09slide.ppt

Liang, Introduction to Java Programming, Eleventh Edition, (c) 2018 Pearson Education, Ltd.
All rights reserved.
1
Chapter 9 Objects and Classes

2
Motivations
After learning the preceding chapters, you are capable of
solving many programming problems using selections,
loops, methods, and arrays. However, these Java features
are not sufficient for developing graphical user interfaces
and large scale software systems. In this chapter, we will
start discussing object-oriented programming concepts.

3
Objectives
 To describe objects and classes, and use classes to model objects (§9.2).
 To use UML graphical notation to describe classes and objects (§9.2).
 To demonstrate how to define classes and create objects (§9.3).
 To create objects using constructors (§9.4).
 To access objects via object reference variables (§9.5).
 To define a reference variable using a reference type (§9.5.1).
 To access an object’s data and methods using the object member access operator (.) (§9.5.2).
 To define data fields of reference types and assign default values for an object’s data fields (§9.5.3).
 To distinguish between object reference variables and primitive data type variables (§9.5.4).
 To use the Java library classes Date, Random, and Point2D (§9.6).
 To distinguish between instance and static variables and methods (§9.7).
 To define private data fields with appropriate get and set methods (§9.8).
 To encapsulate data fields to make classes easy to maintain (§9.9).
 To develop methods with object arguments and differentiate between primitive-type arguments and
object-type arguments (§9.10).
 To store and process objects in arrays (§9.11).
 To create immutable objects from immutable classes to protect the contents of objects (§9.12).
 To determine the scope of variables in the context of a class (§9.13).
 To use the keyword this to refer to the calling object itself (§9.14).

4
OO Programming Concepts
• Object-oriented programming (OOP) involves
programming using objects. An object represents an
entity in the real world that can be distinctly identified.
For example, a student, a desk, a circle, a button, and
even a loan can all be viewed as objects.

5
• The state of an object (also known as its properties or attributes)
is represented by data fields with their current values. A circle
object, for example, has a data field radius, which is the
property that characterizes a circle. A rectangle object has the
data fields width and height, which are the properties that
characterize a rectangle
• The behavior of an object (also known as its actions) is defined
by methods. To invoke a method on an object is to ask the object
to perform an action. For example, you may define methods
named getArea() and getPerimeter() for circle objects. A circle
object may invoke getArea() to return its area and getPerim-
eter() to return its perimeter.

6
• Objects of the same type are defined using a common class. A
class is a template, blueprint, or contract that defines what an
object’s data fields and methods will be. An object is an instance
of a class. You can create many instances of a class. Creating an
instance is referred to as instantiation.
• The terms object and instance are often interchangeable. The
relationship between classes and objects is analogous to that
between an apple-pie recipe and apple pies: You can make as
many apple pies as you want from a single recipe.

7
Objects
An object has both a state and behavior. The state
defines the object, and the behavior defines what
the object does.
Class Name: Circle
Data Fields:
radius is _______
Methods:
getArea
Circle Object 1
Data Fields:
radius is 10
Circle Object 2
Data Fields:
radius is 25
Circle Object 3
Data Fields:
radius is 125
A class template
Three objects of
the Circle class

8
Classes
Classes are constructs that define objects of the
same type. A Java class uses variables to define
data fields and methods to define behaviors.
Additionally, a class provides a special type of
methods, known as constructors, which are invoked
to construct objects from the class.

9
Classes
class Circle {
/** The radius of this circle */
double radius = 1.0;
/** Construct a circle object */
Circle() {
}
/** Construct a circle object */
Circle(double newRadius) {
radius = newRadius;
}
/** Return the area of this circle */
double getArea() {
return radius * radius * 3.14159;
}
}
Data field
Method
Constructors

10
Classes
• The Circle class is different from all of the other
classes you have seen thus far. It does not have a main
method and therefore cannot be run; it is merely a
definition for circle objects. The class that contains the
main method will be referred to in this book, for
convenience, as the main class.
• The illustration of class templates and objects in can be
standardized using Unified Modeling Language (UML)
notation. This notation is called a UML class diagram,
or simply a class diagram.

11
Unified Modeling Language (UML) Class
Diagram
In the class diagram, the data field is denoted as
dataFieldName: dataFieldType
The constructor is denoted as:
ClassName(parameterName: parameterType)
The method is denoted as
methodName(parameterName: parameterType): returnType

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Example: SimpleCircle class

13

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• The program contains two classes. The first of these,
TestSimpleCircle, is the main class. Its sole purpose is to
test the second class, SimpleCircle. Such a program that
uses the class is often referred to as a client of the class.
When you run the program, the Java runtime system
invokes the main method in the main class.
• You can put the two classes into one file, but only one
class in the file can be a public class. Furthermore, the
public class must have the same name as the file name.
Therefore, the file name is TestSimpleCircle.java, since
TestSimpleCircle is public.
• Each class in the source code is compiled into a .class
file. When you compile TestSimpleCircle.java, two class
files TestSimpleCircle.class and SimpleCircle.class
are generated,

Combine
Two
Classes
into
One
15

Combine Two Classes into One
16

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Example: Defining Classes and Creating Objects
TV
channel: int
volumeLevel: int
on: boolean
+TV()
+turnOn(): void
+turnOff(): void
+setChannel(newChannel: int): void
+setVolume(newVolumeLevel: int): void
+channelUp(): void
+channelDown(): void
+volumeUp(): void
+volumeDown(): void
The current channel (1 to 120) of this TV.
The current volume level (1 to 7) of this TV.
Indicates whether this TV is on/off.
Constructs a default TV object.
Turns on this TV.
Turns off this TV.
Sets a new channel for this TV.
Sets a new volume level for this TV.
Increases the channel number by 1.
Decreases the channel number by 1.
Increases the volume level by 1.
Decreases the volume level by 1.
The + sign indicates
a public modifier.
The constructor and methods in the TV class are
defined public so they can be accessed from
other classes.

18

19

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Constructors
Circle() {
}
Circle(double newRadius) {
radius = newRadius;
}
Constructors are a special
kind of methods that are
invoked to construct objects.

21
Constructors, cont.
A constructor with no parameters is referred to as a
no-arg constructor.
· Constructors must have the same name as the class itself.
· Constructors do not have a return type—not even void.
· Constructors are invoked using the new operator when an
object is created. Constructors play the role of initializing objects.

22
Creating Objects Using
Constructors
new ClassName();
Example:
new Circle();
new Circle(5.0);

23
Default Constructor
A class may be defined without constructors. In
this case, a no-arg constructor with an empty body
is implicitly defined in the class. This constructor,
called a default constructor, is provided
automatically only if no constructors are explicitly
defined in the class.

24
Declaring Object Reference Variables
Objects are accessed via the object’s reference
variables, which contain references to the objects.
To reference an object, assign the object to a reference
variable.
A class is a reference type, which means that a variable
of the class type can reference an instance of the class.
To declare a reference variable, use the syntax:
ClassName objectRefVar;
Example:
Circle myCircle;

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Declaring/Creating Objects
in a Single Step
ClassName objectRefVar = new ClassName();
Example:
Circle myCircle = new Circle();
Create an object
Assign object reference

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Accessing Object’s Members
 Referencing the object’s data is done using the dot
operator or object member access operator:
objectRefVar.data
e.g., myCircle.radius
 Invoking the object’s method:
objectRefVar.methodName(arguments)
e.g., myCircle.getArea()

27
Trace Code
Circle myCircle = new Circle(5.0);
Circle yourCircle = new Circle();
yourCircle.radius = 100;
Declare myCircle
no value
myCircle

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Trace Code, cont.
yourCircle.radius = 100; : Circle
radius: 5.0
no value
myCircle
Create a circle

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Trace Code, cont.
radius: 5.0
reference value
myCircle
to myCircle

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Trace Code, cont.
radius: 5.0
reference value
myCircle
no value
yourCircle
Declare yourCircle

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Trace Code, cont.
radius: 5.0
reference value
myCircle
no value
yourCircle
: Circle
radius: 1.0
Create a new
Circle object

32
Trace Code, cont.
radius: 5.0
reference value
myCircle
reference value
yourCircle
: Circle
radius: 1.0
to yourCircle

33
Trace Code, cont.
radius: 5.0
reference value
myCircle
reference value
yourCircle
: Circle
radius: 100.0
Change radius in
yourCircle

34
Caution
The data field radius is referred to as an instance variable, because it
is dependent on a specific instance. For the same reason, the method
getArea is referred to as an instance method, because you can
invoke it only on a specific instance.
The object on which an instance method is invoked is called a
calling object.

35
Caution
Recall that you use
Math.methodName(arguments) (e.g., Math.pow(3, 2.5))
to invoke a method in the Math class. Can you invoke getArea() using
SimpleCircle.getArea()? The answer is no.
All the methods used before this chapter are static methods, which
are defined using the static keyword. However, getArea() is non-
static. It must be invoked from an object using
objectRefVar.methodName(arguments) (e.g., myCircle.getArea()).

36
Reference Data Fields
The data fields can be of reference types. For example,
the following Student class contains a data field name of
the String type.
public class Student {
String name; // name has default value null
int age; // age has default value 0
boolean isScienceMajor; // isScienceMajor has default value false
char gender; // c has default value 'u0000'
}

37
The null Value
If a data field of a reference type does not
reference any object, the data field holds a
special literal value, null.

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Default Value for a Data Field
The default value of a data field is null for a
reference type, 0 for a numeric type, false for a
boolean type, and 'u0000' for a char type.
However, Java assigns no default value to a local
variable inside a method.
public class Test {
public static void main(String[] args) {
Student student = new Student();
System.out.println("name? " + student.name);
System.out.println("age? " + student.age);
System.out.println("isScienceMajor? " + student.isScienceMajor);
System.out.println("gender? " + student.gender);
}
}

39
Example
public class Test {
public static void main(String[] args) {
int x; // x has no default value
String y; // y has no default value
System.out.println("x is " + x);
System.out.println("y is " + y);
}
}
Compile error: variable not
initialized
Java assigns no default value to a local variable
inside a method.

40
Differences between Variables of
Primitive Data Types and Object Types
• Every variable represents a memory location that holds
a value. When you declare a variable, you are telling
the compiler what type of value the variable can hold.
For a variable of a primitive type, the value is of the
primitive type. For a variable of a reference type, the
value is a reference to where an object is located.

41
Copying Variables of Primitive Data Types and Object Types

42
Garbage Collection
As shown in the previous figure, after the
assignment statement c1 = c2, c1 points to
the same object referenced by c2. The object
previously referenced by c1 is no longer
referenced. This object is known as garbage.
Garbage is automatically collected by JVM.

43
Garbage Collection, cont
TIP: If you know that an object is no longer
needed, you can explicitly assign null to a
reference variable for the object. The JVM
will automatically collect the space if the
object is not referenced by any variable.

44
The Date Class
Java provides a system-independent encapsulation of date
and time in the java.util.Date class. You can use the Date
class to create an instance for the current date and time and
use its toString method to return the date and time as a string.

45
The Date Class Example

46
The Random Class
You have used Math.random() to obtain a random double
value between 0.0 and 1.0 (excluding 1.0). A more useful
random number generator is provided in the java.util.Random
class.
java.util.Random
+Random()
+Random(seed: long)
+nextInt(): int
+nextInt(n: int): int
+nextLong(): long
+nextDouble(): double
+nextFloat(): float
+nextBoolean(): boolean
Constructs a Random object with the current time as its seed.
Constructs a Random object with a specified seed.
Returns a random int value.
Returns a random int value between 0 and n (exclusive).
Returns a random long value.
Returns a random double value between 0.0 and 1.0 (exclusive).
Returns a random float value between 0.0F and 1.0F (exclusive).
Returns a random boolean value.

47
The Random Class Example
If two Random objects have the same seed, they will generate
identical sequences of numbers. For example, the following
code creates two Random objects with the same seed 3.
Random random1 = new Random(3);
System.out.print("From random1: ");
for (int i = 0; i < 10; i++)
System.out.print(random1.nextInt(1000) + " ");
Random random2 = new Random(3);
System.out.print("nFrom random2: ");
for (int i = 0; i < 10; i++)
System.out.print(random2.nextInt(1000) + " ");
From random1: 734 660 210 581 128 202 549 564 459 961
From random2: 734 660 210 581 128 202 549 564 459 961

48
The Point2D Class
Java API has a conveninent Point2D class in the
javafx.geometry package for representing a point in a two-
dimensional plane.

49
The Point2D Class

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Instance Variables, and Methods
• Instance variables belong to a specific instance.
• Instance methods are invoked by any instance
of the class.

51
Static Variables, Constants,
and Methods
• Static variables are shared by all the instances of
the class.
• Static methods are not tied to a specific object.
Because of this, a static method cannot access
instance members of the class
• Static constants are final variables shared by all
the instances of the class.
• A non-static (or instance) variable is tied to a
specific instance

52
and Methods
• Static variables store values for the variables in
a common memory location. Because of this
common location, if one object changes the
value of a static variable, all objects of the
same class are affected.
• Java supports static methods as well as static
variables. Static methods can be called without
creating an instance of the class.

53
and Methods, cont.
• To declare static variables, constants, and methods,
use the static modifier. For example, the constant
PI in the Math class is defined as
final static double PI=3.14159265358979323846
• Let’s modify the Circle class by adding a static
variable numberOfObjects to count the number of
circle objects created. When the first object of this class
is created, numberOfObjects is 1. When the second
object is created, numberOfObjects becomes 2. The
UML of the new circle class is shown below

54
and Methods, cont.

55
CircleWithStaticMembers Class

56
TestCircleWithStaticMembers.java
Static variables and
methods can be accessed
without creating objects.
Line 6 displays the number
of objects, which is 0,
since no objects have been
created.

57
Static and Instance Methods
An instance method can invoke an instance or static
method and access an instance or static data field. A static
method can invoke a static method and access a static data
field. However, a static method cannot invoke an instance
method or access an instance data field, since static
methods and static data fields don’t belong to a particular
object.

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Examples

59
Examples

60
Visibility Modifiers and Accessor/Mutator Methods
You can use the public visibility modifier for classes, methods,
and data fields to denote that they can be accessed from any
other classes.
If no visibility modifier is used, then by default the classes,
methods, and data fields are accessible by any class in the same
package. This is known as package-private or package-access.
Packages can be used to organize classes. To do so, you need to
add the following line as the first statement in the program:
package packageName;
If a class is defined without the package statement, it is said to be
placed in the default package.

61
Visibility Modifiers and Accessor/Mutator Methods
By default, the class, variable, or method can be
accessed by any class in the same package.
 public
The class, data, or method is visible to any class in any
package.
 private
The data or methods can be accessed only by the declaring
class.
Public getter (Accessor) and setter (Mutator) methods are
used to read and modify private properties.

62
• The private modifier restricts access to within a class,
• The default modifier restricts access to within a package,
• The public modifier enables unrestricted access.
Examples

63
• The default modifier on a class restricts access to within a
package
• The public modifier enables unrestricted access.
Visibility Modifier of Classes

64
NOTE
• An object cannot access its private members, as shown in
(b). It is OK, however, if the object is declared in its own
class, as shown in (a).

65
NOTE
• The private modifier applies only to the members of a class.
The public modifier can apply to a class or members of a class.
Using the modifiers public and private on local variables
would cause a compile error.
• In most cases, the constructor should be public. However, if you
want to prohibit the user from creating an instance of a class,
use a private constructor. For example, there is no reason to
create an instance from the Math class, because all of its data
fields and methods are static. To prevent the user from creating
objects from the Math class, the constructor in java.lang.Math
is defined as private.

66
Data Field Encapsulation
Making data fields private protect data.
Making data fields private helps to make code easy to
maintain since the client programs cannot modify them.
To prevent direct modifications of data fields, declaring the
data fields private is known as data field encapsulation.
To make a private data field accessible, provide a getter
method to return its value. To enable a private data field to
be updated, provide a setter method to set a new value. A
getter method is also referred to as an accessor and a setter
to a mutator.

67
Example of

68
Example of

69
Example of Data Field Encapsulation

70
Example of Data Field Encapsulation

71
Passing Objects to Methods
 Passing by value for primitive type value
(the value is passed to the parameter)
 Passing by value for reference type value
(the value is the reference to the object)

72

73
Passing a primitive type value and a reference value

74
Passing a primitive type value and a reference value
When passing an argument of a reference type, the reference
of the object is passed. In this case, c contains a reference for
the object that is also referenced via myCircle. Therefore,
changing the properties of the object through c inside the
printAreas method has the same effect as doing so outside the
method through the variable myCircle. Pass-by-value on
references can be best described semantically as pass-by-
sharing; that is, the object referenced in the method is the
same as the object being passed.

75

76
Array of Objects
Circle[] circleArray = new Circle[10];
An array of objects is actually an array of reference
variables. So invoking circleArray[1].getArea() involves
two levels of referencing as shown in the next figure.
circleArray references to the entire array. circleArray[1]
references to a Circle object.

77
Array of Objects
Circle[] circleArray = new Circle[10];

78
Array of Objects
To initialize circleArray, you can use a for loop
for (int i = 0; i < circleArray.length; i++) {
circleArray[i] = new Circle();
}
An array of objects is actually an array of reference
variables.
When an array of objects is created using the new
operator, each element in the array is a reference variable
with a default value of null.

79
Summarizing the areas of the circles

80

81

82
Immutable Objects
• Normally, you create an object and allow its
contents to be changed later. However,
occasionally it is desirable to create an object
whose contents cannot be changed once the
object has been created. We call such an object
as immutable object and its class as immutable
class.
• If a class is immutable, then all its data fields must
be private and it cannot contain public setter
methods for any data fields. A class with all private
data fields and no mutators is not necessarily
immutable.

83
Example

84
Example

85
Scope of Variables
 The scope of instance and static variables is the
entire class. They can be declared anywhere inside
a class.
 The scope of a local variable (i.e. a variable
defined in a method) starts from its declaration
and continues to the end of the block that contains
the variable. A local variable must be initialized
explicitly before it can be used.

86
Scope of Variables

87
Scope of Variables
• If a local variable has the same name as a class’s
variable, the local variable takes precedence and the
class’s variable with the same name is hidden.

88
The this Keyword
 The this keyword is the name of a reference that
refers to an object itself. One common use of the
this keyword is reference a class’s hidden data
fields.
 Another common use of the this keyword to
enable a constructor to invoke another
constructor of the same class.

89
The this Keyword

90
Reference the Hidden Data Fields
The this keyword can be used to reference a class’s hidden data fields.
For example, a data-field name is often used as the parameter name in a
setter method for the data field. In this case, the data field is hidden in
the setter method. You need to reference the hidden data-field name in
the method in order to set a new value to it. A hidden static variable can
be accessed using the keyword this.

91
Reference the Hidden Data Fields
The this keyword gives us a way to reference the object that invokes an instance
method. To invoke f1.setI(10), this.i = i is executed, which assigns the value of
parameter i to the data field i of this calling object f1. The keyword this refers to
the object that invokes the instance method setI, as shown below:

92
Calling Overloaded Constructor
The this keyword can be used to invoke another
constructor of the same class. For example, you can
rewrite the Circle class as follows:

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