Download Chapter 8: User-Defined Classes and ADTs

Chapter 8: User-Defined
Classes and ADTs
Java Programming:
Program Design Including Data Structures
Chapter Objectives
 Learn about classes
 Learn about private, public, and static
members of a class
 Explore how classes are implemented
 Learn about the various operations on classes
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Chapter Objectives (continued)
 Examine constructors and finalizers
 Examine the method toString
 Learn about the abstract data type (ADT)
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Point objects
Point p1 = new Point(5, -2);
Point p2 = new Point();
// origin, (0, 0)
• Data in each Point object:
Field name
Description
x
the point's x-coordinate
y
the point's y-coordinate
• Methods in each Point object:
Method name
Description
setLocation(x, y)
sets the point's x and y to the given values
transform(dx, dy)
adjusts the point's x and y by the given amounts
distance(p)
how far away the point is from point p
draw(g)
displays the point on a drawing panel
Point class as blueprint
Point class
state:
int x,
y
behavior:
setLocation(int x, int y)
transform(int dx, int dy)
distance(Point p)
draw(Graphics g)
Point object #1
state:
x = 5,
y = -2
behavior:
setLocation(int x, int y)
transform(int dx, int dy)
distance(Point p)
draw(Graphics g)
Point object #2
state:
x = -245,
y = 1897
behavior:
setLocation(int x, int y)
transform(int dx, int dy)
distance(Point p)
draw(Graphics g)
Point object #3
state:
x = 18,
y = 42
behavior:
setLocation(int x, int y)
transform(int dx, int dy)
distance(Point p)
draw(Graphics g)
– The class (blueprint) will describe how to create objects.
– Each object will contain its own data and methods.
Point class, version 1
public class Point {
int x;
int y;
}
– Save this code into a file named Point.java.
• The above code creates a new type named Point.
– Each Point object contains two pieces of data:
• an int named x, and
• an int named y.
– Point objects do not contain any behavior (yet).
Accessing fields
• Other classes can access/modify an object's fields.
– access:
variable.field
– modify:
variable.field = value;
• Example:
Point p1 = new Point();
Point p2 = new Point();
System.out.println("the x-coord is " + p1.x);
p2.y = 13;
// access
// modify
A class and its client
• Point.java is not, by itself, a runnable program.
– A class can be used by client programs.
PointMain.java (client program)
public class PointMain {
main(String args) {
Point p1 = new Point();
p1.x = 7;
p1.y = 2;
Point p2 = new Point();
p2.x = 4;
p2.y = 3;
...
}
}
Point.java (class of objects)
public class Point {
int x;
int y;
}
x
7
y
2
x
4
y
3
PointMain client example
public class PointMain {
public static void main(String[] args) {
// create two Point objects
Point p1 = new Point();
p1.y = 2;
Point p2 = new Point();
p2.x = 4;
}
}
System.out.println(p1.x + ", " + p1.y);
// 0, 2
// move p2 and then print it
p2.x += 2;
p2.y++;
System.out.println(p2.x + ", " + p2.y);
// 6, 1
Classes
 class: A collection of a fixed number of
components
 Components: Members of a class
 Members are accessed by name.
 Class categories/modifiers:
– private
– public
– protected (discussed at chapter 11)
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Classes (continued)
 Private: members of class are not accessible
outside class
 Public: members of class are accessible outside
class
 Class members: can be methods or variables
 Variable members are declared like any other
variables
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Syntax
The general syntax for defining a class:
 If a member of a class is a named constant, you declare
it just like any other named constant
 If a member of a class is a variable, you declare it just
like any other variable
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Syntax (continued)
• If a member of a class is a method, you define it
just like any other method
• If a member of a class is a method, it can
(directly) access any member of the class—data
members and methods
• When you write the definition of a method, you
can directly access any data member of the class
(without passing it as a parameter)
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Syntax (continued)
class Clock:
Data Members (Instance Variables):
• private int hr; //store hours
• private int min; //store minutes
• private int sec; //store seconds
Methods:
• public void setTime(int hours, int minutes,
int seconds)
• public int getHours()
• public int getMinutes()
• public int getSeconds()
• public void printTime()
• public void incrementSeconds()
• public void incrementMinutes()
• public void incrementHours()
• public boolean equals(Clock otherClock)
• public void makeCopy(Clock otherClock)
• public Clock getCopy()
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Instance methods
• instance method (or object method): Exists inside each object
of a class and gives behavior to each object.
public type name(parameters) {
statements;
}
– same syntax as static methods, but without static keyword
Example:
public void shout() {
System.out.println("HELLO THERE!");
}
Instance method example
public class Point {
int x;
int y;
// Draws this Point object with the given pen.
public void transform(int dx, int dy) {
...
}
}
– The transform method no longer has a Point p parameter.
– How will the method know which point to transform?
• How will the method access that point's x/y data?
• Implicit parameter, this
Constructors
 constructor: Initializes the state of new objects.
public type(parameters) {
statements;
}
 runs when the client uses the new keyword
 no return type is specified;
it implicitly "returns" the new object being created
 If a class has no constructor, Java gives it a default
constructor with no parameters that sets all fields to 0.
 Two types of constructors:
 With parameters
 Without parameters (default constructor)
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Constructors (continued)
• Constructors have the following properties:
– The name of a constructor is the same as the name
of the class
– A constructor has no return type
– A class can have more than one constructor
– All constructors of a class have the same name
– If a class has more than one constructor, any two
constructors must have different signatures
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Constructors (continued)
– Constructors are automatically executed when a
class object is instantiated
– With multiple constructors, which constructor
executes depends on the type of values passed to
the class object when the class object is
instantiated
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Constructors (continued)
class Clock: Constructors
 Default constructor is public Clock()
Constructor with parameters:
 public Clock(int hours, int
minutes,int seconds)
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Unified Modeling Language Class
Diagrams
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Variable Declaration and Object
Instantiation
 General syntax for using the operator new is:
new className()
OR
new className(argument1, argument2, ...,
argumentN)
Clock myClock;
Clock yourClock;
myClock = new Clock();
yourClock = new Clock(9, 35, 15);
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Variable Declaration and Object
Instantiation (continued)
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Accessing Class Members
• To access a data member of a class object or method:
referenceVariableName.memberName
Example 8-1
myClock.setTime(5, 2, 30);
myClock.printTime();
yourClock.setTime(x, y, z);
if (myClock.equals(yourClock))
…
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Assignment Operator: A Precaution
 Consider:
myClock = yourClock;
 Copies the value of the reference variable yourClock
into the reference variable myClock
 After execution, both variables refer to same object
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Assignment Operator: A Precaution
(continued)
 Shallow copying: two or more reference variables of
the same type point to the same object
 Deep copying: each reference variable refers to its
own object
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The this keyword
•this : Refers to the implicit parameter
inside your class.
(a variable that stores the object on which a method is called)
– Refer to a field:
this.field
– Call a method:
this.method(parameters);
– One constructor
can call another:
this(parameters);
The Copy Constructor
 Executes when an object is instantiated
 Initialized using an existing object
 Syntax:
public ClassName(ClassName otherObject)
…
Clock theClock = new Clock(myClock);
…
public Clock(Clock otherClock) {
this(otherClock.hr, otherClock.min, otherClock.sec);
} // or
public Clock(Clock otherClock) {
hr = otherClock.hr;
min = otherClock.min;
sec = otherClock.sec;
}
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Example: class Clock
myClock.makeCopy(yourClock);
…
public void makeCopy(Clock otherClock) {
hr = otherClock.hr;
min = otherClock.min;
sec = otherClock.sec;
}
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The Method toString




Public value-returning method
Takes no parameters
Returns address of a String object
Output using print, println, printf
methods
 Default definition creates String with name of
object’s class name followed by hash code of
object
System.out.println(myClock);
is:
Clock@11b86e7
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Example: toString()
System.out.pringln (myClock.toString());
…
public String toString(Clock clockObject) {
String str;
…
return str;
}
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The Modifier static Keyword
 In the method heading, it specifies that the method can
be invoked by using the name of the class
Arrays.toString(), Arrays.sort(),
Math.sqrt(), Math.min()
 If it isused to declare data member  Data member
invoked by using the class name
Math.PI, Bank.count
 Static data members of a class exist even when no
object of class type instantiated
 Static variables are initialized to their default values
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The Modifier static
(continued)
Example 8-3
public class Illustrate
{
private int x;
private static int y;
public static int count;
public Illustrate()
{
x = 0;
}
public Illustrate(int a)
{
x = a;
}
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The Modifier static
(continued)
void setX(int a)
{
x = a;
}
public String
{
return("x
+ ",
}
public static
{
y++;
}
toString()
= " + x + ", y = " + y
count = " + count);
void incrementY()
}
Illustrate illusObject = new Illustrate();
Illustrate.incrementY();
Illustrate.count++;
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The Modifier static
(continued)
Illustrate illusObject1 = new Illustrate(3);
Illustrate illusObject2 = new Illustrate(5);
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The Modifier static
(continued)
Illustrate.incrementY();
Illustrate.count++;
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Finalizers
 Automatically execute when class object goes out
of scope
 Have no parameters
 Only one finalizer per class
 Name of finalizer: finalize
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Accessor and Mutator Methods
 Accessor method: A method of a class that only
accesses (that is, does not modify) the value(s) of
the data member(s)
 Mutator method: modifies the value(s) of the data
member(s)
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Creating Packages
You can create packages using a reserved word package
 Define the class to be public. (If class is not
public, it can only be used within package.)
 Choose name for package
 Organize package (create subdirectories)
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Creating Package for class Clock
package jpfpatpd.ch08.clockPackage;
public class Clock
{
//put instance variables and methods,
//as before, here
}
import jpfpatpd.ch08.clockPackage.Clock;
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The Reference this
 Refers to instance variables and methods of a
class
 Used to implement cascaded method calls
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Inner Classes
 Defined within other classes
 Can be either a complete class definition or an
anonymous inner class definition
 Used to handle events
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Abstract Data Type
A data type that specifies the logical properties
without the implementation details
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Programming Example:
Candy Machine (Problem
Statement)
A new candy machine is bought for the gym, but it is
not working properly. The machine sells candies,
chips, gum, and cookies. In this programming
example, we will write a program to create a Java
application program for this candy machine so that it
can be put into operation. We will divide this
program in two parts. In the first part, we will design
a non-GUI application program. In the second part,
we will design an application program that will
create a GUI, as described in the second part.
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Programming Example:
Candy Machine (Problem
Statement continued)
• The non-GUI application program should do the
following:
1.
2.
3.
4.
5.
Show the customer the different products sold
Let the customer make the selection
Show the customer the cost of the item selected
Accept money from the customer
Release the item
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Programming Example:
Candy Machine (Input and
Output)
 Input: The item selection and the cost of the item
 Output: The selected item
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Programming Example:
Candy Machine
Components:
 Cash register
 Dispenser
 Machine
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Programming Example:
Candy Machine (continued)
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Programming Example:
Candy Machine (continued)
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Programming Example:
Candy Machine (continued)
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Programming Example:
Candy Machine (continued)
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Programming Example:
Candy Machine (continued)
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Programming Example:
Candy Machine (continued)
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Chapter Summary
 Creating classes
 Members of a class:




private
protected
public
static
 Implementing classes
 Various operations on classes
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Chapter Summary (continued)
 Constructors
 Finalizers
 Method toString
 Abstract data types
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