Notesale: Turn your study into money

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Chapter 3
Object Interaction
Objects cooperating with other objects –
objects calling other objects methods
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BlueJ Book: Sections 3
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6
1

Abstraction and Modularisation
• Abstraction is the ability to ignore details of parts
to focus attention on a higher level of a problem
...

• Object-oriented design and development
provides strong support for both Abstraction
and Modularisation
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Barnes, Michael Kölling

2

A Digital Clock

Range 00:00  23:59
Going to examine the design and implementation of this
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Barnes, Michael Kölling

Modularising the Clock
Display
One four-digit display?

Or two two-digit
displays?

4
Objects First with Java - A Practical Introduction using BlueJ, © David J
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Barnes, Michael Kölling

5

Implementation - ClockDisplay
public class ClockDisplay
{
private NumberDisplay hours;
private NumberDisplay minutes;
Constructor and
methods omitted
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g
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• Classes define Types
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Barnes, Michael Kölling

Class NumberDisplay Methods
public NumberDisplay(int rollOverLimit)
public int getValue()
public String getDisplayValue()
public void setValue(int replacementValue)
public void increment()
This is an abstract view of NumberDisplay
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Barnes, Michael Kölling

7

Object Diagram

Object Diagram shows
objects and their
relationships at one
moment in time during
the execution of a
program (known as a run
time or dynamic view)
Objects First with Java - A Practical Introduction using BlueJ, © David J
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Objects First with Java - A Practical Introduction using BlueJ, © David J
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}
value is the current value of number to display
limit is the „limit‟ at which it „rolls over‟ to zero
Objects First with Java - A Practical Introduction using BlueJ, © David J
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The 'modulo' operator (%) returns the remainder of an
integer division
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g
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Barnes, Michael Kölling

11

Source Code: NumberDisplay
public voidsetValue(int replacementValue)
{
if((replacementValue >= 0) &&
(replacementValue < limit))
value = replacementValue;
}
&& is the boolean “and” operator
a && b evaluates to true if both a is true AND b is true
|| is “or”: a || b evaluates to true if either a OR b is true
! is “not”: !a evaluates to true if a is false
Objects First with Java - A Practical Introduction using BlueJ, © David J
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)
!
* / %
Brackets ‘( …)’ have the
+ strongest precedence
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Barnes, Michael Kölling

13

Source Code: NumberDisplay
public String getDisplayValue()
{
if(value < 10) {
return "0" + value;
}
else {
return "" + value;
}
}
Reminder: „+‟ concatenates strings when at least
one of the two operands is a string
What does this method do?
Objects First with Java - A Practical Introduction using BlueJ, © David J
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Only require brackets if more than one statement
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Barnes, Michael Kölling

15

Key points so far …
•
•
•
•
•
•

Abstraction
Modularisation
Classes defines types
Class Diagram
Object Diagram
Short Circuit Operators: ! * / % + - && ||

Objects First with Java - A Practical Introduction using BlueJ, © David J
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7 – 3
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12 – 3
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Barnes, Michael Kölling

17

Primitive Types vs
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g
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3

See Appendix B
P469 (4th ed
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Barnes, Michael Kölling

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Primitive Types vs
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(A „reference‟ is the memory address of the object
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This is the case before the constructer is called
obj null
e
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after just: SomeObjectType obj;
Objects First with Java - A Practical Introduction using BlueJ, © David J
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Object Types
int
int
a =
b =

a;
b;
32;
a;

a

32

b

32

ObjType a;
a
ObjType b;
a = new ObjType();
b
b = a;
With shared object references changing one, a or b
above, changes both – see the Quiz on the next slide
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Barnes, Michael Kölling

20

Quiz: What is the output?
int a;
int b;
a = 32;
b = a;
a = a + 1;
System
...
println(b);
Person a;
Person b;
a = new Person(“Pascali”);
b = a;
a
...
out
...
getName());
Objects First with Java - A Practical Introduction using BlueJ, © David J
...
Barnes, Michael Kölling

ClockDisplay Object Diagram

Objects First with Java - A Practical Introduction using BlueJ, © David J
...
Barnes, Michael Kölling

24

Multiple Constructors
There are two alternative ways for creating and initialising a
ClockDisplay object:
new ClockDisplay()
new ClockDisplay(15, 23)
The ClockDisplay class contains two constructors - one with
no parameters, the other with two
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ClockDisplay()
ClockDisplay(int hour, int minute)
Overloading: Providing more than one constructor or method of the same
name is permissible provided each has a distinctive list of parameter types
(signature)
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Barnes, Michael Kölling

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Method Calls
When a method call is encountered:
e
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minutes
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g
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Barnes, Michael Kölling

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Method Calls
• External method calls - methods can call methods of
other objects using the “dot” notation:
objectName
...
g
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increment();
• The method is sought in class of object referenced by
minutes i
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NumberDisplay
• increment() must be public (or protected) in
class NumberDisplay

Objects First with Java - A Practical Introduction using BlueJ, © David J
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g
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updateDisplay();
Can make internal method calls to public or private
(or protected) methods
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Barnes, Michael Kölling

28

Method Calls
In class ClockDisplay:
Which class?
public void timeTick()
{
minutes
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getValue() == 0) {
// it just rolled over!
hours
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Barnes, Michael Kölling

29

Internal Methods
Notice „private‟

/**
* Update the internal string that
* represents the display
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getDisplayValue() + ":" +
minutes
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8
...
6)
Objects First with Java - A Practical Introduction using BlueJ, © David J
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• You can assign and test for null:
private NumberDisplay hours;
if(hours == null) {
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Barnes, Michael Kölling

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this
• Special keyword in Java
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updateDisplay();
• Can be used to refer to field of current object, thus
public getAge(int age)
{
this
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Barnes, Michael Kölling

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Key Points…
• object references
• primitive types
• classes define types e
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ClockDisplay, NumberDisplay
• object creation using new and constructors
• overloading – more than one constructor
• internal / external method call
Most important overall concept in Chapter 3 is the way that
objects cooperate
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We‟ll be using it in the Practical
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1 – 4
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Barnes, Michael Kölling

33



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