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Java Programming:Inheritance,Inheritance,Polymorphism
Example: PacMan and SuperPacMan
public class PacMan{
private int x;private int y;
bli
P
M
(i t
i t
){
}
public PacMan(int x, int y){..}public void eatDot(){ ..}public void move(){.. }public void turnLeft(){.. } }
public class SuperPacMan extends PacMan{
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… public void turnRight(){
turnLeft();turnLeft();turnLeft();
}
}
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Inheritance
One of the key concepts of OOP
A hierarchical relationship among classes
Establishes a superclass/subclass relationship
Establishes
“is a” relationships
Establishes
is a
relationships
¾
e.g. a SuperPacMan “is a” PacMan
Benefits:
Reusability of code
¾
Put code in one class, use it in all the subclasses ¾
Revisions only needs to be done in 1 place
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¾
Revisions
only needs to be done in 1 place
Polymorphic code (works on “many forms”)
¾
Write general purpose code designed for a supertype thatworks for all subtypes
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The “extends” keyword
public class PacMan{}
Inheritance is established via the “extends” keyword
public class SuperPacMan extends PacMan{}
Now we say
SuperPacMan
inherits
from PacMan
A SuperPacMan “is a” PacMan
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SuperPacMan is a
subclass
/ subtype
of PacMan
PacMan is the
superclass/supertype
of SuperPacMan
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What can you inherit?
Visibility modifiers determine which classmembers are accessible and which do not
Members (variables and methods) declaredwith
public
visibility are accessible, and those
with
private
visibility are not
Problem: How to make class instance variables
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Problem:
How to make class instance variables
visible only to its subclasses?^
Solution: Java provides a third visibility modifier thathelps in inheritance situations:
protected
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Inheritance Tree
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Java has
single inheritance
; each node has one parent
Except for Object which has no parent
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The Object Class
All classes inherit from the Object class
The Object class is the root of the class hierarchy
When we create a new class, “extends Object” is implied/implicit public class Car {} public class Car extends Object{}
The Object class has several methods which all object
inherit,
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j^
j^
most notably: toString() and equals()
-^
Once we inherit these, we can also override the behavior i.e. make itconform to what the object of subclass will do when this method iscalled.
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Need for equals(): Comparison of Strings
If the == operator is used for Strings
Java compares the addresses where the String objects arestored, not the letters in the String
For example:
String a = "hi";String a =
hi ;
String b = "hi";result = a == bresult: false
Use the String class'
equals
method to compare two
Strings for equality
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a.equals(b)
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Method Overloading
Method
overloading
occurs when
A class has two or more methods with the same
name
but
different
signatures
g
¾
Different signature -> the number, order, or types of theirparameters differ
// the foo method is overloadedpublic void foo() {.. }
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public void foo(int x) { ..}Public void foo(double x){..}public void foo(int x, double y) {..}
When the foo(..) method is called, Java picks the one that “matches”. E.g.
foo(10, 350.5);
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Overriding
Overriding
occurs if
There are two or more methods with the samename
and the same signature
in an inheritance
h i
chain
For example, the Object class has a toString()method
¾
It can be
overriden
in a subclass simply by creating a
method with the same signature
public String toString() {. .}
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Java picks the “lowest” method in the inheritancechain possible
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Type Rules
A reference variable of type t may hold a value of its own type orany subtype (but not of a supertype).
Given the following variable declaration:
PacMan b;PacMan b = new PacMan(0, 0);b = new String();b = new SuperPacMan(0, 0);b = new Object();
b;
Which of the following assignments are valid (compile)?How about these? ESE
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SuperPacMan bb;bb = new PacMan(0, 0);bb = new String();bb = new SuperPacMan(0, 0);bb = new Object();
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The “instanceof” Operator
PacMan b = new PacMan(0, 0);b instanceof PacManAnswer: trueb instanceof ObjectAnswer: trueb instanceof SuperPacManAnswer: false
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The “instanceof” Operator
SuperPacMan
bb
=^
new
SuperPacMan(0,
0);
bb
instanceof SuperPacMan
Answer:
true
bb
instanceof Object
Answer:
true
bb
instanceof PacMan
Answer:
true
PacMan bbb
=^
new
SuperPacMan
(0, 0);
//a variable can store a subtype
bbb
instanceof
SuperPacMan(0, 0)
Answer:
true
bbb
instanceof
PacMan
Answer:
true
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