Transcript 9-inheritance

What is Inheritance?
A form of software reuse
Create new class from existing class
Absorb existing class’ data and
behaviors
Enhance with new or modified
capabilities
Used to eliminate redundant code
Example
Circle class inherits from Shape class
Circle extends Shape
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Subclass and Superclass
Subclass extends superclass
Subclass
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Also called child class or derived class
More specialized group of objects
Inherits data and behaviors from superclass
Can add or modify behaviors (methods)
Superclass
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Also called parent class or base class
Typically represents larger group of objects
Supplies data and behaviors to subclass
Direct and indirect
Single and multiple inheritance
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The Object class
Top of the Java class hierarchy
Located in package java.lang
Class from which every other Java
class inherits
A class implicitly extends Object if no
other class is specified
.toString(), .clone(), .equals()
equals() compares addresses, not the contents of objects
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“is-a” vs. “has-a” relationships
“is-a”
Represents inheritance
subclass object is an example of the
superclass object
Example: a Car is a Vehicle
Car is subclass; Vehicle is superclass
Keywords: extends, implements
“has-a”
Represents composition
Object contains one or more objects of
other classes as members
Example: Car has a Steering Wheel
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Composition
Dog “has-a” owner, mother, father,
leash…
public class Dog
{
private String name;
private int age;
private Person owner;
private Dog mother, father;
private Leash dogLeash;
}
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Inheritance
Dog extends (“is-a”) Animal
Animal
Animal’s Stuff
Dog
Animal’s Stuff
Dog’s Stuff
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public class Animal
{
private int numLegs;
public int getLegs()
{
return numLegs;
}
public void setLegs(int legs)
{
numLegs = legs;
}
public String noise()
{
return “?”;
}
public class Dog extends Animal
{
public static void main (String[] args)
{
Dog d = new Dog();
d.setLegs(4);
System.out.println ("A dog has " +
d.getLegs() + " legs");
System.out.println ("A dog says "
+ d.noise());
}
public String noise()
{
return "WOOF!";
}
}
}
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Inheritance Examples
Superclass
Student
Subclasses
GraduateStudent,
UndergraduateStudent
Shape
Circle, Triangle, Rectangle
Loan
CarLoan, HomeImprovementLoan,
MortgageLoan
Employee
Faculty, Staff
BankAccount CheckingAccount,
SavingsAccount
Fig. 9.1 Inheritance examples.
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An Inheritance (Class) Hierarchy
CommunityMember
Employee
Faculty
Administrator
Fig. 9.2
Student
Alumnus
Staff
Teacher
Inheritance hierarchy for university CommunityMembers.
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Draw an Inheritance Hierarchy for
Triangle
these classes:
Sphere
TwoDimensionalShape
Shape
Pyramid
Square
ThreeDimensionalShape
Cube
Circle
Sphere
Shape
TwoDimensionalShape
Circle
Square
ThreeDimensionalShape
Triangle
Sphere
Cube
Pyramid 10
Try one more!
WalkingBird
Owl
Ostrich
Parrot
Bird
FlyingBird
Chicken
TalkingParrot
Bird
WalkingBird
Ostrich
Chicken
FlyingBird
Parrot
Owl
TalkingParrot
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Strategy
Design classes for objects
Identify characteristics classes have
in common
Abstraction: focus on commonalities
among objects in a system
Design superclasses to store
common characteristics
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Bird
WalkingBird
call: ?
color:?
food:?
movement:walk
call: ?
color:?
food:?
movement:?
Chicken
Ostrich
call: cluck
color: white
food: bugs
call: neek-neek
color: brown
food: grass
FlyingBird
call: ?
color:?
food:?
movement:fly
Parrot
Owl
call: Squawk
color:?
food: fruit
call: hoo
color: brown
food:mice
TalkingParrot
...
protected Members
Variables or methods available only to derived
classes
Intermediate level of protection between
public and private
Accessible to
superclass members
subclass members
class members in the same package
Use super. to access a superclass method that
has been overridden by a subclass method.
Don’t use protected instance variables!
“Fragile” software can “break” if superclass changes
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protected Members
ClassC
ClassA.m1()
ClassA.m3()
is-a ClassA
public m1()
private m2()
protected m3()
has-a ClassB
ClassA.m1()
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Constructors in Subclasses
Constructors are not inherited!
Chain of constructor calls
subclass constructor invokes superclass
constructor
– Implicitly or explicitly
– To call explicitly, use super()
– Superclass constructor call must be first
statement in subclass constructor
Object constructor is always fired last
Example: Student extends Person
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Software Engineering with
Inheritance
Customizing existing software
Inherit from existing classes
Include additional members
Redefine superclass members
No direct access to superclass source code
Independent software vendors (ISVs)
Develop proprietary code for sale/license
Users derive new classes
– Without accessing ISV proprietary source code
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Quiz
Private/Static/
Constructor
_________
methods are not inherited.
Object class is at the top of the Java
The _____
hierarchy.
extends keyword is used in Java to
The _____
represent inheritance.
A Java class _____
cannot inherit from more than
one class.
A variable or method that is available only
to derived classes is called a
protected member
____________.
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