Inheritance

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Inheritance
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Inheritance

• Early programmers often wrote code very similar
  to existing code
• Example A human resources system might handle
  different types of personnel. Much of the code
  for different classifications of personnel would
  be identical
• This was often handled by cutting and pasting
  code and then modifying
• Needed a way to capture and formalize the
  similarity

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Inheritance
Natural, hierarchical way of organizing things.
(superclass)
Staff Member
Employee Volunteer Hourly
Salaried Consultant
(subclass of Staff)
(subclass of Employee)
(subclass of Hourly)
Think in terms of is a relationships An
Employee is a Staff Member, as is a
Volunteer. An Hourly worker is a(n) Employee. A
Consultant is a(n) Hourly employee.
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Classes and Subclasses
Dont worry about private and public for now
class Animal public String name ""
public String noise "" public int
numTimesPerformed 0 // constructors,
accessors modifiers go here public void
identifySelf( ) System.out.println(My
name is name) // of identifySelf
public void perform( ) doYourThing( )
numTimesPerformed // of perform
public void doYourThing( ) // no-op
method // of doYourThing // of Animal
So, animals have a name and noise and they can
identify themselves, perform and do their thing.
Animal harpo new Animal() harpo.setName(Harpo
) harpo.doYourThing() // says nothing
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Subclasses (Dog extends Animal i.e. A dog is an
animal or All dogs are animals)
class Dog extends Animal public Dog()
noise Woof // of constructor
public void doYourThing ( )
identifySelf() System.out.println(I am a
dog) System.out.println(noise) //
of doYourThing // of Dog
Recall The Animal class had a no-op method for
doYourThing()
Dog pickles new Dog() pickles.setName(Pickles
) pickles.doYourThing() // output // My name
is Pickles // I am a dog // Woof
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Subclasses (Cat extends Animal i.e. A cat is an
animal or All cats are animals)
class Cat extends Animal public Cat()
noise Miaow // of constructor
public void doYourThing ( )
identifySelf() System.out.println(I am a
cat) System.out.println(noise) //
of doYourThing // of Cat
Cat abby new Cat() abby.setName(Abby) abby.d
oYourThing() // output // My name is Abby //
I am a cat // Miaow
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Subclasses (Human extends Animal i.e. A human is
an animal or All humans are animals)
class Human extends Animal public Human()
noise I think therefore I am //
of constructor public void doYourThing ( )
identifySelf() System.out.println(
I am a sentient being)
System.out.println(noise) // of
doYourThing // of Human
Human descartes new Human() descartes.setName(
Rene) descartes.doYourThing() // output //
My name is Rene // I am a sentient being //
I think therefore I am
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Questions?
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Inheritance Scope

• Using super this

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Inheritance and Scope

• Variables (e.g. noise)
• Java first examines current method, looks for
  local variable or parameter
• Java then examines current class (e.g. Dog)
• Java then examines superclass (e.g. Animal)
• Java continues up the class hierarchy until no
  more superclasses to examine.
• Methods (e.g. doYourThing() or identifySelf())
• Java first examines current class
• Java then examines superclass
• Java continues up inheritance hierarchy until no
  more superclasses to examine.

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Specifying Scope
Java allows you to override the scope rules by
saying which variable/method youre referring to
Keyword super keyword for specifying method or
variable from superclass, e.g.,
super.doYourThing( ) Keyword this keyword for
specifying method or variable in current object,
e.g., this.doYourThing( )
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Using super this
(but why??)
Same (in this case) as noise Woof and this.no
ise Woof
class Dog extends Animal public Dog()
super.noise Woof // of constructor
public void doYourThing ( )
super.identifySelf() System.out.println(I
am a dog) System.out.println(noise)
// of doYourThing // of Dog
Same (in this case) as identifySelf() or this.ide
ntifySelf()
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Using super and this

• class Animal
• String name
• class Dog extends Animal
• String name / Just so I don't forget! /
• void twoNames()
• System.out.println
• ("My dog name is " name)
• System.out.println
• ("My animal name is " super.name)

Good idea?
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Using super
class Dog extends Animal // constructor as
before public void doYourThing()
identifySelf() System.out.println(noise)
// of doYourThing public void
identifySelf() super.identifySelf() Sys
tem.out.println(I am a dog) // of
identifySelf // of Dog
I.e. this.identifySelf() (newly defined below)
I.e. the identifySelf() (defined in Animal)
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A geometry example
class Shape public String name
public String getName () return
(this.name) // getName public
int area () return (0) // area
// Shape
Each extending object would override the area()
method.
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Constructor chaining
class Rectangle extends Shape private int
length, width Rectangle () this(0,
0) // constructor Rectangle (int l,
int w) this( l, w, rectangle) //
constructor Rectangle (int l, int
w, String n) length l width l
name n // constructor public int area ()
return (length width) // area
public String getName () if (length
width) return "square" else return
super.getName()) // getName public
String toString () String s
s new String ("A " getName() "
with length " length " and width "
width) return (s)
// toString // Rectangle
The Circle class implementation is left as an
exercise to the reader.
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Constructors and Inheritance

• Javas rule
• If first line of constructor is not an explicit
  call to a superclass constructor, Java will
  implicitly put super( ) as the first line,
  calling the superclass default constructor.publi
  c Dog()
• strNoise Woof
• // of constructor
• An exception to this rule chained constructor
  call to
• this(params) will defer(ertelemek) super( )
  call
• To use superclass constructors with params, call
  them explicitly, e.g., super(strName)

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Look Closer...
class Rectangle extends Shape private int
length, width Rectangle ()
this(0, 0) // constructor
Rectangle (int l, int w)
this( l, w, rectangle) // constructor
Rectangle (int l, int w, String n)
length l width
l name n //
constructor public int area ()
return (length width) // area
What if I want a Shape constructor run here?
Shape()
What if there is no Shape()???
Error
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Inheritance and Scoping
Examples
super(xxx) // calls a superclass
constructor super.xxx // accesses superclass
variable super.xxx( ) // calls superclass
method this(xxx) // calls a current-class
constructor this.xxx // accesses current
classs variable this.xxx( ) // calls current
class method Note cannot do
super.superltsomethinggt (can achieve this effect
via casting, but rarely should details later...)
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Chaining, superclass example
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• class Parent
• String name
• public Parent()
• setName("NONE") cp(1)
• public Parent(String name)
• setName(name) cp(2)
• public void setName(String name)
• this.name name
• cp(3)
• public void cp(int n)
• System.out.println("At checkpoint "n"
  "name)
• public static void main(String args)
• Child c new Child()
• // Parent
• class Child extends Parent
• public Child()
• this("NONAME")
• cp(4)
• public Child(String name)

Output At checkpoint 3 NONE At checkpoint 1
NONE At checkpoint 5 NONAME At checkpoint 4
NONAME
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• class Parent
• String name
• public Parent()
• setName("NONE") cp(1)
• public Parent(String name)
• setName(name) cp(2)
• public void setName(String name)
• this.name name
• cp(3)
• public void cp(int n)
• System.out.println("At checkpoint "n"
  "name)
• public static void main(String args)
• Child c new Child("Bob")
• // Parent
• class Child extends Parent
• public Child()
• this("NONAME")
• cp(4)
• public Child(String name)

Output At checkpoint 3 NONE At checkpoint 1
NONE At checkpoint 5 Bob
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Recall Class Object

• Java provides a base class, Object
• All classes that do not have an extends clause
  implicitly inherit directly from class
  java.lang.Object
• Examples utilizing this fact
• public boolean equals (Object o)
• public boolean String toString ()
• When you create your own toString( ) method for
  a class, you are overriding the toString( )
  provided by Object.

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Object Hierarchy
class Object methods String toString() boolean
equals(Object obj) and a few others...
Or how about...
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Questions?
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Wrapper Classes
Primitive types (e.g., int) are not classes But
sometimes, we may have need to make use of
primitive types in a context that requires that
we manipulate objects, not primitives e.g. many
collection classes are collections of
Objects Java provides a set of wrapper classes
(a.k.a. type wrappers, a.k.a. envelope classes)
to support treating primitives as objects. It
does this by providing a specific class that
corresponds to each primitive data type They are
in java.lang, so the names are universally
available The names are mostly identical to
primitive types, but capitalized...
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Wrapper Classes
Class corresponds to Primitive
Boolean
boolean Character
char Byte
byte Short
short Integer
int Long
long Float
float Double
double

• Each one
• allows us to manipulate primitives as objects
• contains useful conversion methods. E.g.
  Integer contains static Integer valueOf(String
  s) Integer.valueOf("27") is the object
  corresponding to int 27
• contains useful utility methods (e.g. for
  hashing)

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Wrapper Classes
Using wrappers to bridge between objects and
primitives // create and initialize an int
int i 7 // create an Integer object and
convert the int to it Integer intObject new
Integer( i ) // retrieve the int by unwrapping
it from the object System.out.println(
intObject.intValue() ) // convert a
string into an Integer object String strS
"27" Integer intObject intObject new
Integer (strS) // then to an int int a
intObject.intValue() // one way int b
Integer.parseInt(strS) // second way
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Questions?