Discovering Classes

• A class represents a single concept from the problem domain
• Name for a class should be a noun that describes concept
• Concepts from mathematics:

  Point
  Rectangle
  Ellipse

• Concepts from real life:

  BankAccount
  CashRegister

Discovering Classes

• Actors (end in -er, -or) — objects do some kinds of work for you:

  Scanner
  Random // better name: RandomNumberGenerator

• Utility classes — no objects, only static methods and constants:

  Math

• Program starters: a class with only a main method
• The class name should indicate what objects of the class will do: Paycheck is a better name than PaycheckProgram.
• Don't turn a single operation action into a class: Paycheck is a better name than ComputePaycheck

Self Check 8.1

What is the rule of thumb for finding classes?

• Answer: Look for nouns in the problem description.

Self Check 8.2

Your job is to write a program that plays chess. Might ChessBoard be an appropriate class? How about MovePiece?

• Answer: Yes (ChessBoard) and no (MovePiece).

Designing Good Methods - Cohesion

• A class should represent a single concept
• The public interface of a class is cohesive if all of its features are related to the concept that the class represents
• The members of a cohesive team have a common goal.
  

Designing Good Methods - Cohesion

• This class lacks cohesion.

  public class CashRegister
  {
     public static final double QUARTER_VALUE = 0.25;
     public static final double DIME_VALUE = 0.1;
     public static final double NICKEL_VALUE = 0.05;
     . . .
     public void receivePayment(int dollars, int quarters,
           int dimes, int nickels, int pennies)
     . . .
  
  }

• It contains two concepts
  • A cash register that holds coins and computes their total
  • The values of individual coins.

Designing Good Methods - Cohesion

• Solution: Make two classes:

  public class Coin
  {
     public Coin(double aValue, String aName) { . . . }
     public double getValue() { . . . }
     . . .
  }
  
  
  }

  public class CashRegister
  {
     . . .
     public void receivePayment(int coinCount, Coin coinType) 
     {
        payment = payment + coinCount * coinType.getValue();
     }
     . . .
  }

• Now CashRegister class can handle any type of coin

Minimizing Dependencies

• A class depends on another class if its methods use that class in any way.
  • CashRegister depends on Coin
• UML: Unified Modeling Language
  • Notation for object-oriented analysis and design
• UML class diagram showing dependency relationship between the CashRegister and Coin Classes
  
• The Coin class does not depend on the CashRegister class

Minimizing Dependencies

• Example: printing BankAccount balance
• Recommended

  System.out.println("The balance is now $" + momsSavings.getBalance());

• Don't add a printBalance method to BankAccount

  public void printBalance() // Not recommended
  {
  System.out.println("The balance is now $" + balance);
  }

  • The method depends on System.out
  • Not every computing environment has System.out
  • Violates the rule of minimizing dependencies
• Best to decouple input/output from the work of your classes
  • Place the code for producing output or consuming input in a separate class.

Separating Accessors and Mutators

• A mutator method changes the state of an object
• An accessor method asks an object to compute a result, without changing the state
• An immutable class has no mutator methods.
• String is an immutable class
  • No method in the String class can modify the contents of a string
• References to objects of an immutable class can be safely shared.

 

Separating Accessors and Mutators

• In a mutable class, separate accessors and mutators
• A method that returns a value should not be a mutator
• In general, all mutators of your class should have return type void.
• Sometimes a mutator method can return an informational value
• ArrayList remove method returns true if the removal was successful
• To check the temperature of the water in the bottle, you could take a sip, but that would be the equivalent of a mutator method.
  

Minimizing Side Effects

• A side effect of a method is any externally observable data modification.
• Mutator methods have a side effect, namely the modification of the implicit parameter.
• In general, a method should not modify its parameter variables.

  /**
  Computes the total balance of the given accounts.
  @param accounts a list of bank accounts
  */
  public double getTotalBalance(ArrayList<String> accounts)
  {
     double sum = 0;
     while (studentNames.size() > 0)
     {
        BankAccount account = accounts.remove(0); // Not recommended
        sum = sum + account.getBalance();
     }
     return sum;
  

• Such a side effect would not be what most programmers expect

Minimizing Side Effects

• The following method mutates the System.out object, which is not a part of the BankAccount object.

  public void printBalance() // Not recommended
  {
     System.out.println("The balance is now $" + balance);
  }

• That is a side effect
• Keep most of your classes free from input and output operations
• This taxi has an undesirable side effect, spraying bystanders with muddy water
  
• When designing methods, minimize side effects.

Self Check 8.3

Why is the CashRegister class from Chapter 4 not cohesive?

• Answer: Some of its features deal with payments, others with coin values.

Self Check 8.4

Why does the Coin class not depend on the CashRegister class?

• Answer: None of the coin operations require the CashRegister class.

Self Check 8.5

Why is it a good idea to minimize dependencies between classes?

• Answer: If a class doesn't depend on another, it is not affected by interface changes in the other class.

Self Check 8.6

Is the substring method of the String class an accessor or a mutator?

• Answer: It is an accessor – calling substring doesn't modify the string on which the method is invoked. In fact, all methods of the String class are accessors.

Self Check 8.7

Is the Rectangle class immutable?

• Answer: No – translate is a mutator.

Self Check 8.8

If a refers to a bank account, then the call a.deposit(100) modifies the bank account object. Is that a side effect?

• Answer: It is a side effect; this kind of side effect is common in object-oriented programming.

Self Check 8.9

Consider the Student class of Chapter 7. Suppose we add a method

void read(Scanner in)
{
   while (in.hasNextDouble())
      addScore(in.nextDouble());
}

Does this method have a side effect other than mutating the data set?

• Answer: Yes – the method affects the state of the Scanner argument.

Consistency

While it is possible to eat with mismatched silverware, consistency is more pleasant.

Problem Solving: Patterns for Object Data - Keeping a Total

All classes that manage a total follow the same basic pattern

• Keep an instance variable that represents the current total

  private double purchase;

• Provide these methods as necessary
  • A method to increase the total by a given amount

    public void recordPurchase(double amount)
    {
       purchase = purchase + amount;
    }

  • A method that reduces or clears the total

    public void clear()
    {
       purchase = 0;
    }

  • A method that yields the current total

    public double getAmountDue()
    {
       return purchase;
    }

Problem Solving: Patterns for Object Data - Counting Events

A counter that counts events is incremented in methods that correspond to the events.

• Keep a counter
  

  private int itemCount;

• Increment the counter in those methods that correspond to the events that you want to count

  public void recordPurchase(double amount)
  {
     purchase = purchase + amount;
     itemCount++;
  }

• Provide a method to clear the counter if necessary

  public void clear()
  {
     purchase = 0;
     itemCount = 0;
  }

• You may need a method to report the count to the user of the class

Problem Solving: Patterns for Object Data - Collecting Values

An object can collect other objects in an array or array list.

• A shopping cart object needs to manage a collection of items.
  
• An array list is usually easier to use than an array

  public class Question
  {
     private ArrayList<String> choices;
     . . .
  }

Problem Solving: Patterns for Object Data - Collecting Values

• Initialize the instance variable to an empty collection

  public Question()
  {
     choices = new ArrayList<String>();
  }

• Supply a mechanism for adding values

  public void add(String option)
  {
     choices.add(option);
  }

  • The user of a Question object can call this method multiple times to add the choices

Problem Solving: Patterns for Object Data - Managing Properties of an Object

• A property is a value of an object that an object user can set and retrieve
• Provide an instance variable to store the property’s value and methods to get and set it.

  public class Student
  {
     private String name;
     . . .
     public String getName() { return name; }
     public void setName(String newName) { name = newName; }
     ...
  }
  

• It is common to add error checking to the setter method

  public void setName(String newName)
  {
     if (newName.length() > 0) { name = newName; }
  }

• Some properties should not change after they have been set in the constructor
  • Don’t supply a setter method

    public class Student
    {
       private int id;
       . . .
       public Student(int anId) { id = anId; }
       public String getId() { return id; }
       // No setId method
       . . .
    }

Problem Solving: Patterns for Object Data - Modeling Objects with Distinct States

• Some objects have behavior that varies depending on what has happened in the past
• If a fish is in a hungry state, its behavior changes.
  
• Supply an instance variable for the current state

  public class Fish
  {
     private int hungry;
  . . .
  }

• Supply constants for the state values

  public static final int NOT_HUNGRY = 0;
  public static final int SOMEWHAT_HUNGRY = 1;
  public static final int VERY_HUNGRY = 2;

Problem Solving: Patterns for Object Data - Modeling Objects with Distinct States

• Determine which methods change the state

  public void eat()
  {
     hungry = NOT_HUNGRY;
  . . .
  }
  
  public void move()
  {
     . . .
     if (hungry < VERY_HUNGRY) { hungry++; }
  }

• Determine where the state affects behavior

  public void move()
  {
     if (hungry == VERY_HUNGRY)
     {
        Look for food.
     }
     . . .
  }

Problem Solving: Patterns for Object Data - Describing the Position of an Object

• To model a moving object:
  • You need to store and update its position.
  • You may also need to store its orientation or velocity
• If the object moves along a line, you can represent the position as a distance from a fixed point

  private double distanceFromTerminus;

• If the object moves in a grid, remember its current location and direction in the grid

  private int row;
  private int column;
  private int direction; // 0 = North, 1 = East, 2 = South, 3 = West

• A bug in a grid needs to store its row, column, and direction.
  

Problem Solving: Patterns for Object Data - Describing the Position of an Object

• There will be methods that update the position
• You may be told how much the object moves

  public void move(double distanceMoved)
  {
     distanceFromTerminus = distanceFromTerminus + distanceMoved;
  }

• If the movement happens in a grid, you need to update the row or column, depending on the current orientation.

  public void moveOneUnit()
  {
     if (direction == NORTH) { row--; }
     else if (direction == EAST) { column++; }
     else if (direction == SOUTH) { row++; }
     else if (direction == WEST) { column––; }
  }

• Your program will simulate the actual movement in some way
• Locate the methods that move the object, and update the positions according to the rules of the simulation

Self Check 8.10

Suppose we want to count the number of transactions in a bank account in a statement period, and we add a counter to the BankAccount class:

public class BankAccount
{
   private int transactionCount;
   ...
}

In which methods does this counter need to be updated?

• Answer: It needs to be incremented in the deposit and withdraw methods. There also needs to be some method to reset it after the end of a statement period.

Self Check 8.11

In How To 3.1, the CashRegister class does not have a getTotalPurchase method. Instead, you have to call receivePayment and then giveChange. Which recommendation of Section 8.2.4 does this design violate? What is a better alternative?

• Answer: The giveChange method is a mutator that returns a value that cannot be determined any other way. Here is a better design. The receivePayment method could decrease the purchase instance variable. Then the program user would call receivePayment, determine the change by calling getAmountDue, and call the clear method to reset the cash register for the next sale.

Self Check 8.12

In the example in Section 8.3.3, why is the add method required? That is, why can’t the user of a Question object just call the add method of the ArrayList<String> class?

• Answer: The ArrayList<String> instance variable is private, and the class users cannot acccess it.

Self Check 8.13

Suppose we want to enhance the CashRegister class in How To 3.1 to track the prices of all purchased items for printing a receipt. Which instance variable should you provide? Which methods should you modify?

• Answer: You need to supply an instance variable that can hold the prices for all purchased items. This could be an ArrayList<Double> or ArrayList<String>, or it could simply be a String to which you append lines. The instance variable needs to be updated in the recordPurchase method. You also need a method that returns the receipt.

Self Check 8.14

Consider an Employee class with properties for tax ID number and salary. Which of these properties should have only a getter method, and which should have getter and setter methods?

• Answer: The tax ID of an employee does not change, and no setter method should be supplied. The salary of an employee can change, and both getter and setter methods should be supplied.

Self Check 8.15

Suppose the setName method in Section 8.3.4 is changed so that it returns true if the new name is set, false if not. Is this a good idea?

• Answer: Section 8.2.3 suggests that a setter should return void, or perhaps a convenience value that the user can also determine in some other way. In this situation, the caller could check whether newName is blank, so the change is fine.

Self Check 8.16

Look at the direction instance variable in the bug example in Section 8.3.6. This is an example of which pattern?

• Answer: It is an example of the “state pattern” described in Section 8.3.5. The direction is a state that changes when the bug turns, and it affects how the bug moves.

Static Variables and Methods - Variables

• A static variable belongs to the class, not to any object of the class.
• To assign bank account numbers sequentially
  • Have a single value of lastAssignedNumber that is a property of the class, not any object of the class.
• Declare it using the static reserved word

  public class BankAccount
  {
     private double balance;
     private int accountNumber;
     private static int lastAssignedNumber = 1000;
     
     public BankAccount()
     {
        lastAssignedNumber++;
        accountNumber = lastAssignedNumber;
     }
     . . .
  }

Static Variables and Methods

• Every BankAccount object has its own balance and accountNumber instance variables
• All objects share a single copy of the lastAssignedNumber variable
  • That variable is stored in a separate location, outside any BankAccount objects
• Static variables should always be declared as private,
  • This ensures that methods of other classes do not change their values
• static constants may be either private or public

  public class BankAccount
  {
     public static final double OVERDRAFT_FEE = 29.95;
     . . .
  }

• Methods from any class can refer to the constant as BankAccount.OVERDRAFT_FEE

Static Variables and Methods

A Static Variable and Instance Variables

Static Variables and Methods - Methods

• Sometimes a class defines methods that are not invoked on an object
  • Called a static method
• Example: sqrt method of Math class
  • if x is a number, then the call x.sqrt() is not legal
  • Math class provides a static method: invoked as Math.sqrt(x)
  • No object of the Math class is constructed.
  • The Math qualifier simply tells the compiler where to find the sqrt method.

Static Variables and Methods

• You can define your own static methods

  public class Financial
  {
     /**
     Computes a percentage of an amount.
     @param percentage the percentage to apply
     @param amount the amount to which the percentage is applied
     @return the requested percentage of the amount
     */
     public static double percentOf(double percentage, double amount)
     {
        return (percentage / 100) * amount;
     }
  }

• When calling such a method, supply the name of the class containing it

  double tax = Financial.percentOf(taxRate, total);

• The main method is always static.
  • When the program starts, there aren’t any objects.
  • Therefore, the first method of a program must be a static method.
• Programming Tip: Minimize the Use of Static Methods

Self Check 8.17

Name two static variables of the System class.

• Answer: System.in and System.out.

Self Check 8.18

Name a static constant of the Math class

• Answer: Math.PI

Self Check 8.19

The following method computes the average of an array of numbers:

public static double average(double[] values)

Why should it not be defined as an instance method?

• Answer: The method needs no data of any object. The only required input is the values argument.

Self Check 8.20

Harry tells you that he has found a great way to avoid those pesky objects: Put all code into a single class and declare all methods and variables static. Then main can call the other static methods, and all of them can access the static variables. Will Harry's plan work? Is it a good idea?

• Answer: Yes, it works. Static methods can access static variables of the same class. But it is a terrible idea. As your programming tasks get more complex, you will want to use objects and classes to organize your programs.

Packages

• Package: Set of related classes
• Important packages in the Java library:

  Package	Purpose	Sample Class
  java.lang	Language support	Math
  java.util	Utilities	Random
  java.io	Input and output	PrintStream
  java.awt	Abstract Windowing Toolkit	Color
  java.applet	Applets	Applet
  java.net	Networking	Socket
  java.sql	Database Access	ResultSet
  javax.swing	Swing user interface	JButton
  omg.w3c.dom	Document Object Model for XML documents	Document

Organizing Related Classes into Packages

 

In Java, related classes are grouped into packages.

Organizing Related Classes into Packages

• To put classes in a package, you must place a line

  package packageName;

  as the first instruction in the source file containing the classes
• Package name consists of one or more identifiers separated by periods
• To put the Financial class into a package named com.horstmann.bigjava, the Financial.java file must start as follows:

  package com.horstmann.bigjava;
  public class Financial
  {
     . . .
  }

• A special package: default package
  • Has no name
  • No package statement
  • If you did not include any package statement at the top of your source file
    • its classes are placed in the default package.

Importing Packages

• Can use a class without importing: refer to it by its full name (package name plus class name)

  java.util.Scanner in = new java.util.Scanner(System.in);

• Inconvenient
• import directive lets you refer to a class of a package by its class name, without the package prefix

  import java.util.Scanner;

• Now you can refer to the class as Scanner without the package prefix
• Can import all classes in a package:

  import java.util.*;

• Never need to import java.lang
• You don't need to import other classes in the same package

Package Names

• Use packages to avoid name clashes

  java.util.Timer

  vs.

  javax.swing.Timer

• Package names should be unique
• To get a package name: turn the domain name around:

  com.horstmann.bigjava

• Or write your email address backwards

  edu.sjsu.cs.walters

Syntax 8.1 Package Specification

Packages and Source Files

• The path of a class file must match its package name.
• The parts of the name between periods represent successively nested directories
• Base directory: holds your program's files
• Place the subdirectory inside the base directory
• If your homework assignment is in a directory /home/britney/hw8/problem1
  • Place the class files for the com.horstmann. bigjava package into the directory:
    • /home/britney/hw8/problem1/com/horstmann/bigjava (UNIX)
    • Or c:\Users\Britney\ hw8\problem1\com\horstmann\ bigjava (Windows)
• Base Directories and Subdirectories for Packages
  
  

Self Check 8.21

Which of the following are packages?
a. java
b. java.lang
c. java.util
d. java.lang.Math

• Answer: (a) No; (b) Yes; (c) Yes; (d) No

Self Check 8.22

Is a Java program without import statements limited to using the default and java.lang packages?

• Answer: No — you simply use fully qualified names for all other classes, such as java.util.Random and java.awt.Rectangle.

Self Check 8.23

Suppose your homework assignments are located in the directory /home/me/cs101 (c:\Users\me\cs101 on Windows). Your instructor tells you to place your homework into packages. In which directory do you place the class hw1.problem1.TicTacToeTester?

• Answer: /home/me/cs101/hw1/problem1 or, on Windows, c:\Users\me\cs101\hw1\problem1

Unit Test Frameworks

• Unit test frameworks simplify the task of writing classes that contain many test cases
• JUnit: http://junit.org
  • Built into some IDEs like BlueJ and Eclipse
• Philosophy: whenever you implement a class, also make a companion test class. Run all tests whenever you change your code

Unit Test Frameworks

• Customary that name of the test class ends in Test:

  import org.junit.Test; 
  import org.junit.Assert; 
  public class CashRegisterTest 
  { 
     @Test public void twoPurchases() 
     { 
        CashRegister register = new CashRegister(); 
        register.recordPurchase(0.75); 
        register.recordPurchase(1.50); 
        register.enterPayment(2, 0, 5, 0, 0); 
        double expected = 0.25; 
        Assert.assertEquals(expected, register.giveChange(), EPSILON); 
     } 
     // More test cases 
     . . . 
  }

• If all test cases pass, the JUnit tool shows a green bar:
  
  

Self Check 8.24

Provide a JUnit test class with one test case for the Earthquake class in Chapter 5.

• Answer: Here is one possible answer.

  public class EarthquakeTest
  {
     @Test public void testLevel4()
     {
        Earthquake quake = new Earthquake(4);
        Assert.assertEquals("Felt by many people, no destruction",
           quake.getDescription());
     }
  }

Self Check 8.25

What is the significance of the EPSILON parameter in the assertEquals method?

• Answer: It is a tolerance threshold for comparing floating-point numbers. We want the equality test to pass if there is a small roundoff error.