Objects and
Classes
Jericho Jover V. Magpantay
Department of Information Technology
Cavite State University – Trece Campus
TOPICS
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Objects and Classes
What is Object in Java?
Characteristics of Object
What is a Class in Java?
Instance Variable in Java
Objects and Classes
In Java, classes and objects are basic concepts of Object Oriented
Programming (OOPs) that are used to represent real-world concepts and
entities. The class represents a group of objects having similar properties
and behavior. For example, the animal type Dog is a class while a particular
dog named Tommy is an object of the Dog class.
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Java Classes
A class in Java is a set of objects which shares common characteristics/
behavior and common properties/ attributes. It is a user-defined blueprint
or prototype from which objects are created.
For example, Student is a class while a particular student named Ravi is an
object.
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Properties of Java Classes
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Class is not a real-world entity. It is just a template or blueprint or
prototype from which objects are created.
Class does not occupy memory.
Class is a group of variables of different data types and a group of
methods.
A Class in Java can contain:
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Data member
Method
Constructor
Nested Class
Interface
Class Declaration in Java
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Example of Java Class
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Components of Java Classes
In general, class declarations can include these components, in order:
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Modifiers : A class can be public or has default access.
Class keyword: class keyword is used to create a class.
Class name: The name should begin with an initial letter (capitalized by convention).
Superclass(if any): The name of the class’s parent (superclass), if any, preceded by the keyword
extends. A class can only extend (subclass) one parent.
Interfaces(if any): A comma-separated list of interfaces implemented by the class, if any, preceded by
the keyword implements. A class can implement more than one interface.
Body: The class body is surrounded by braces, { }.
Java Objects
An object in Java is a basic unit of Object-Oriented Programming and
represents real-life entities. Objects are the instances of a class that are
created to use the attributes and methods of a class. A typical Java program
creates many objects, which as you know, interact by invoking methods.
An object consists of :
1. State : It is represented by attributes of an object. It also reflects the
properties of an object.
2.
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Behavior : It is represented by the methods of an object. It also reflects
the response of an object with other objects.
Identity : It gives a unique name to an object and enables one object to
interact with other objects.
Java Objects : Example
Note: When we create an object which is a non primitive data type, it’s
always allocated on the heap memory.
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Declaring Objects (Also called instantiating a class)
When an object of a class is created, the class is said to be instantiated . All
the instances share the attributes and the behavior of the class. But the
values of those attributes, i.e. the state are unique for each object. A single
class may have any number of instances.
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Declaring Objects : Example
As we declare variables like (type name;). This notifies the compiler that we
will use the name to refer to data whose type is type. With a primitive
variable, this declaration also reserves the proper amount of memory for the
variable. So for reference variables , the type must be strictly a concrete
class name. In general, we can’t create objects of an abstract class or an
interface.
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Initializing a Java object
The new operator instantiates a class by allocating memory for a new object
and returning a reference to that memory. The new operator also invokes
the class constructor.
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Initializing a Java object: Example
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Initializing a Java object: Example
This class contains a single constructor. We can recognize a constructor because its
declaration uses the same name as the class and it has no return type. The Java compiler
differentiates the constructors based on the number and the type of the arguments. The
constructor in the Student class takes four arguments. The following statement provides
“Stephen Navarro”, “General Trias, Cavite”,35, and “2015-100-450” as values for those
arguments:
An Address
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Name: Stephen Navarro
Address: General Trias, Cavite
Age: 25
Student Number: 2015-100-450
Initialize by using method/function: Example
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Initializing a Java object
Note: All classes have at least one constructor. If a class does not explicitly declare any,
the Java compiler automatically provides a no-argument constructor, also called the default
constructor. This default constructor calls the class parent’s no-argument constructor (as it
contains only one statement i.e super();), or the Object class constructor if the class has no
other parent (as the Object class is the parent of all classes either directly or indirectly).
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Ways to Create an Object of a Class
There are four ways to create objects in Java. Strictly speaking, there is only
one way (by using a new keyword), and the rest internally use a new
keyword.
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Ways to Create an Object of a Class
1. Using new keyword - It is the most common and general way to create an
object in Java.
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Ways to Create an Object of a Class
3. Using clone() method - clone() method is present in the Object class. It
creates and returns a copy of the object.
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Ways to Create an Object of a Class
4. Deserialization - De-serialization is a technique of reading an object from
the saved state in a file.
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Instance variable as final in Java
The instance variable is declared inside a class but not within any method,
constructor, block etc. If we don’t initialize an instance variable, then JVM
automatically provide default value according to the data type of that
instance variable.
But if we declare an instance variable as final, then we must have to take
care about the behavior of instance variable.
• There is no requirement as such to make the variable final. However,
when it is true that you explicitly intend to never change the variable, it is
often good practice to make it final
• We have to use instance variable as final in the creation of immutable
class.
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Important points about final instance variable:
1. Initialization of variable Mandatory : If the instance variable declared as
final, then we have to perform initialization explicitly whether we are using it
or not and JVM won’t provide any default value for the final instance
variable.
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Important points about final instance variable:
2. Initialization before constructor completion : For final instance variable
we have to perform initialization before constructor completion. We can
initialize a final instance variable at the time of declaration.
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Important points about final instance variable:
3. Initialize inside a non-static or instance block : We can also initialize a
final instance variable inside a non-static or instance block also.
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Important points about final instance variable:
4. Initialization in default constructor : Inside default constructor we can
also initialize a final instance variable
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Objects and Classes : Exercise
1. Create a Java project named Objects_and_Classes.
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Objects and Classes : Exercise
2. Create a class Car with attributes and methods that simulate real-life
characteristics of a car.
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Objects and Classes : Exercise
2. Create a class Car with attributes and methods that simulate real-life
characteristics of a car.
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Objects and Classes : Exercise
3. Class Definition: A class is like a
blueprint for creating objects. The class
Car will define the attributes (such as
color, brand, and speed) and methods
(such as drive and brake) of the car.
4. Attributes: These are characteristics
of the car. In Java, attributes are
defined as variables inside the class. For
example, color, brand, and speed.
5. Methods: These are actions that a
car can perform. Methods like drive,
brake, and honk will be defined inside
the class
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Objects and Classes : Exercise
6. Main Class
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The user will input the color, brand and
speed.
Main Class: Car is the main class in this
program because it contains the main
method.
main Method: This is where the
program begins execution. In the main
method:
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We create an object myCar of the Car class.
We use methods like drive(), brake(), and
honk() to interact with the car object.
Execution: When you run the program,
the JVM starts at the main method,
executing the code in order, which
tests the functionality of the Car class.
Objects and Classes : Exercise
6. Output
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