee() 
     {  
        this.withdraw(10); // Withdraw $10 from this account 
     } 
  }

Self Check 3.24

How many implicit and explicit parameters does the withdraw method of the BankAccount class have, and what are their names and types?

• Answer: One implicit parameter, called this, of type BankAccount, and one explicit parameter, called amount, of type double.

Self Check 3.25

In the deposit method, what is the meaning of this.amount? Or, if the expression has no meaning, why not?

• Answer: It is not a legal expression. this is of type BankAccount and the BankAccount class has no instance variable named amount.

Self Check 3.26

How many implicit and explicit parameters does the main method of the BankAccountTester class have, and what are they called?

• Answer: No implicit parameter — the main method is not invoked on any object — and one explicit parameter, called args.

Shape Classes

• Good practice: Make a class for each part of a drawing that occurs more than once.

  public class Car
  {
     public Car(int x, int y)
     {
        // Remember position
        . . .
     }
     public void draw(Graphics2D g2)
     {
        // Drawing instructions
        . . .
     }
  }

Drawing Cars

Goal: draw two cars - one in top-left corner of window, and another in the bottom right.

Figure 8 The Car Component Draws Two Car Shapes

Plan Complex Shapes on Graph Paper

Figure 9 Using Graph Paper to Find Shape Coordinates

Drawing Cars

The program that produces the drawing is composed of three classes:

• The Car class is responsible for drawing a single car.
  • Two objects of this class are constructed, one for each car.
• The CarComponent class displays the drawing.
• The CarViewer class shows a frame that contains a CarComponent.

Drawing Cars

• The paintComponent method of the CarComponent class draws the two cars.
• To compute bottom right position:

  Car car1 = new Car(0, 0);
  int x = getWidth() - 60;
  int y = getHeight() - 30;
  Car car2 = new Car(x, y);

  • getWidth and getHeight return the dimensions of the CarComponent
  • Subtract the dimensions of the car to determine the position of car2:
• When window is resized
  • paintComponent is called
  • car position is recomputed using current dimensions
    

section_8/Car.java

section_8/CarComponent.java

section_8/CarViewer.java

Self Check 3.27

Which class needs to be modified to have the two cars positioned next to each other?

• Answer: CarComponent

Self Check 3.28

Which class needs to be modified to have the car tires painted in black, and what modification do you need to make?

• Answer: In the draw method of the Car class, call

  g2.fill(frontTire);
  g2.fill(rearTire);

Self Check 3.29

How do you make the cars twice as big?

• Answer: Double all measurements in the draw method of the Car class.