Chapter 3:

Chapter 3
Object orientation

Overview

Chapter 2 presented a first look at Java source code and details on fields, constructors, parameters, methods, assignment and conditional statements. Here we will examine fundamental problem solving approaches of modularization and abstraction.

3.1 The clock example

A 24-hour clock displays time in hours:minutes format from 00:00 (midnight) to 23:59 (one minute before midnight).

Exercise 1 - Clock example
In BlueJ

Open project chapter03\clock-display.
Create a NumberDisplay object with rollOverLimit = 3.

Inspect the object but don't close.
Call increment() method at least 6 times.
What behavior does a NumberDisplay object exhibit.

Create a ClockDisplay object with hour=23 and minute = 58.

Inspect the object but don't close.
Call timeTick() method at least 3 times.
Notice the displayString field. What behavior does a ClockDisplay object exhibit?

3.2 Abstraction and modulation

Abstraction - Ignores details of object implementation, concerned only with object use.

Abstraction is the standard means humans use to handle complexity, we learn to ignore it. Consider the chair on which you are setting. A chair implementation is the modules:

legs - 4 vertical metal bars about 20 inches long, 1 1/2 inch square, painted black
seat - 2 horizontal metal bars 20 inches in length and 2 horizontal metal bars 18 inches in length
padding - clothe covering foam rubber, about 24x18 inches, originally light gray color
etc.

We use the chair as an abstraction of these parts, ignoring the implementation details to set on it. The details of the chair are important for understanding how chairs are implemented but not needed to use a chair.

Modularization - Dividing an object into well-defined components.

Modularization is somewhat the complement of abstraction. It is the standard means humans use to divide complexity into smaller, simpler components. Consider again the chair on which you are setting.

To implement a chair we first divide the chair abstraction into smaller modules, for example the seat. We may then divide the seat into smaller modules, the padding and seat. We can divide the padding into foam rubber and clothe modules. Eventually we divide modules to its simplest level. We can then focus on implementing each module separate of the others; the foam rubber module can be made separate from the clothe module.

Modularization divides the whole into related detail groups; abstraction combines the details into the whole.

3.3 Abstraction in software

You have already seen that seemingly simple software has considerable complexity. Designing correct software is hard primarily due to the large number of details that must each be correct. Abstraction allows us to think about details as a group.

Recall the TicketMachine method printTicket, the details are listed below. We abstract or ignore the details and think only that printTicket method prints a ticket not about each println statement.

public void printTicket()
{
       System.out.println("##################");
       System.out.println("# The BlueJ Line");
       System.out.println("# Ticket");
       System.out.println("# " + price + " cents.");
       System.out.println("##################");
       System.out.println();

       total += balance;
       balance = 0;
}

3.4 Modularization in the clock example

Single clock display - 4 digits for time
Modular display - 2 modules of 2 digits where hour digits range from 00 to 23, minute from 00 to 59.
Modularization divides the single time display to two modules, one for hours and one for minutes. Display of hours is now separate from the display of minutes.
Abstract display - 2 digits where digits range from 00 to an upper limit. Abstraction ignores the detail of the number range difference between hour (00-23) and minute (00-59) module using a variable maximum number range. The hour maximum is 23 and the minute maximum is 59 is the only difference.

From this brief analysis, we see the clock display has 2 number displays for hours and minutes.

3.5 Implementing the clock display

The clock display has 2 number displays for hours and minutes. A number display needs two fields for the state of a NumberDisplay object:

value - the value of the number
limit - the maximum of the value

public class NumberDisplay
{

private int limit;
private int value;

Constructor and methods omitted

}

The clock display has 2 number displays for hours and minutes. A clock display needs two fields for the state of a ClockDisplay object:

hours - the NumberDisplay for hours
minutes - the NumberDisplay for minutes

public class ClockDisplay
{

private NumberDisplay hours;
private NumberDisplay minutes;

Constructor and methods omitted

}

3.6 Class diagrams versus object diagrams

Class diagram - Shows the classes and static relationships between them.

Static view - The class diagram at right shows the class definition relationships. ClockDisplay depends on the NumberDisplay.

Object diagram - Shows the objects and dynamic relationships between them.

Dynamic view - The object diagram a right shows when the program runs, the dynamic view.

A ClockDisplay object and two NumberDisplay objects have been created.

The arrow implies the ClockDisplay hours object references the 11 NumberDisplay object.

The ClockDisplay minutes object references the 03 NumberDisplay object.

Exercise 2 - Class diagrams and object diagrams

Sketch the static Class diagram for the Exercise2A class below.

Show any dependency arrows.

Sketch the dynamic Object diagram for the person object below after execution of:

Exercise2A person = new Exercise2A("John", "Crab");

Show reference arrows.

Sketch the Class diagram for the Exercise2B class below.

Show any dependency arrows.

Sketch the Object diagram for the couple object below after execution of:

Exercise2B couple = new Exercise2B();

Show reference arrows.

public class Exercise2A
{

private String name;
private String favoriteFood;

public Exercise2A(String newName, String food) {
      favoriteFood = food;
      name = newName;
}

}

public class Exercise2B
{

private Exercise2A husband;
private Exercise2A wife;

public Exercise2B( ) {
      husband = new Exercise2A("John", "Crab");
      wife = new Exercise2A("Sally", "tripe");
}

}

3.7 Primitive types and object types

Primitive type - Non-object types.
- int - Integer (whole number). Examples: 34, -25, 9999999, 0.
- boolean - Either true or false.
- char - One character enclosed in single quotes. Examples: 'a', '4'.
- double - Real or floating pointing numbers. Examples: 123.0, 45.67

3.8 The ClockDisplay source code

3.8.1 Class NumberDisplay

Logic operators - Operate on boolean (true or false) to produce a boolean result.

Truth tables - Define boolean operations.

! - not. Examples: !true is false, !false is true.

X !X

true false

false true

&& - and. Result true when both operands are true.

A B A && B

false false false

false true false

true false false

true true true

|| - or. Result true when either operand is true

A B A || B

false false false

false true true

true false true

true true true

Exercise 3 - Logical operators

What is the result of the following?

true && false
false || true
true && (false || true)
!true || false
(4 < 5)
(4 < 5) && (6 >= 5)
(4 > 5) || (6 <= 5)

NumberDisplay Source code

public class NumberDisplay
{

private int limit;
private int value;

public NumberDisplay(int rollOverLimit)
{
     limit = rollOverLimit;
     value = 0;
}

public int getValue()
{
     return value;
}

public String getDisplayValue()
{
     if(value < 10)
          return "0" + value;
     else
          return "" + value;
}

public void setValue(int replacementValue)
{
     if((replacementValue >= 0) && (replacementValue < limit))
          value = replacementValue;
}

public void increment()
{
     value = (value + 1) % limit;
}

}

Exercise 3 - Class NumberDisplay Analysis

Give the Java code to create a new NumberDisplay object named miles with a rollOverLiimit of 5.

What is the state of miles?

Give the Java code to set the object miles to 5 using method setValue.

What is the new state of miles?
What would happen if the following were used in setValue:
if((replacementValue >= 0) || (replacementValue < limit))

Give the Java code to set the object miles to 4 using method setValue.

What is the new state of miles?

Give the Java code to increment the object miles one time.

What is the new state of miles?

3.8.2 String Concatenation

Concatenation operator - + with one String operand results in a concatenated String.

Examples:
- "abc" + "efg" is "abcefg"
- 1 + "a" is "1a"
- 1 + "2" is "12"
- 1 + "" is "1"

public String getDisplayValue()
{
     if(value < 10)
          return "0" + value;
     else
          return "" + value;
}

Exercise 4 - getDisplayValue() Analysis

For value = 5, what is: "0" + value
For value = 5, what is: "" + value
Assume NumberDisplay object miles has state:

limit = 10
value = 5

What is: miles.getDisplayValue();

Assume NumberDisplay object miles has state:

limit = 120
value = 105

What is: miles.getDisplayValue();

3.8.3 The modulo operator

Modulo operator - % is the remainder of division.

Division operator - / is the quotient of division.

Examples:
- 25 % 4 is 1
- 25 / 4 is 6
- 17 % 3 is 2
- ( 16 + 1 ) % 3 is 2
- 17 / 3 is 5
- (16 + 1 ) / 3 is 5

public void increment()
{
value = (value + 1) % limit;
}

Exercise 5 - increment() Analysis

What are all the possible results of:

n % 4

What are all the possible results of:

n % m

Assume NumberDisplay object miles has state:

limit = 7
value = 5

What is: miles.increment();

Assume NumberDisplay object miles has state:

limit = 7
value = 6

What is: miles.increment();

Rewrite value = (value + 1) % limit using an if-statement.

3.8.4 Class ClockDisplay

public class ClockDisplay
{

private NumberDisplay hours;
private NumberDisplay minutes;
private String               displayString;

public ClockDisplay()
{
     hours = new NumberDisplay(24);
     minutes = new NumberDisplay(60);
     updateDisplay();
}

public ClockDisplay(int hour, int minute)
{
     hours = new NumberDisplay(24);
     minutes = new NumberDisplay(60);
     setTime(hour, minute);
}

public void timeTick()
{
     minutes.increment();
     if(minutes.getValue() == 0) {
          hours.increment();
     }
     updateDisplay();
}

public void setTime(int hour, int minute)
{
     hours.setValue(hour);
     minutes.setValue(minute);
     updateDisplay();
}

public String getTime()
{
     return displayString;
}

private void updateDisplay()
{
     displayString = hours.getDisplayValue() + ":" + minutes.getDisplayValue();
}

}

Exercise 6 - ClockDisplay Analysis
In BlueJ

Open project chapter03\clock-display.
Create a ClockDisplay object that corresponds to the object diagram at right.

Inspect the object but don't close.
Call each method.

Create a ClockDisplay object with time at 23:58.
Test that can be used to test that the clock display rolls over hours and minutes correctly.

3.9 Objects creating objects

public class NumberDisplay
{

private int limit;
private int value;

public NumberDisplay( int rollOverLimit )
{
     limit = rollOverLimit;
     value = 0;
}

}

public class ClockDisplay
{

private NumberDisplay hours;
private NumberDisplay minutes;

public ClockDisplay()
{
     hours = new NumberDisplay( 24 );
     minutes = new NumberDisplay( 60 );
     updateDisplay();
}

}



Exercise 7 - Objects creating objects
In your head

Match each of the Java code below with one of the objects created at right.

hours = new NumberDisplay( 24 );
new ClockDisplay( );
minutes = new NumberDisplay( 60 );

Follow the Java code above as it executes:

public ClockDisplay()
{
           hours = new NumberDisplay( 24 );
           minutes = new NumberDisplay( 60 );

3.10 Multiple constructors

Overloading - When more than one constructor (or method) have the same name. The signature and call are matched to determine which is called.

public class ClockDisplay
{

private NumberDisplay hours;
private NumberDisplay minutes;
private String               displayString;

public ClockDisplay()
{
     hours = new NumberDisplay(24);
     minutes = new NumberDisplay(60);
     updateDisplay();
}

public ClockDisplay(int hour, int minute)
{
     hours = new NumberDisplay(24);
     minutes = new NumberDisplay(60);
     setTime(hour, minute);
}

public void setTime(int hour, int minute)
{
     hours.setValue(hour);
     minutes.setValue(minute);
     updateDisplay();
}

}

public class NumberDisplay
{

private int limit;
private int value;

public NumberDisplay(int rollOverLimit)
{
     limit = rollOverLimit;
     value = 0;
}

public void setValue(int replacementValue)
{
     if((replacementValue >= 0) &&
        (replacementValue < limit))
          value = replacementValue;
}

}

Exercise 8 - Multiple constructors
Match the objects created below with the corresponding Java code, the signature must match the call:

ClockDisplay myDisplay = ClockDisplay();
ClockDisplay myDisplay = ClockDisplay(15, 23 );
ClockDisplay myDisplay = ClockDisplay(24, 60);

3.11 Method calls

3.11.1 Internal method calls

Internal method call - Calling a method defined within the class from a constructor or method defined within the same class. The current object is used.

Example:
- ClockDisplay
  - Definition - public void setTime(int hour, int minute)
  - Internal Call - setTime(hour, minute);

3.11.2 External method calls

External method call - Calling a method defined within a different class. The dotted object is used.

Example - ClockDisplay setTime method calls the setValue method of hours and minutes, two NumberDisplay objects.
- ClockDisplay

public void setTime(int hour, int minute)
{
     hours.setValue(hour);
     minutes.setValue(minute);
     updateDisplay();
}

NumberDisplay

public void setValue(int replacementValue)
{
     if((replacementValue >= 0) && (replacementValue < limit))
          value = replacementValue;
}

public class ClockDisplay
{

private NumberDisplay hours;
private NumberDisplay minutes;
private String               displayString;

public ClockDisplay()
{
     hours = new NumberDisplay(24);
     minutes = new NumberDisplay(60);
     updateDisplay();
}

public ClockDisplay(int hour, int minute)
{
     hours = new NumberDisplay(24);
     minutes = new NumberDisplay(60);
     setTime(hour, minute);
}

public void timeTick()
{
     minutes.increment();
     if(minutes.getValue() == 0) {
          hours.increment();
     }
     updateDisplay();
}

public void setTime(int hour, int minute)
{
     hours.setValue(hour);
     minutes.setValue(minute);
     updateDisplay();
}

private void updateDisplay()
{
     displayString = hours.getDisplayValue() + ":" + minutes.getDisplayValue();
}

}

Exercise 8a - Internal and External method calls

Categorize the method calls of ClockDisplay as internal or external.

Example of an external method call:
hours.setValue(hour);

Example of an internal method call:
updateDisplay( );

3.11.3 Summary of the clock display

Abstraction:

ClockDisplay - Focus on display of NumberDisplay objects hours (00-23) and minutes (00-59).
NumberDisplay - Focus on representing, incrementing, and getting the value of a modulus counter.

Exercise 9
In BlueJ

Change ClockDisplay (not NumberDisplay) to display a 12 hour clock rather than a 24 hour clock.

Modify the ClockDisplay constructors to a 12 hour NumberDisplay object.
Modify updateDisplay method to display hour 12 when the object hours value is 0, hour 13 is 1, etc.
Hints: Use hours.getValue() to get the hours integer value. Use modulus operator (i.e. %).

Test appropriately to verify that values of object hours are displayed as 1-12 only.

3.11.4 Using a debugger

Debugger - Tool for examining a program state (variables and field values) and statements as it executes.

3.11.4.1 Setting breakpoints

breakpoint - A flag that will stop program execution during debugging.

Exercise 9.1 - Breakpoints
In BlueJ

Create a ClockDisplay instance with hour=13 and minute=58.
Set a breakpoint in ClockDisplay method timeTick.

Open the editor for the ClockDisplay class.
Click on the line to breakpoint, the statement: minutes.increment();
In the BlueJ menu, click Tools | Set/Clear Breakpoint.

Call ClockDisplay method timeTick. The debugger should highlight the first breakpointed statement encountered in the program.
What are the names of the instance variables (or fields) of a ClockDisplay object. See the Debugger window below.
Double-click the minutes field to inspect. What is the value?
Close the inspection window.

3.11.4.2 Single stepping

Single stepping - Executing a single statement.

Exercise 9.2 - Single stepping
In BlueJ

The minutes.increment(); should be highlighted in the Debugger window.
Click Step in Debugger to execute the statement.
What changed in the Debugger?
Double-click the minutes field to inspect. What changed?
Click Step in Debugger to execute the if-statement.

Which statement was executed?
Why?

Click Continue to complete the method timeTick.

Exercise 9.3 - Single stepping
In BlueJ

Call ClockDisplay method timeTick again. The debugger should highlight the first breakpointed statement encountered in the program.
What is the statement at the breakpoint again?
Double-click the minutes field to inspect. What is the value?
Close the inspection window.
Click Step in Debugger to execute the statement.
Double-click the minutes field to inspect. What is the value?
Close the inspection window.
Click Step in Debugger to execute the if-statement.

Which statement was executed?
Why?

Click Continue to complete the method timeTick.

3.11.4.3 Stepping into methods

executes one statement but will not follow the execution of a method.

executes one statement but follows the execution of a method.

Exercise 9.4 - Single stepping into methods
In BlueJ

Set a breakpoint at the first statement of method setTime.
Call method setTime with hour=23 and minute=59. The debugger should highlight the first breakpointed statement encountered in the program.
What local variables (or parameters) are listed?
Click Step to execute until the statement below is highlighted:

updateDisplay();

Click Step Into to execute into the updateDisplay method.
Click Step to follow the execution.
Does the updateDisplay method return to the point where it was called in timeTick?

3.12 Another example of object interaction

Debugger - Tool for examining a program state (variables and field values) and statements as it executes.

3.12.1 The mail system example

Exercise 10

What are the static dependencies shown by the class diagram?

In BlueJ

Experiment with the mail-system project.

Open the mail-system project.
Create a MailServer object.
Create two MailClient objects:

name the two objects sophie and juan.
supply each the MailServer object name just created
give a user name that matches the object's name (i.e. "Sophie" and "Juan").

Use the MailClient objects to send and receive messages using:

sendMessage
getNextMailItem or printNextMailItem

Sketch the object diagram from part 1 similar to the ClockDisplay.

3.12.2 The this keyword

this - The name of the object used in an internal method. Needed when a name is overloaded (same name used simultaneously in different ways).

The following MailTerm constructors behave identically. The first constructor requires this because the field and parameter names are the same in the scope of the MailItem constructor.

public class MailItem
{

private String from;
private String to;
private String message;

public MailItem(String from, String to, String message)
{
     this.from = from;
     this.to = to;
     this.message = message;
}

public MailItem(String newFrom, String newTo, String newMessage)
{
     from = newFrom;
     to = newTo;
     message = newMessage;
}

}

3.13 Using a debugger

3.13.1 Setting breakpoints

breakpoint - A flag that will stop program execution during debugging.

Exercise 11 - Breakpoints
In BlueJ

Use sophie's sendMessage to send a message from "Sophie" to "Juan".
Don't read the message yet.
Set a breakpoint in MailClient method printNextMailItem.

Open the editor for the MailClient class and set a breakpoint at the first line of method printNextMailItem.
Click on the line to breakpoint.
In the BlueJ menu, click Tools | Set/Clear Breakpoint.

Call MailClient method printNextMailItem on juan. The debugger should highlight the first breakpointed statement encountered in the program.
What are the names of the instance variables (or fields) of a MailClient object. See the Debugger window below.

3.13.2 Single stepping

Single stepping - Executing a single statement.

Exercise 11 - Single stepping
In BlueJ

Note there are no local variables listed in the Debugger window.
What does the statement do at the breakpoint?

MailItem item = server.getNextMailItem(user);

Click Step in Debugger to execute the statement.
What changed in the Debugger?
item is a local variable and references the MailItem object for user "Juan".

Since "Juan" has mail, which statement would you expect the if-statement to execute next?
Is the value of item listed as null or something else?

Click Step in Debugger to execute the statement.

Which statement was executed?
Why?

Click Continue to complete the method printNextMailItem.

Exercise 12 - Single stepping
In BlueJ

Call MailClient method printNextMailItem on juan. The debugger should highlight the first breakpointed statement encountered in the program.
Is the statement at the breakpoint again?

MailItem item = server.getNextMailItem(user);

Click Step in Debugger to execute the statement.
item is a local variable and references the MailItem object for what user?

Since "Juan" has mail, which statement would you expect the if-statement to execute next?
Is the value of item listed as null?

Click Step in Debugger to execute the statement.

Which statement was executed?
Why?

Click Continue to complete the method printNextMailItem.

3.13.3 Stepping into methods

executes one statement but will not follow the execution of a method.

executes one statement but follows the execution of a method.

Exercise 13 - Single stepping into methods
In BlueJ

The breakpoint should still be set in method printNextMailItem.
Use sophie's sendMessage to send a message from "Sophie" to "Juan".
Don't read the message yet.
Call MailClient method printNextMailItem on juan. The debugger should highlight the first breakpointed statement encountered in the program.
What is the variable item?
Click Step to execute until the statement below is highlighted:

item.print();

Click Step Into to execute into the print method.
Continue to click Step Into to follow the execution.

3.14 Method calling revisited

Exercise 14 - Method calling revisited
In BlueJ

Set a breakpoint in the first statement of method sendMessage:

MailItem mess = new MailItem(user, to, message);

Use sophie's sendMessage to send a message from "Sophie" to "Juan".
What are the formal parameters (i.e. local variables) listed?
Click Step to execute new MailItem(user, to, message) method.
What changed and why?
Click Continue to complete the execution.
Use juan's sendMessage to send a message from "Juan" to "Sophie".

Repeat 3-6 again.

A	B	A && B
false	false	false
false	true	false
true	false	false
true	true	true

A	B	A \|\| B
false	false	false
false	true	true
true	false	true
true	true	true

Chapter 3Object orientation