Agenda
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About Homework for BPJ lesson 36
About practice of last class
Review –superclass vs subclass
Encapsulation
Homework
Quiz on 12/6
public class Red extends Green
{
public int blue(double x)
{...}
public String s;
private int i;
}
public class Green
{
public double peabody(double y)
{
return mm;
}
private boolean crunch( )
{...}
private double mm;
public long xx;
}
Long data type
long: The long data type is a 64-bit signed two's
complement integer. It has a minimum value of 9,223,372,036,854,775,808 and a maximum
value of 9,223,372,036,854,775,807 (inclusive).
Use this data type when you need a range of
values wider than those provided by int.
super.peabody(11);
Is this legal?
Red myObj = new Red();
boolean bb = myObj.Crunch();
Is this legal? the two above)
Red myObj = new Red( );
int bb = myObj.blue(105.2);
Write code for the blue method that
will printout the mm state variable.
public int blue(double x)
{ double mmValue =
super.peabody(x);
System.out.println(mmValue);
}
Write code for the blue method that
will printout the xx state variable.
public int blue(double x)
{ double xxValue = super.xx;
System.out.println(xxValue);
return 0;
}
this keyword
• Within an instance method or a constructor,
this is a reference to the current object.
public class Point {
public int x = 0;
public int y = 0;
public Point(int a, int b) { x = a; y = b; } }
public class Point {
public int x = 0;
public int y = 0;
public Point(int x, int y)
{ this.x = x; this.y = y; } }
Using this with a Constructor
public class Rectangle {
private int x, y;
private int width, height;
public Rectangle() {
this(0, 0, 0, 0); }
public Rectangle(int width, int height) {
this(0, 0, width, height); }
public Rectangle(int x, int y, int width, int height)
{
this.x = x;
this.y = y;
this.width = width;
this.height = height; } ... }
Encapsulation
• Protecting an object’s data from code
outside the class.
• Those instance variables marked as private.
• They can only be seen or modified through
the use of public accessor and mutator
methods.
• Encapsulation = information hiding
public class Actor
{
private Grid<Actor> grid;
private Location location;
private int direction;
private Color color;
public Actor()
{
color = Color.BLUE;
direction = Location.NORTH;
grid = null;
location = null;
}
public Color getColor() {
return color;
}
public class Flower extends Actor
{
private static final Color DEFAULT_COLOR = Color.PINK;
private static final double DARKENING_FACTOR = 0.05;
public Flower()
{
setColor(DEFAULT_COLOR);
}
public Flower(Color initialColor)
{
setColor(initialColor);
}
public void act(){….}
public class Cube{
private int length;
public int breadth;
private int height;
public Cube(int l, int b, int h) {
length = l;
breadth = b;
height = h; }
public int getVolume() {
return (length * breadth * height); }
public int getBreadth(){
return breadth; }
public int getHeight(){
return height; }
}
Golden nuggets of wisdom
• Private state variables and methods
are not accessible from outside the
class.
• But private things can only be
accessed from within the class itself.
How to compare objects?
• Circle cir1 = new Circle(3.0);
• Circle cir2 = new Circle(3.0);
• cir1 == cir2 ??????
• Circle cir3;
• cir3 = cir1;
• cir1 == cir3 ??????
• String str1 = “Hello”;
• String str2 = “Hello”;
• str 1 == str2 ????
• String str3 = new String(“Hello”);
• str3 == str1?????
Inheritance
• Inheritance enables you to define a new class
based upon an existing class. The new class is
similar to the existing class, but has additional
member variables and methods.
• This makes programming easier because you
can build upon an existing class instead of
starting out from scratch.
Is-a relationship
In diagrams that show inheritance, an
arrow points from the new class to the
class it is based upon. The arrow is
sometimes labeled "is a".
superclass
subclass
The class that is used to define a new class
is called a parent class (or superclass or
base class.)
The class based on the parent class is called
a child class (or subclass or derived class.)
Inheritance example
BankAccount
SavingsAccount
checkingAccount
Subclasses need all the methods and state
variables of the superclass - BankAccount
BankAccount class
public class BankAccount{
private double balance;
public BankAccount(double amt){
balance = amt; }
public void deposit(double amt){
balance += amt; }
private void withdraw(double amt){
balance -= amt;
}
public class SavingsAccount extends BankAccount
{
private double interestRate;
public SavingsAccount(double amt, double rate){
super(amt);
interestRate = rate;
}
Public void addInterest(){
double interest = getBalance()* interestRate/100;
deposit(interest);
}
}
1. Private members of the superclass are not
inherited by the subclass and can only be
indirectly accessed.
2. Members that have default accessibility in the
superclass are also not inherited by subclasses in
other packages, as these members are only
accessible by their simple names in subclasses
within the same package as the superclass.
3. Since constructors and initializer blocks are
not members of a class, they are not inherited
by a subclass.
4. A subclass can extend only one superclass
GridWorld
Actor
Flower
Rock
Bug
Bug class
• If you don’t have the import statement for ex.
Bug class, you have to write the whole path in
order to use the class like
• Bug oBug = new info.gridworld.actor.Bug();
instead of
Bug oBug = new Bug();
Abstract class
• An abstract class models an abstract
concept. For example, a musical
instrument is an abstract concept.
• An instrument is something that can be
played, but there is no such thing an
“instrument” instrument. There are
however, flutes, drums, and cymbals.
• Abstract classes cannot be instantiated
because they should not represent
objects. They instead describe the more
general details and actions of an object.
Homework
Based on the BankAccount and
SavingsAccount classes on this slide and
answer all the questions in the word
document homework-nov26 posted on the
web