Review

1
Review

• View classes as modules
• Encapsulate operations
• View classes as struct types
• Encapsulate data
• View classes as abstract data types
• Encapsulate both data and operations
• Inheritance
• Defining classes in terms of other classes

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Inheritance
superclass
ParkingMeter
subclass or extended class
DigitalParkingMeter
AnalogParkingMeter
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Syntax
A
B
class B extends A ....
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An Example of Inheritance

• Variety of heating controllers.
• Basic (shared) functionality.
• On or off.
• More sophisticated functionality.
• Temperature setting.
• On/off time periods.
• Possibly others.

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HeaterController Super Class
class HeaterController public void
switchOn() setOn(true)
public void switchOff() setOn(false)
public boolean isOn() return
getOn() ...
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Extending HeaterController
class VariableController extends HeaterController
public static final int DefaultLevel 16,
... public int
getLevel() return getHeater().getTemperat
ure() public void setLevel(int level)
throws RuntimeException if((MinLevel lt
level) (level lt MaxLevel))
getHeater().setTemperature(level)
else throw new RuntimeException(
"Illegal level setting
"level)
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A VariableController Object

• Use like a VariableController or
  HeaterController.

VariableController v new VariableController() .
.. // Warm up the room. v.switchOn() if(v.getLeve
l() v.MinLevel) v.setLevel(v.DefaultLevel)
... // The room is warm enough - switch the
heater off. v.switchOff()
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The Is-A Relationship

• Sub class/super class often characterized in this
  way.
• VariableController is a HeaterController.
• Circle is a Shape, Rectangle is a Shape, Square
  is a Rectangle.
• The relationship is one way.
• Rectangle is not necessarily a Square.

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Is-A versus Has-A

• A common linguistic confusion.
• Often responsible for inappropriate class
  structures.
• VariableController has a level attribute.
• VariableController is a HeaterController.

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Issues in Inheritance

• Inappropriate Inheritance.
• Multiple Inheritance.
• Downcasts and Upcasts.
• Inheritance for Specialization.
• Structural Similarity of Classes.
• Order of Initialization and Super Class
  Constructors.

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Inappropriate Inheritance
import java.util.Random // Inappropriate
inheritance class Die extends Random //
Return an integer in the range 1..6 public
int roll() final int range 6
// Use the inherited nextInt method.
return 1nextInt(range)

• java.util.Stack extends java.util.Vector

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Multiple Inheritance?

• A class may extend only a single class.
• A class may implement multiple interfaces.
• In addition to extending a single class.
• An interface may extend multiple interfaces.
• interface ParentTeacher extends Parent, Teacher

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Downcasts and Upcasts

• The terminology relates to the inheritance
  hierarchy up or down.

// Explicit downcasting. // Downcast Object -gt
String. String s (String)h.get(key) //
Implicit upcasting VariableController v new
VariableController() // Upcast HC lt-
VC. HeaterController h v
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Inheritance for Specialization

• Sub classes are often more specialized versions
  of their super classes.
• President extends Citizen
• A supermarket's SellableItem class extended to
  add a sell-by date.
• class PerishableItem extends SellableItem
• Sub class has everything (and more).

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Sub Class Initialization

• The super class elements of a sub class object
  must be properly initialized.
• VariableController on/off state.
• Super class elements initialized first.
• Super class constructor selection is necessary.
• Argument passing must be arranged.

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Super Class Construction (cont.)

• super must be called as first statement.
• No-arg super class constructor called by default.
• Sub class error if the super class does not have
  one.
• All classes without any constructor have a
  default no-arg constructor.

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Default No-Arg Constructor
class VariableController extends HeaterController
public VariableController(int initialLevel)
throws ... setLevel(initialLevel)
... // Implicit extension of Object super
class. class HeaterController extends Object
// Default (implicit) no-arg constructor.
public HeaterController() // (Implicit)
super() ...
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Access Control Issues

• Inheritance means we need to revisit issues of
  access control.
• What rights does a sub class have over its super
  class members?
• Classes in one package might extend classes
  defined elsewhere.

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Access Control Rules

• public access is global.
• private access is class only.
• Sub classes have no rights.
• package access is whole package.
• protected access is package plus sub classes.

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Overriding Methods

• Sub class specializations often need to modify
  super class behavior.
• The default behavior of equals only mimics
  reference equality ().
• clone only creates a shallow copy, not a deep
  copy.

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class Point ... public String
toString() return "("getX()","getY()")"
public boolean equals(Object o) if(o
this) return true else
if(o null) return false
else if(o instanceof Point) Point
p (Point) o return getX()
p.getX() getY() p.getY() else
return false ...
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Rules on Overriding

• Return type and arguments must match.
• Sub class version must not be less visible.
• final disregarded in matching arguments.
• Checked exceptions in sub class must be
  compatible with those in super class.
• Sub class version may throw none.

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Method Selection

• Messages always arrive at the outermost layer for
  non-private methods.
• The outermost matching method is selected.
• Compilers generate efficient code to handle
  selection.
• An inner version may be selected.

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Selecting the Super Class Version of a Method
public void method(int arg) ... //
Invoke the closest super class version.
super.method(arg) ...
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Usage of super

• Distinct from usage of super in constructors.
• May be used from anywhere within a method.
• Only possible to select the nearest version from
  an inner layer.

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Restrictions on Overriding

• Overridden method may not be private.
• Method signatures must have identical name,
  return type, argument types.
• Hence, Object argument of equals.
• Sub class version may be more visible.
• Sub class version's exceptions must be a subset
  of those thrown by super class version.

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Super Class Behavior and Overriding

• When you invoke a method on an object, the actual
  class of the object governs which implementation
  is used
• When you access a field, the declared type of the
  reference is used

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Example
class CC2 extends C int getX() return
x2 int x2 2 class TestExtend
public static void main(String s) C o1
new CC1() C o2 new CC2()
System.out.println(o1.toString())
System.out.println(o2.toString())
class C public String toString()
return "x"getX() int getX() return
defaultX int defaultX 0 class CC1
extends C int getX() return x1 int
x1 1
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Example
class CC2 extends C int getX() return
x int x 2 class TestExtend
public static void main(String s) C o1
new CC1() C o2 new CC2()
System.out.println(o1.toString())
System.out.println(o2.toString())
class C public String toString()
return "x"x int getX() return x
int x 0 class CC1 extends C int
getX() return x int x 1
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Example
class CC2 extends C int getX() return
x int x 2 class TestExtend
public static void main(String s) C o1
new CC1() C o2 new CC2()
System.out.println(o1.x((CC1)o1).x)
System.out.println(o2.x((CC2)o2).x)
class C public String toString()
return "x"getX() int getX() return
x int x 0 class CC1 extends C
int getX() return x int x 1
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Final Classes and Methods

• A class defined as final cannot be sub classed.
• public final class String ...
• Often done to protect complex internal state
  representations.
• A method defined as final may not be overridden.

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Abstract Classes and Methods
abstract class Name ...

• Abstract classes can be extended but not
  instantiated
• A class that contains an abstract method must be
  also abstract