CS12320 Lecture 16

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CS12320 - Lecture 16

• Reyer Zwiggelaar
• rrz_at_aber.ac.uk

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In This Lecture

• Inheritance

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Terminology

• A subclass inherits from a superclass
• A subclass a derived class
• A subclass is a specialisation of the superclass
• A superclass a base class
• A superclass is a generalisation of a subclass

Stevens and Pooley, pp 22 - 24
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Definition

• A subclass is an extended, specialized version of
  its superclass. It includes the operations and
  attributes of the superclass, and possibly some
  more.

Stevens and Pooley, pp 22 - 24
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Pure Versus Extension

• Pure only methods in the base class are to be
  overridden in the derived class (see interfaces
  and implements)
• Subclass is a superclass
• Extension additional attributes and methods
  allowed (see extends)
• Subclass is like a superclass

Eckel, p 316
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extends
class SubClassName extends SuperClassName
//declaration of additional instance variables
//definitions of additional methods
Skansholm, p 223
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Generalisation

• .... relationship between a more general element
  and a more specific element. The more specific
  element is fully consistent with the more general
  element (it has all of its properties, members
  and relationships) and may contain additional
  information.

Bennett et al., pp 68-73
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Generalisation - Example
Vehicle speed weight changeSpeed
Car enginePower changeGear
Train noCarriages coupleCar
Bicycle noGears pedal
Skansholm, p 102
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Composition

• Instead of inheritance it is possible to achieve
  generalisation through using attributes of type
  superclass
• Does not use extends/implements
• This might be the first step to inheritance
• First composition then inheritance
• Typically indicated by a has a relationship
  between classes

Eckel, pp 226/238/315
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Composition - Example
GearBox
Engine
Car gearbox engine changeSpeed()
Skansholm, p 102
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Coupling

• Criteria for Good Design
• Inheritance Coupling describes the degree to
  which a subclass actually needs the features it
  inherits from its superclass.

Bennett et al., p. 353
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Coupling - Poor Example
LandVehicle numberOfAxles registrationDate regis
ter()
Vehicle description serviceDate maximumAltitude t
akeOffSpeed checkAltitude() takeOff()
Bennett et al., p. 353
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Coupling - Improved Example
LandVehicle numberOfAxles registrationDate regis
ter()
Vehicle description serviceDate
Aircraft maximumAltitude takeOffSpeed checkAltit
ude() takeOff()
Bennett et al., p. 353
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Hierarchies

• Number of levels in class diagram
• Not too deep nor to shallow
• More than 4/5 levels is too much
• Others say that up to 9 levels might be
  acceptable
• Do not over-specialise
• Adds complexity

Bennett et al., p. 355/356
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Hidden Instance Variables

• An instance variable v in a subclass that has the
  same name as an instance variable in the
  superclass hides the instance variable in the
  superclass
• We can write super.v to access the instance
  variable in the superclass

Skansholm, p 227
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Method Overriding

• Methods in the subclass with the same name and
  parameters as in the superclass
• A form of polymorphism
• The objects class determines which method is used

Skansholm, p 228, Eckel, p 231
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Method Overriding
class MySC String s From MySC void
get() println(s) class MyDC extends MySC
String s From MydC void get()
println(s) void getS() println(super.s)

Eckel, p 231
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Method Overloading
class MySC String s From MySC void
get() println(s) class MyDC extends MySC
String s From MydC void get()
println(s) void getS() println(super.s)
void get(String t) s t get()
Eckel, p 231
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Method Over
class Test static void main(String args)
MySC s new MySC() s.get() MyDC d
new MyDC() d.get() d.getS()
d.get("new text") d.get() d.getS()

C\gtjava Test From MySC From MydC From MySC new
text new text From MySC C\gt
Eckel, p 231
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Initialisation

• How Inheritance determines Attributes and Methods
• In which order are aspects executed

Eckel, p 246
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Subclass Constructor

• The instance variables for the superclass with
  explicit initialization expressions are
  initialized to these values
• A constructor for the superclass is called
• The instance variables for the subclass with
  explicit initialization expressions are
  initialized to these values
• The statements in the subclasss constructor are
  executed

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Initialisation Example
public class Insect protected int i 9
protected int j Insect() prt(i
i , j j) j 39
protected int x1 prt(Insect.x1
initialized) public int prt(String s)
System.out.println(s) return 47
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Initialisation Example
public class Beetle extends Insect private
int k prt(Beetle.k initialized) Beetle()
prt(k k) prt(j j) private
int x2 prt(Beetle.x2 initialized) public
int prt(String s) System.out.println(s)
return 63 public static void
main(String args) System.out.println(Be
etle constructor) Beetle b new
Beetle()
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Initialisation Example
Beetle constructor Insect.x1 initialized i 9,
j 0 Beetle.k initialized Beetle.x2 initialized
k 63 j 39
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finalize()

• Method associated with the Object class and is
  inherited by all classes
• The method is called when an object is to be
  garbaged
• Can be redefined and used to clean up
• Needs to be called for the superclass

Skansholm, p 234
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clone

• To copy whole objects
• The method clone() needs to be publicly
  implemented
• Only available downwards in inheritance tree

Eckel, p 561
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Synchronized

• Just for information
• If a method is synchronized in the superclass it
  is not necessarily synchronized in the subclass
  overridden version

Eckel, p 785
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Books Used

• Skansholm Java From the Beginning
• Eckel Thinking in Java
• Stevens and Pooley Using UML
• Bennett, McRobb and Farmer Object-Oriented
  Systems Analysis and Design

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In This Lecture

• Inheritance

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In The Next Lecture

• Input/Output