Transcript Java Methods 3rd AP edition

Java Methods
Object-Oriented Programming
and Data Structures
3rd AP edition
Maria Litvin ● Gary Litvin
Objects and Classes
Copyright © 2015 by Maria Litvin, Gary Litvin, and Skylight Publishing. All rights reserved.
Objectives:
• See an example of a program, written in
OOP style, and discuss the types of objects
used in it
• Learn about the general structure of a
class, its fields, constructors, and methods
• Get a feel for how objects are created and
how to call their methods
• Learn a little about inheritance in OOP
4-2
OOP
• An OO program models the application as a
world of interacting objects.
• An object can create other objects.
• An object can call another object’s (and its
own) methods (that is, “send messages”).
• An object has data fields, which hold values
that can change while the program is running.
4-3
Objects
• Can model real-world objects
• Can represent GUI (Graphical User
Interface) components
• Can represent software entities (events,
files, images, etc.)
• Can represent abstract concepts (for
example, rules of a game, a location on a
grid, etc.)
4-4
Objects in the BalloonDraw program
Balloons
BalloonDraw
window
Control panel
Colors
Buttons
4-5
Classes and Objects
• A class is a piece of the program’s source
code that describes a particular type of
objects. OO programmers write class
definitions.
• An object is called an instance of a class.
A program can create and use more than
one object (instance) of the same class.
4-6
Class
Object
• A “blueprint” for
objects of a
particular type
Attributes
• Defines the
structure (number,
types) of the
attributes
Behaviors
• Defines available
behaviors of its
objects
4-7
Class: Car
Attributes:
String model
Color color
int numPassengers
double amountOfGas
Object: a car
Attributes:
model = "Mustang"
color = Color.YELLOW
numPassengers = 0
amountOfGas = 16.5
Behaviors:
Behaviors:
Add/remove a passenger
Get the tank filled
Report when out of gas
4-8
Class
• A piece of the
program’s source
code
• Written by a
programmer
vs.
Object
• An entity in a
running program
• Created when the
program is running
(by the main method
or another method)
4-9
Class
vs.
• Specifies the
structure (the
number and types)
of its objects’
attributes — the
same for all of its
objects
• Specifies the
possible behaviors
of its objects
Object
• Holds specific
values of attributes;
these values can
change while the
program is running
• Behaves
appropriately when
called upon
4-10
CRC Card
• A preliminary description of a class at the
initial stage of program design
Class
Balloon
Responsibilities
Occupies a location on the
canvas
Moves
Stretches
Canvas
Collaborators
4-11
Classes and Source Files
• Each class is stored in a separate file
• The name of the file must be the same as the
name of the class, with the extension .java
Car.java
public class Car
{
...
}
By convention, the
name of a class
(and its source file)
always starts with
a capital letter.
(In Java, all names are case-sensitive.)
4-12
Libraries
• Java programs are usually not written from
scratch.
• There are hundreds of library classes for all
occasions.
• Library classes are organized into packages.
For example:
java.util — miscellaneous utility classes
java.awt — windowing and graphics toolkit
javax.swing — GUI development package
4-13
import
• Full library class names include the package
name. For example:
java.awt.Color
javax.swing.JButton
• import statements at the top of the source file
let you refer to library classes by their short
names:
Fully-qualified
import javax.swing.JButton;
name
...
JButton go = new JButton("Go");
4-14
import (cont’d)
• You can import names for all the classes in a
package by using a wildcard .*:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
Imports all classes
from awt, awt.event,
and swing packages
• java.lang is imported automatically into all
classes; defines System, Math, Object,
String, and other commonly used classes.
4-15
SomeClass.java
import ...
import statements
public class SomeClass
{
• Fields
• Constructors
• Methods
}
Class header
Attributes / variables that define the
object’s state; can hold numbers,
characters, strings, other objects
Procedures for constructing
a new object of this class
and initializing its fields
Actions that an object
of this class can take
(behaviors)
4-16
public class Balloon
{
private int xCenter, yCenter, radius;
Fields
private Color color;
public Balloon()
{
xCenter = 0;
yCenter = 0;
radius = 50;
color = Color.BLUE;
}
...
...
public void move(int x, int y)
{
xCenter = x;
yCenter = y;
}
...
Constructor(s)
Methods
}
4-17
Fields
• A.k.a. instance variables
• Constitute “private memory” of an object
• Each field has a data type (int, double, String,
Color, etc.)
• Each field has a name given by the
programmer
4-18
Fields (cont’d)
You name it!
private [static] [final] datatype name;
Usually
private
May be present:
means the field is
shared by all
objects in the class
int, double, etc., or an
object: String, Color
May be present:
means the field
is a constant
private Color color;
4-19
Constructors
• Short procedures for creating objects of a
class
•
•
•
•
Always have the same name as the class
Initialize the object’s fields
May take parameters
A class may have several constructors that
differ in the number and/or types of their
parameters
4-20
Constructors (cont’d)
public class Circle
{
private int xCenter, yCenter;
private int radius;
The name of a constructor
is always the same as the
name of the class
...
public Circle (int x, int y, int r)
{
A constructor can take parameters
xCenter = x;
yCenter = y;
radius = r;
}
...
}
Initializes the fields
4-21
Constructors (cont’d)
...
Balloon b = new Balloon(50, 100, 30, Color.GREEN);
...
public class Balloon
{
An object is
created with
the new
operator
The number, order, and
types of parameters must
match
...
public Balloon (int x, int y, int r, Color c)
{
...
}
...
Constructor
}
4-22
Constructors (cont’d)
JButton go = new JButton("Go");
4-23
Methods
• Call them for a particular object:
Balloon bigBlue = new Balloon(50, 100,
30, Color.BLUE);
bigBlue.move( 55, 105);
4-24
Methods (cont’d)
• The number and types of parameters (a.k.a.
arguments) passed to a method must match
method’s parameters:
public void drawString ( String msg, int x, int y )
{
...
}
g.drawString ("Welcome", 120, 50);
4-25
Methods (cont’d)
• A method can return a value to the caller
• The keyword void in the method’s header
indicates that the method does not return any
value
public void drawString ( ... )
{
...
}
4-26
Encapsulation and
Information Hiding
• A class interacts with other classes only
through constructors and public methods
• Other classes do not need to know the
mechanics (implementation details) of a class
to use it effectively
• Encapsulation facilitates team work and
program maintenance (making changes to
the code)
4-27
Methods (cont’d)
• Constructors and methods can call other
public and private methods of the same class.
• Constructors and methods can call only
public methods of another class.
Class X
Class Y
private field
public method
public method
private method
4-28
Inheritance
• In OOP a programmer can create a new class
by extending an existing class
Superclass
(Base class)
subclass extends
superclass
Subclass
(Derived class)
4-29
Inheritance (cont’d)
Balloon
RoundBalloon
extends Balloon
RoundBalloon
4-30
A Subclass...
• inherits fields and methods of its
superclass
• can add new fields and methods
• can redefine (override) a method of the
superclass
• must provide its own constructors, but
calls superclass’s constructors
• does not have direct access to its
superclass’s private fields
4-31
public class InflatableBalloon extends Balloon
{
...
public void inflate(int percentage)
{
...
}
...
}
A new
method
4-32
Review:
• Name a few objects used in BalloonDraw.
• Name a few library classes used in
BalloonDraw.
• What are import statements used for?
• What is a field? A constructor? A method?
• Which operator is used to construct an
object?
4-33
Review (cont’d):
• What is the difference between private and
public methods?
• Why are fields usually private?
• What is inheritance?
• Can a subclass add new fields? New
methods?
4-34