Chapter Goals

• To learn how to discover appropriate classes for a given problem
• To understand the concepts of cohesion and coupling
• To minimize the use of side effects
• To document the responsibilities of methods and their callers with preconditions and postconditions
• To understand static methods and variables
• To understand the scope rules for local variables and instance variables
• To learn about packages
• To learn about unit testing frameworks

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: only have a main method
• Don't turn actions into classes: 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).

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
• This class lacks cohesion:

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

Cohesion

• CashRegister, as described above, involves two concepts: cash register and coin
• Solution: Make two classes:

  public class Coin
  {
     public Coin(double aValue, String aName) { . . . }
     public double getValue() { . . . }
     . . .
  }
  
  public class CashRegister
  {
     public void enterPayment(int coinCount, Coin coinType) { . . . }
     . . .
  }

Coupling

• A class depends on another if it uses objects of that class
• CashRegister depends on Coin to determine the value of the payment
• Coin does not depend on CashRegister
• High Coupling = many class dependencies
• Minimize coupling to minimize the impact of interface changes
• To visualize relationships, draw class diagrams
• UML: Unified Modeling Language
  • Notation for object-oriented analysis and design

Dependency

High and Low Coupling Between Classes

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 should coupling be minimized between classes?

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

Immutable Classes

• Accessor: does not change the state of the implicit parameter:

  double balance = account.getBalance();

• Mutator: modifies the object on which it is invoked:

  account.deposit(1000);

• Immutable class: has no mutator methods (e.g., String):

  String name = "John Q. Public";
  String uppercased = name.toUpperCase(); // name is not changed

• It is safe to give out references to objects of immutable classes; no code can modify the object at an unexpected time

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.

Side Effects

• Side effect of a method: any externally observable data modification:

  harrysChecking.deposit(1000);

• Modifying explicit parameter can be surprising to programmers — avoid it if possible:

  public void addStudents(ArrayList<String> studentNames) 
  { 
     while (studentNames.size() > 0) 
     { 
        String name = studentNames.remove(0); // Not recommended 
        ...
     } 
  }

Side Effects

• This method has the expected side effect of modifying the implicit parameter and the explicit parameter other:

  public void transfer(double amount, BankAccount other)
  {
     balance = balance - amount;
     other.balance = other.balance + amount;
  }

Side Effects

• Another example of a side effect is output:

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

  Bad idea: message is in English, and relies on System.out
• Decouple input/output from the actual work of your classes
• Minimize side effects that go beyond modification of the implicit parameter

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 DataSet class of Chapter 6. Suppose we add a method

void read(Scanner in)
{
   while (in.hasNextDouble())
      add(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 parameter.

Common Error: Trying to Modify Primitive Type Parameters

• void transfer(double amount, double otherBalance)
  {
     balance = balance - amount;
     otherBalance = otherBalance + amount;
  }

• Won't work
• Scenario:

  double savingsBalance = 1000;
  harrysChecking.transfer(500, savingsBalance);
  System.out.println(savingsBalance);

• In Java, a method can never change parameters of primitive type

Common Error: Trying to Modify Primitive Type Parameters

double savingsBalance = 1000;
harrysChecking.transfer(500, savingsBalance);
System.out.println(savingsBalance);
...
void transfer(double amount, double otherBalance)
{
   balance = balance - amount;
   otherBalance = otherBalance + amount;
}

Common Error: Trying to Modify Primitive Type Parameters

double savingsBalance = 1000;
harrysChecking.transfer(500, savingsBalance);
System.out.println(savingsBalance);
...
void transfer(double amount, double otherBalance)
{
   balance = balance - amount;
   otherBalance = otherBalance + amount;
}

Common Error: Trying to Modify Primitive Type Parameters

double savingsBalance = 1000;
harrysChecking.transfer(500, savingsBalance);
System.out.println(savingsBalance);
...
void transfer(double amount, double otherBalance)
{
   balance = balance - amount;
   otherBalance = otherBalance + amount;
}

Common Error: Trying to Modify Primitive Type Parameters

double savingsBalance = 1000;
harrysChecking.transfer(500, savingsBalance);
System.out.println(savingsBalance);
...
void transfer(double amount, double otherBalance)
{
   balance = balance - amount;
   otherBalance = otherBalance + amount;
}

Animation 8.1 – A Method Cannot Modify a Numeric Parameter

Link to Flash animation

Call by Value and Call by Reference

• Call by value: Method parameters are copied into the parameter variables when a method starts
• Call by reference: Methods can modify parameters
• Java has call by value
• A method can change state of object reference parameters, but cannot replace an object reference with another:

  public class BankAccount
  {
     public void transfer(double amount, BankAccount otherAccount)
     {
        balance = balance - amount;
        double newBalance = otherAccount.balance + amount;
        otherAccount = new BankAccount(newBalance); // Won't work
     }
  }

Call by Value Example

harrysChecking.transfer(500, savingsAccount);

Preconditions

• Precondition: requirement that the caller of a method must meet
• Publish preconditions so the caller won't call methods with bad parameters:

  /**
     Deposits money into this account.
     @param amount the amount of money to deposit
     (Precondition: amount >= 0)
  */

• Typical use:
  1. To restrict the parameters of a method
  2. To require that a method is only called when the object is in an appropriate state

Preconditions

• If precondition is violated, method is not responsible for computing the correct result. It is free to do anything
• Method may throw exception if precondition violated — more in Chapter 11:

  if (amount < 0) throw new IllegalArgumentException();
  balance = balance + amount;

• Method doesn't have to test for precondition (Test may be costly):

  // if this makes the balance negative, it's the caller's fault
  balance = balance + amount;

Preconditions

• Method can do an assertion check:

  assert amount >= 0;
  balance = balance + amount;

• To enable assertion checking:

  java -enableassertions MainClass

• You can turn assertions off after you have tested your program, so that it runs at maximum speed
• Many beginning programmers silently return to the caller:

  if (amount < 0)
     return; // Not recommended; hard to debug
  balance = balance + amount;

Syntax 8.1 Assertion

Postconditions

• Postcondition: requirement that is true after a method has completed
• If method call is in accordance with preconditions, it must ensure that postconditions are valid
• Two kinds of postconditions:
  1. The return value is computed correctly
  2. The object is in a certain state after the method call is completed

• /**
     Deposits money into this account.
     (Postcondition: getBalance() >= 0)
     @param amount the amount of money to deposit
     (Precondition: amount >= 0)
  */

• Don't document trivial postconditions that repeat the @return clause
• Formulate pre- and postconditions only in terms of the interface of the class:

  amount <= getBalance() // this is the way to state a postcondition
  amount <= balance // wrong postcondition formulation

• Contract: If caller fulfills preconditions, method must fulfill postconditions

Self Check 8.10

Why might you want to add a precondition to a method that you provide for other programmers?

• Answer: Then you don't have to worry about checking for invalid values – it becomes the caller's responsibility.

Self Check 8.11

When you implement a method with a precondition and you notice that the caller did not fulfill the precondition, do you have to notify the caller?

• Answer: No — you can take any action that is convenient for you.

Static Methods

• Every method must be in a class
• A static method is not invoked on an object
• Why write a method that does not operate on an object?
• Common reason: encapsulate some computation that involves only numbers.
  • Numbers aren't objects, you can't invoke methods on them. E.g., x.sqrt() can never be legal in Java
• Example:

  public class Financial
  {
     public static double percentOf(double p, double a)
     {
        return (p / 100) * a;
     }
     // More financial methods can be added here.
  }

• Call with class name instead of object:

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

Static Methods

• If a method manipulates a class that you do not own, you cannot add it to that class
• A static method solves this problem:

  public class Geometry 
  { 
     public static double area(Rectangle rect) 
     { 
        return rect.getWidth() * rect.getHeight(); 
     } 
     // More geometry methods can be added here. 
  }

• main is static — there aren't any objects yet

Self Check 8.12

Suppose Java had no static methods. How would you use the Math.sqrt method for computing the square root of a number x?

• Answer:

  Math m = new Math();
  y = m.sqrt(x);

Self Check 8.13

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

public static double average(ArrayList<Double> values)

Why must it be a static method?

• Answer: You cannot add a method to the ArrayList class — it is a class in the standard Java library that you cannot modify.

Static Variables

• A static variable belongs to the class, not to any object of the class:

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

  If lastAssignedNumber was not static, each instance of BankAccount would have its own value of lastAssignedNumber

• public BankAccount()
  {
     lastAssignedNumber++; // Updates the static variable
     accountNumber = lastAssignedNumber; // Sets the instance variable
  }

A Static Variable and Instance Variables

Static Variables

• Three ways to initialize:
  1. Do nothing. Variable is initialized with 0 (for numbers), false (for boolean values), or null (for objects)
  2. Use an explicit initializer, such as

     public class BankAccount
     {
        private static int lastAssignedNumber = 1000;
        . . .
     }

  3. Use a static initialization block
• Static variables should always be declared as private
• Exception: Static constants, which may be either private or public:

  public class BankAccount
  {
     . . .
     public static final double OVERDRAFT_FEE = 5; // Refer to it as BankAccount.OVERDRAFT_FEE
  }

• Minimize the use of static variables (static final variables are ok)

Self Check 8.14

Name two static variables of the System class.

• Answer: System.in and System.out.

Self Check 8.15

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.

Scope of Local Variables

• Scope of variable: Region of program in which the variable can be accessed
• Scope of a local variable extends from its declaration to end of the block that encloses it
• Sometimes the same variable name is used in two methods:

  public class RectangleTester
  {
     public static double area(Rectangle rect)
     {
        double r = rect.getWidth() * rect.getHeight();
        return r;
     }
     public static void main(String[] args)
     {
        Rectangle r = new Rectangle(5, 10, 20, 30);
        double a = area(r);
        System.out.println(r);
     }
  }

• These variables are independent from each other; their scopes are disjoint

Scope of Local Variables

• Scope of a local variable cannot contain the definition of another variable with the same name:

  Rectangle r = new Rectangle(5, 10, 20, 30);
  if (x >= 0)
  {
     double r = Math.sqrt(x);
     // Error - can't declare another variable called r here
     . . .
  }

• However, can have local variables with identical names if scopes do not overlap:

  if (x >= 0)
  {
     double r = Math.sqrt(x);
     . . .
     } // Scope of r ends here
  else
  {
     Rectangle r = new Rectangle(5, 10, 20, 30);
     // OK - it is legal to declare another r here
     . . .
  }

Overlapping Scope

• A local variable can shadow a variable with the same name:
• Local scope wins over class scope:

  public class Coin
  {
     private String name;
     private double value; // Instance variable 
     . . .
     public double getExchangeValue(double exchangeRate)
     {
        double value; // Local variable with the same name
        . . .
        return value;
     }
  }

• Access shadowed variables by qualifying them with the this reference:

  value = this.value * exchangeRate;

Overlapping Scope

• Generally, shadowing an instance variable is poor code — error-prone, hard to read
• Exception: when implementing constructors or setter methods, it can be awkward to come up with different names for instance variables and parameters
• OK:

  public Coin(double value, String name) 
  { 
     this.value = value; 
     this.name = name; 
  }

Self Check 8.16

Consider the following program that uses two variables named r. Is this legal?

public class RectangleTester 
{ 
   public static double area(Rectangle rect) 
   {  
      double r = rect.getWidth() * rect.getHeight(); 
      return r; 
   } 
   public static void main(String[] args) 
   { 
      Rectangle r = new Rectangle(5, 10, 20, 30); 
      double a = area(r); 
      System.out.println(r); 
   } 
}

• Answer: Yes. The scopes are disjoint.

Self Check 8.17

What is the scope of the balance variable of the BankAccount class?

• Answer: It starts at the beginning of the class and ends at the end of the class.

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

• 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
• For example, to put the Financial class introduced into a package named com.horstmann.bigjava, the Financial.java file must start as follows:

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

• Default package has no name, no package statement

Syntax 8.2 Package Specification

Importing Packages

• Can always use class without importing:

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

• Tedious to use fully qualified name
• Import lets you use shorter class name:

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

• 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 unambiguous
• Recommendation: start with reversed domain name:

  com.horstmann.bigjava

• edu.sjsu.cs.walters for Britney Walters' classes (walters@cs.sjsu.edu)

Packages and Source Files

• Base directory: holds your program's Files
• Path name, relative to base directory, must match package name:

  com/horstmann/bigjava/Financial.java

  

Self Check 8.18

Which of the following are packages?

1. java
2. java.lang
3. java.util
4. java.lang.Math

• Answer:
  1. No
  2. Yes
  3. Yes
  4. No

Self Check 8.19

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 8.20

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

The Explosive Growth of Personal Computers

Unit Testing 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 Testing 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 8.21

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

• Answer: Here is one possible answer, using the JUnit 4 style.

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

Self Check 8.22

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.