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. An object has a unique identity, state, and
behaviors. The state of an object consists of a set of
data fields (also known as properties) with their
current values. The behavior of an object is defined
by a set of methods.
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Objects
Class Name: Circle

A class template

Data Fields:
radius is _______
Methods:
getArea

Circle Object 1

Circle Object 2

Circle Object 3

Data Fields:
radius is 10

Data Fields:
radius is 25

Data Fields:
radius is 125

Three objects of
the Circle class

An object has both a state and behavior. The state

defines the object, and the behavior defines what
the object does.
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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.

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Classes
class Circle {
/** The radius of this circle */
double radius = 1.0;
/** Construct a circle object */
Circle() {
}

Data field

Constructors

/** Construct a circle object */

Circle(double newRadius) {
radius = newRadius;
}

/** Return the area of this circle */

double getArea() {
return radius * radius * 3.14159;
}

Method

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UML Class Diagram

Circle

UML Class Diagram

Class name

radius: double

Data fields

Circle()

Constructors and
Methods

Circle(newRadius: double)
getArea(): double

circle1: Circle
radius: 10

circle2: Circle
radius: 25

circle3: Circle

UML notation
for objects

radius: 125

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Constructors
Circle() {
}

Constructors are a special

kind of methods that are
invoked to construct objects.

Circle(double newRadius) {
radius = newRadius;
}

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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 typenot

even void.

Constructors are invoked using the new

operator when an object is created. Constructors
play the role of initializing objects.
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Creating Objects Using

Constructors
new ClassName();

Example:
new Circle();

new Circle(5.0);

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

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Declaring Object Reference Variables

To reference an object, assign the object to a reference
variable.
To declare a reference variable, use the syntax:
ClassName objectRefVar;

Example:
Circle myCircle;
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10

Declaring/Creating Objects
in a Single Step
ClassName objectRefVar = new ClassName();

Example:

Assign object reference

Create an object

Circle myCircle = new Circle();

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11

Accessing Objects

Referencing the objects data:

objectRefVar.data

e.g., myCircle.radius

Invoking the objects method:

objectRefVar.methodName(arguments)

e.g., myCircle.getArea()

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12

A Simple Circle Class

Objective:

Demonstrate creating objects,

accessing data, and using methods.

TestCircle1

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animation

Trace Code
Declare myCircle

Circle myCircle = new Circle(5.0);

myCircle

no value

SCircle yourCircle = new Circle();

yourCircle.radius = 100;

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animation

Trace Code, cont.

Circle myCircle = new Circle(5.0);

myCircle

no value

Circle yourCircle = new Circle();

: Circle

yourCircle.radius = 100;

radius: 5.0

Create a circle

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animation

Trace Code, cont.

Circle myCircle = new Circle(5.0);

myCircle reference value

Circle yourCircle = new Circle();

yourCircle.radius = 100;

Assign object
reference to myCircle

: Circle
radius: 5.0

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animation

Trace Code, cont.

Circle myCircle = new Circle(5.0);

myCircle reference value

Circle yourCircle = new Circle();

yourCircle.radius = 100;

: Circle
radius: 5.0

yourCircle

no value

Declare yourCircle

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animation

Trace Code, cont.

Circle myCircle = new Circle(5.0);

myCircle reference value

Circle yourCircle = new Circle();

: Circle

yourCircle.radius = 100;

radius: 5.0

no value

yourCircle

: Circle
Create a new
Circle object

radius: 0.0

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animation

Trace Code, cont.

Circle myCircle = new Circle(5.0);

myCircle reference value

Circle yourCircle = new Circle();

: Circle

yourCircle.radius = 100;

radius: 5.0

yourCircle reference value

Assign object
reference to yourCircle

: Circle
radius: 1.0

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animation

Trace Code, cont.

Circle myCircle = new Circle(5.0);

myCircle reference value

Circle yourCircle = new Circle();

: Circle

yourCircle.radius = 100;

radius: 5.0
yourCircle reference value

: Circle

Change radius in
yourCircle

radius: 100.0

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20

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
Circle1.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()).

More explanations will be given in Section 7.7, Static Variables,

Constants, and Methods.
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21

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'
}

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22

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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23

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);
}
}

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Example
Java assigns no default value to a local variable inside a method.

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);
}
}

Compilation error: variables not

initialized
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25

Differences between Variables of

Primitive Data Types and Object Types
Created using new Circle()

Primitive type

int i = 1

Object type

Circle c

reference

c: Circle
radius = 1

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26

Copying Variables of Primitive

Data Types and Object Types
Primitive type assignment i = j
Before:

After:

2
Object type assignment c1 = c2
Before:

After:

c1

c1

c2

c2

c1: Circle

C2: Circle

c1: Circle

C2: Circle

radius = 5

radius = 9

radius = 5

radius = 9

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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.

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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.

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29

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.
The + sign indicates
public modifer

java.util.Date
+Date()

Constructs a Date object for the current time.

+Date(elapseTime: long)

Constructs a Date object for a given time in

milliseconds elapsed since January 1, 1970, GMT.

+toString(): String

Returns a string representing the date and time.

+getTime(): long

Returns the number of milliseconds since January 1,

1970, GMT.

+setTime(elapseTime: long): void

Sets a new elapse time in the object.

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The Date Class Example

For example, the following code

java.util.Date date = new java.util.Date();

System.out.println(date.toString());

displays a string likeSun Mar 09 13:50:19

EST 2003.

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31

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()

Constructs a Random object with the current time as its seed.

+Random(seed: long)

Constructs a Random object with a specified seed.

+nextInt(): int

Returns a random int value.

+nextInt(n: int): int

Returns a random int value between 0 and n (exclusive).

+nextLong(): long

Returns a random long value.

+nextDouble(): double

Returns a random double value between 0.0 and 1.0 (exclusive).

+nextFloat(): float

Returns a random float value between 0.0F and 1.0F (exclusive).

+nextBoolean(): boolean

Returns a random boolean value.

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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
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Instance
Variables, and Methods

Instance variables belong to a specific instance.

Instance methods are invoked by an instance of
the class.

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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.
Static constants are final variables shared by all the
instances of the class.

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Static Variables, Constants,

and Methods, cont.
To declare static variables, constants, and methods,
use the static modifier.

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Static Variables, Constants,

and Methods, cont.
instantiate

circle1
radius = 1
numberOfObjects = 2

Circle
radius: double
numberOfObjects: int
getNumberOfObjects(): int
+getArea(): double

UML Notation:
underline: static variables or methods

instantiate

Memory
1

radius

numberOfObjects

radius

circle2
radius = 5
numberOfObjects = 2

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Example of
Using Instance and Class Variables
and Method
Objective: Demonstrate the roles of
instance and class variables and their
uses. This example adds a class variable
numberOfObjects to track the number of
Circle objects created.
Circle2
TestCircle2
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38

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.
The get and set methods are used to read and modify private
properties.
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package p1;

package p2;

public class C1 {
public int x;
int y;
private int z;

public void m1() {

}
void m2() {
}
private void m3() {
}

public class C2 {
void aMethod() {
C1 o = new C1();
can access o.x;
can access o.y;
cannot access o.z;

can invoke o.m1();

can invoke o.m2();
cannot invoke o.m3();

package p1;
class C1 {
...
}

public class C3 {
void aMethod() {
C1 o = new C1();
can access o.x;
cannot access o.y;
cannot access o.z;
can invoke o.m1();
cannot invoke o.m2();
cannot invoke o.m3();
}

package p2;
public class C2 {
can access C1
}

public class C3 {
cannot access C1;
can access C2;
}

The private modifier restricts access to within a class, the default

modifier restricts access to within a package, and the public
modifier enables unrestricted access.
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40

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).
public class Foo {
private boolean x;
public static void main(String[] args) {
Foo foo = new Foo();
System.out.println(foo.x);
System.out.println(foo.convert());
}

public class Test {

public static void main(String[] args) {
Foo foo = new Foo();
System.out.println(foo.x);
System.out.println(foo.convert(foo.x));
}
}

private int convert(boolean b) {

return x ? 1 : -1;
}
}
(a) This is OK because object foo is used inside the Foo class

(b) This is wrong because x and convert are private in Foo.

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Why Data Fields Should Be

private?
To protect data.
To make class easy to maintain.

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42

Example of
Data Field Encapsulation
The - sign indicates
private modifier

Circle
-radius: double

The radius of this circle (default: 1.0).

-numberOfObjects: int

The number of circle objects created.

+Circle()

Constructs a default circle object.

+Circle(radius: double)

Constructs a circle object with the specified radius.

+getRadius(): double

Returns the radius of this circle.

+setRadius(radius: double): void

Sets a new radius for this circle.

+getNumberOfObject(): int

Returns the number of circle objects created.

+getArea(): double

Returns the area of this circle.

Circle3

TestCircle3

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Immutable Objects and Classes

If the contents of an object cannot be changed once the object
is created, the object is called an immutable object and its class
is called an immutable class. If you delete the set method in
the Circle class in the preceding example, the class would be
immutable because radius is private and cannot be changed
without a set method.
A class with all private data fields and without mutators is not
necessarily immutable. For example, the following class
Student has all private data fields and no mutators, but it is
mutable.

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Example
public class Student {
private int id;
private BirthDate birthDate;

public class BirthDate {

private int year;
private int month;
private int day;

public Student(int ssn,

int year, int month, int day) {
id = ssn;
birthDate = new BirthDate(year, month, day);
}

public BirthDate(int newYear,

int newMonth, int newDay) {
year = newYear;
month = newMonth;
day = newDay;
}

public int getId() {

return id;
}
public BirthDate getBirthDate() {
return birthDate;
}
}

public void setYear(int newYear) {

year = newYear;
}
}

public class Test {

public static void main(String[] args) {
Student student = new Student(111223333, 1970, 5, 3);
BirthDate date = student.getBirthDate();
date.setYear(2010); // Now the student birth year is changed!
}
}
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What Class is Immutable?

For a class to be immutable, it must mark all data fields private
and provide no mutator methods and no accessor methods that
would return a reference to a mutable data field object.

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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)
TestPassObject

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Passing Objects to Methods, cont.

Stack
Space required for the
printAreas method
int times: 5
Circle c: reference

Pass by value (here

the value is 5)
Pass by value
(here the value is
the reference for
the object)

Space required for the

main method
int n: 5
myCircle: reference

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Heap

A circle
object

48

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 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.

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The this Keyword

Use this to refer to the object that invokes

the instance method.

Use this to refer to an instance data field.

Use this to invoke an overloaded

constructor of the same class.

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Serving as Proxy to the Calling Object

class Foo {
int i = 5;
static double k = 0;
void setI(int i) {
this.i = i;
}

Suppose that f1 and f2 are two objects of Foo.

Invoking f1.setI(10) is to execute
f1.i = 10, where this is replaced by f1
Invoking f2.setI(45) is to execute
f2.i = 45, where this is replaced by f2

static void setK(double k) {

Foo.k = k;
}

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Calling Overloaded Constructor

public class Circle {
private double radius;
public Circle(double radius) {
this.radius = radius;
}
this must be explicitly used to reference the data
field radius of the object being constructed

public Circle() {
this(1.0);
}

this is used to invoke another constructor

public double getArea() {

return this.radius * this.radius * Math.PI;
}
Every instance variable belongs to an instance represented by this,
which is normally omitted
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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.
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Array of Objects, cont.

Circle[] circleArray = new Circle[10];

circleArray

reference

circleArray[0]
circleArray[1]

Circle object 0

Circle object 1

circleArray[9]

Circle object 9

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Array of Objects, cont.

Summarizing the areas of the circles

TotalArea

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Class Abstraction and Encapsulation

Class abstraction means to separate class implementation
from the use of the class. The creator of the class provides
a description of the class and let the user know how the
class can be used. The user of the class does not need to
know how the class is implemented. The detail of
implementation is encapsulated and hidden from the user.
Class implementation
is like a black box
hidden from the clients

Class

Class Contract
(Signatures of
public methods and
public constants)

Clients use the

class through the
contract of the class

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56