Transcript ECE 122 - University of Massachusetts Amherst

ECE 122
March 24
Motivation
• I am tasked to write two classes, one for cat,
one for sheep.
• That is easy. I roll up my sleeve and get it
done!
Here is my class for cat
import java.io.*;//importing java I/O package
public class Cat1
{
File imageFile; //The filename storing the image of the cat
String food; //"meat"
int hunger;//hunger level of the cat. It changes depending on when and how much the cat
//eats
int locationX;//X coordinate of the cat in space
int locationY; //Y coordinate of the cat in space
public void makeNoise(){}
public void eat(){}
public void sleep(){}
public void roam(){}
}
Here is my class for sheep
import java.io.*;//importing java I/O package
public class Sheep1
{
File imageFile; //The filename storing the image of the sheep
String food; //“grass"
int hunger;//hunger level of the sheep. It changes depending on when and how much the
//sheep eats
int locationX;//X coordinate of the sheep in space
int locationY; //Y coordinate of the sheep in space
public void makeNoise(){}
public void eat(){}
public void sleep(){}
public void roam(){}
}
Do you notice anything?
Observations
•
•
•
•
Sheep1 class and Cat1 class are very much alike.
They have same instance variables
They have same methods
Can we save some typing and maintenance efforts
by sharing?
• My supervisor asks me to write more classes for
lion, hippo, tiger, cow, wolf, dog,…
• Where is my vacation???
Inheritance
• Put common codes in a super class.
• Subclass inherits from super class, and REUSE codes from
super class.
• An inheritance relationship means subclass inherits the
members (both instance variables and methods) of super
class.
• Subclass can add new methods and variables of its own.
• Subclass can override the methods it inherits from super
class.
• Instance variables are not overriden.
IS-A Relationship
• Inheritance is not just for reusing code.
• A subclass should be a specific kind of
super class. E.g. A cat IS A specific kind of
Animal.
What am I going to do?
• I need a vacation.
• I need to finish my work before going to
vacation.
• I want to redesign my objects.
• I have identified IS-A relationship. Cat is a
specific kind of Animal. So do Sheep, …
• I will use inheritance that I just learned.
Add a super class, Animal
• It contains all the common instance
variables
• It contains all the common methods with a
default implementation.
Here is my super class, Animal
import java.io.*;//importing java I/O package
public class Animal
{
File imageFile; //The filename storing the image of the animal
String food; //"meat" or "grass"
int hunger;//hunger level of the animal. It changes depending on
//when and how much the animal eats
int locationX;//X coordinate of the animal in space
int locationY; //Y coordinate of the animal in space
public void makeNoise(){}
public void eat(){}
public void sleep(){}
public void roam(){}
}
Simplify my Cat1 class
• Rename my cat class from Cat1 to Cat
• Cat class inherits all instance variables from
Animal class, so I will not write it.
• Cat class inherits all methods from Animal class,
so I overwrite methods that should be
implemented differently.
• Cat makes noise differently, so I overwrite
makeNoise() method.
• Cat eats meat, not grass, so I need to overwrite
eat() method to specify this.
What is the syntax for
inheritance?
public class Cat extends Animal
{
}
Here is my Simplified Cat class
public class Cat extends Animal
{
public void makeNoise()
{
//Cat's meow
}
public void eat()
{
//Eat meat
}
}
Sheep, dog, tiger, ….???
• I will do the same thing!
• I am going to vacation in one week!
Here is my simplified Sheep class
public class Sheep extends Animal
{
public void makeNoise()
{
//Sheep's meeeeier ?
}
public void eat()
{
//Eat grass
}
}
Which method is called actually?
• Both Animal (super class) and Cat
(subclass) have makeNoise() method. After
I instantiate a Cat object, I call makeNoise()
method. Which method is actually called?
Super class? Or subclass?
• Let’s test it out!
Which method is called, actually?
• The method of the most specific
class/object.
• If a method is not overriden in subclass, the
super class method is called.
• If a method is overriden in subclass, the
subclass’s method is called.
Assignments
• Read “Java 2”, Chapter 7
• Read “Head First Java”, Chapter 7