Chapter 5:

Chapter 5
More Sophisticated Behavior

Overview

Chapter 4 examined some of the ways objects may be grouped together into collections. Here we will look at several types of collections, random behavior, and writing program documentation.

5.1 Documentation for library classes

The Java class library is extensive. Documentation of the classes in HTML is on the text CD for installation onto your computer or is available by searching the web.

5.2 The TechSupport system

User enters question at keyboard (we will see how to do user input).
TechSupport system responds with useful information. Version 1 has only 1 possible response.

Exercise 1 - TechSupport class
In BlueJ

Open project chapter05\tech-support1.
Create a SupportSystem object.

Call the start() method.
Ask questions of SupportSystem.
Type "Bye" to finish. Did it work?
Type " bye" to finish. Did that work?

5.2.2 Reading the code

  public class SupportSystem
{

   private InputReader reader;
   private Responder responder;

   public SupportSystem()
{
     reader = new InputReader();
     responder = new Responder();
   }

    public void start()
   {
      boolean finished = false;
      printWelcome();

      while(!finished) {
          String input = reader.getInput();
          if(input.startsWith("bye")) {
               finished = true;
          }
          else {
               String response = responder.generateResponse();
               System.out.println(response);
          }
      }
      printGoodbye();
}

    private void printWelcome()
   {
          System.out.println("Welcome to the DodgySoft Technical Support System.");
          System.out.println();
          System.out.println("Please tell us about your problem.");
          System.out.println("We will assist you with any problem you might have.");
          System.out.println("Please type 'bye' to exit our system.");
   }

    private void printGoodbye()
   {
          System.out.println("Nice talking to you. Bye...");
   }

}

  public class Responder
{

   public Responder()
{   }

   public String generateResponse()
   {
        return "That sounds interesting. Tell me more...";
   }

}

String comparisions

"Edward".startsWith("Ed") - True because "Edward" starts with "Ed".
"Edward".startsWith("rd") - False because "Edward" does not starts with "rd".
input.startsWith("bye") - True when "bye" is at the beginning of input String, otherwise false.
"Edward".equals("Edward") - True.
"Edward".equals("edward") - False.

Exercise 2 - Reading the code
In your head

What will the following Java statements do? Will printGoodbye() be executed?

    public void start()
   {
      boolean finished = false;

     while(!finished)
     { }
     printGoodbye();
}

What will the following Java statements do? When will printGoodbye() be executed?

    public void start()
   {
      boolean finished = false;

      while(!finished)
     {
          String input = reader.getInput();
          if(input.startsWith("bye"))
         {
               finished = true;
          }
      }
      printGoodbye();
}

In BlueJ

Type bye in the Terminal Window to stop execution from Exercise 1.

The start() method will not finish until bye is read.
If BlueJ seems to have gone on vacation, close BlueJ and Open the project again.

Open class SupportSystem.
Place a breakpoint at the first executable statement of method start().

boolean finished = false;

Call the start() method.

Use Step to trace the execution of the method.

What is your response when the following is executed?

String input = reader.getInput();
Type something into Terminal Window.
What is the value of input?

Use Step Into to execute completely:

String response = responder.generateResponse();
What is the value of response?

5.3 Reading class documents

Java standard library documentation - Details all classes in Java library.

Exercise 3 - Java Documentation
In BlueJ

Click Help in upper-right corner.
Click on Java Class Libraries.

A browser window should open to the Sun Java website.

Find help on the String class.
Find help on the startsWith method.

5.3.2 Using library-class methods

Immutable objects - An object, that once created, its state cannot be changed.

Improvements to TechSupport

trim - Allow spaces around "bye" to stop iteration.
- " a b c ".trim is "a b c"
- (input.trim()).startsWith("bye") trims the blanks and tests that result starts with "bye"
toLowerCase - Allow "Bye", "BYE", etc. to stop iteration.
- "AbC".toLowerCase is "abc"
- " AbC ".toLowerCase().trim() is "abc"
- input.trim().toLowerCase().startsWith("bye") trims the blanks, converts to lowercase and tests that result starts with "bye".

Exercise 4 - trim and toLowerCase
In BlueJ

Change TechSupport method start() to finish when leading, trailing blanks and uppercase characters entered for bye.
Test.
Can the improvement be written another way?

5.3.3 Checking string equality

Testing for "bye" at the start of input, even with our improvements, behaves incorrectly on input such as "Byelection". We will finish execution only on input of "bye" by testing for equality.

equals - Returns true when two strings are equal.
- "AbC".equals("AbC") is true.
- "AbC".equals("ABC") is false.
- input.trim().toLowerCase().equals("bye") trims the blanks, converts to lowercase and tests that result is equal "bye".
== - Tests for equality of primitive types such as int, double, boolean, char and whether the same object is referenced.
- 4 == 3 is false.
- 4 == 4 is true.
- private TicketMachine t1;
  private TicketMachine t2;
  t1 = new TicketMachine(50);
  t2 = t1;
  
  t1 == t2 is true because both reference the same TechSupport object.

Exercise 5 - equals and ==
In your head

'3' == 3?
private TicketMachine t1;
private TicketMachine t2;
t1 = new TicketMachine(50);
t2 = new TicketMachine(50);

t2 == t1?

In BlueJ

Change TechSupport method start() to finish when input is only bye using the equals method.
Test.

5.4 Adding random behavior

Random behavior is observable in games of chance and real-life. Randomness in computer models is produced by generating pseudorandom numbers. True random numbers never recur in the same sequence but pseudorandom numbers do repeat eventually.

5.4.1 The Random class

The following simulates throwing two dies in a dice game.

import java.util.Random; - Uses the Java library class Random.
generator = new Random(); - The object of class Random is named generator.
generator.nextInt(6) - Generates the next integer between 0 and 5 inclusive of the pseudorandom sequence.

import java.util.Random;

public class Dice
{

private Random generator;

   public Dice()
{
       generator = new Random();
}

   public void twoDie()
{
       System.out.println( oneDie() + " " + oneDie() );
   }

   public int oneDie()
{
        return generator.nextInt(6)+1;
}

}

Exercise 6 - Random
In BlueJ

Create a new project named Exercise6.
Create a new class named Dice.

Open the Dice class.
Copy and paste the above class into the Dice class source code.

Compile and test by calling the methods:

oneDie()
twoDie()

Do the results appear random?
What does the following do?

generator.nextInt(6)+1

Exercise 6.5 - Random
In BlueJ

Add the method throw() that throws the die 100 times and prints the number of 1, 2, 3, etc.

Create an array of 6 integers named count.
Initialize the array variables to 0.
For each throw of the die, use the die number as the index of the array variable to increment.

int n = generator.nextInt(6);
count[ n ] = count[ n ] + 1;

Print all 6 array variables.

5.4. Generating random responses

Exercise 7 - Random responses
In BlueJ

Change oneDie() method to return Strings: one, two, three, four, five, or six.

Hint: int n = generator.nextInt( 6 )+1;
        if ( n == 1 ) return "one"
        else ...

Change oneDie() method to use an ArrayList rather than if-statement.

Hint: ArrayList words = new ArrayList();
        words.add("one");                       // "one" is at index 0
                 :
        int n = generator.nextInt(6);        // n is 0-5

        return words.get( n );                  // Return object at index n

Responder class with random responses, see project tech-support2.

  public class Responder
{

private Random randomGenerator;
private ArrayList responses;

public Responder()
{
     randomGenerator = new Random();
     responses = new ArrayList();
     fillResponses();
}

public String generateResponse()
{
    int index = randomGenerator.nextInt( responses.size() );
    return (String) responses.get(index);
}

private void fillResponses()
{
    responses.add("That sounds odd. Could you describe that problem in more detail?");
    responses.add("No other customer has ever complained about this before. \n" +
                           "What is your system configuration?");
    responses.add("That sounds interesting. Tell me more...");
    responses.add("I need a bit more information on that.");
    responses.add("Have you checked that you do not have a dll conflict?");
    responses.add("That is explained in the manual. Have you read the manual?");
    responses.add("Your description is a bit wishy-washy. Have you got an expert\n" +
                           "there with you who could describe this more precisely?");
    responses.add("That's not a bug, it's a feature!");
    responses.add("Could you elaborate on that?");
}

}

5.5 Packages and imports

Packages contain libraries of Java classes not automatically part of the Java language.
import class name makes the package of classes available in a program. For example:
- import java.util.ArrayList; Makes just the ArrayList class available.
- import java.util.*; Makes all classes in the package available.

5.6 Using maps for associations

Map - A collection that associates a fixed pair consisting of a key and a related value. The key maps to the value.

Examples:

Dictionary associates the word as a key and the definition as the value.
Phone book associates the name as a key and the phone number as the value.

ArrayList is not a map since only the object is stored at an index. Removing an object can change the index of other objects, hence no fixed pairing is defined.

HashMap uses a key rather than an index. The key is mapped to an object. Removing an object does not change the key-to-object mapping.

put - The key "one" is defined to map to "I" using put method by:

     HashMap englishToroman = new HashMap();

     englishToroman.put("one", "I");

get - The object "one" is mapped to "I" which is retrieved using get method by:

     englishToroman.get("one");

The following translates English words for Strings "one" to "six" to Roman "I" to "VI".

import java.util.HashMap;

public class EnglishToRoman
{

HashMap englishToroman;

public EnglishToRoman()
{
     englishToroman = new HashMap();

     englishToroman.put("one", "I");
     englishToroman.put("two", "II");
     englishToroman.put("three", "III");
     englishToroman.put("four", "IV");
     englishToroman.put("five", "V");
     englishToroman.put("six", "VI");
}

public String translate(String english)
{
      return (String) englishToroman.get(english);
}

}

Exercise 8 - Mappings
In your head

From the object diagram, are the HashMap keys in any obvious order?
What is the result of the following?

englishToroman.get("five");
englishToroman.get("four");
englishToroman.get("english");

Is (String) really necessary for:

return (String) englishToroman.get(english);

In BlueJ

Create a new project named Exercise8.
Create a new class named Mappings.

Open the Mappings class.
Copy and paste the above EnglishToRoman class into the source code.
Modify to map SSN (social security number) as Strings to peoples names.

Compile:

Test by calling the methods to map SSN to a name.
Test by calling the methods to map name to a SSN.
What happened?

Add the same SSN twice to the mapping but with different names.

Test by calling the methods to map that SSN to a name.
What happened?

Use the Help in upper-right corner. A browser window should open to the Sun Java website.

Find help on the HashMap class.
Find out how to check whether a given key is in a map.

5.6.3 Using a map for the TechSupport system

Responder class with HashMap, responds to a given word in the map by returning an associated string.

  public class Responder
{

private HashMap responseMap;

public Responder()
{
     randomGenerator = new Random();
     responsesMap = new HashMap();
     fillResponses();
}

public String generateResponse(String word)
{
    return (String) responsesMap.get(word);
}

private void fillResponses()
{
     responseMap.put("crash",
        "Well, it never crashes on our system. It must have something\n" +
        "to do with your system. Tell me more about your configuration.");
    responseMap.put("crashes",
        "Well, it never crashes on our system. It must have something\n" +
        "to do with your system. Tell me more about your configuration.");
    responseMap.put("slow",
        "I think this has to do with your hardware. Upgrading your processor\n" +
        "should solve all performance problems. Have you got a problem with\n" +
        "our software?");
}

}

TechSupport start() method modified to generate a response to specific words as input.

    public void start()
   {
      boolean finished = false;
      printWelcome();

      while(!finished) {
          String input = reader.getInput();
          if(input.startsWith("bye")) {
               finished = true;
          }
          else {
               String response = responder.generateResponse(input);
               System.out.println(response);
          }
      }
      printGoodbye();
}

Exercise 9 - Maps
For the Terminal input of:

"crashes"
"crashs"

What is input:

String input = reader.getInput();

What is:

input.startsWith("bye")
responder.generateResponse(input);

5.10 Writing class documentation

Documentation - Should provide enough details that other programmers do not need to examine the source code.

5.10.1 Using javadoc in BlueJ

javadoc -
listNotes - We need a NoteBook method that prints all notes along with their index numbers to the terminal window.

Exercise 10 - Documentation

Open the project Exercise8 and the Mappings class.
On the BlueJ menu, select Tools | Project Documentation | Regenerate
Note the documentation generated.
Add the following comments along with your name; documentation must be placed after the import:
import java.util.HashMap;

/**
* This class implements an English number to Roman numerical translation.
*
* @version 1.0
* @author Your Name
*/

public class EnglishToRoman

Regenerate the documentation.

5.11 Public versus private

Access modifiers - Define the visibility of a field, constructor or method.
Public - Defines the element accessible inside the same class or outside the class.
Private - Defines the element accessible only inside the same class

Exercise 11 - Public/Private
In BlueJ

Open the TechSupport class.

Note private and public methods.

Create a TechSupport object.

Which of the methods can be called?

5.11.1 Information hiding

Information hiding - Principle that internal class implementation details should be hidden from outside the class.

The private keyword hides information making it only accessible inside a class. This prevents accidental or purposeful access to class fields, methods and constructors.

Need to know - One should not need to know class implementation details to use a class.
Not allowed to know - Does not allow access from outside the class.

Exercise 12 - Public/Private
In your head

In the TechSupport class.

Do you need to know that a Responder class object is one of the fields?
When calling TechSupport methods, are you allowed to know about fields or certain methods?
When calling TechSupport methods, are you allowed to know about constructors?

5.11.2 Private methods and public fields

private - methods that are private can only be accessed by other methods of the same class. This hides the methods from external calls. Methods not intended for external use should be private.
public - Fields that are public can be accessed and changed externally without using mutator methods. The values of the fields cannot be controlled, hence the state of the object cannot be controlled.

5.12 Learning about classes from their interfaces

interface - Describes what a class does and how it is used without examining the implementation.

We'll examine the class interfaces in the ball project to understand how to use the classes without understanding how they are implemented.

Exercise 13 - Class interface
In BlueJ

Open the chapter05\balls project.
Create a BallDemo object.

Call drawdemo method.
Call bounce method.

In BlueJ menu:

Open Tools
Project Documentation
Regenerate

Read Canvas documentation and from it:

Draw a circle.
Draw a rectangle.

5.13 Class variables and constants

class or static variable - A field having one copy used by all objects. Each object has own version of regular (non-static) field.
instance variable - A field having one copy per object.

5.13.1 The static keyword

public class Test
{
    private static int count = 0;
    private           int i = 0;

    public Test()
    {
        count = count + 1;
        i = i + 1;
    }

    public int getCount()
    {
        return count;
    }
}
After:

new Test( );
new Test( );

class variable - A single count variable is accessible by all Test objects.
instance variable - Each object has its own variable i.

5.13.2 Constants

final - Defines a constant, a value that cannot change.

Exercise 14 - Constants
In BlueJ

Open a new project named static.
Create a new class named Test.
Copy and paste the above Test class definition.
Create three Test objects.

Inspect two objects.
Show static fields.
What are the values of i and count?

Change:
private static int count = 0;

to

private static final int count = 0;

Why the error?

Chapter 5More Sophisticated Behavior