Java is an Object-Oriented Language. As a language that has
the Object Oriented feature, Java supports the following fundamental concepts:
Polymorphism
Inheritance
Encapsulation
Abstraction
Classes
Objects
Instance
Method
Message Parsing
In this chapter, we will look into the concepts Classes and
Objects.
Object - Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
Class - A class can be defined as a template/blue print that describes the behaviors/states that object of its type support.
Objects in Java:
Let us now look deep into what are objects. If we consider
the real-world we can find many objects around us, Cars, Dogs, Humans, etc. All
these objects have a state and behavior.
If we consider a dog, then its state is - name, breed,
color, and the behavior is - barking, wagging, running
If you compare the software object with a real world object,
they have very similar characteristics.
Software objects also have a state and behavior. A software
object's state is stored in fields and behavior is shown via methods.
So in software development, methods operate on the internal
state of an object and the object-to-object communication is done via methods.
Classes in Java:
A class is a blue print from which individual objects are
created.
A sample of a class is given below:
public class Dog{
String breed;
int age;
String color;
void barking(){
}
void hungry(){
}
void sleeping(){
}
}
A class can contain any of the following variable types.
Local variables:
Variables defined inside methods, constructors or blocks are called local
variables. The variable will be declared and initialized within the method and
the variable will be destroyed when the method has completed.
Instance
variables: Instance variables are variables within a class but outside any
method. These variables are instantiated when the class is loaded. Instance
variables can be accessed from inside any method, constructor or blocks of that
particular class.
Class variables:
Class variables are variables declared with in a class, outside any method,
with the static keyword.
A class can have any number of methods to access the value
of various kinds of methods. In the above example, barking(), hungry() and
sleeping() are methods.
Below mentioned are some of the important topics that need
to be discussed when looking into classes of the Java Language.
Constructors:
When discussing about classes, one of the most important sub
topic would be constructors. Every class has a constructor. If we do not
explicitly write a constructor for a class the Java compiler builds a default
constructor for that class.
Each time a new object is created, at least one constructor
will be invoked. The main rule of constructors is that they should have the
same name as the class. A class can have more than one constructor.
Example of a constructor is given below:
public class Puppy{
public Puppy(){
}
public Puppy(String
name){
// This
constructor has one parameter, name.
}
}
Java also supports Singleton Classes where you would be able
to create only one instance of a class.
Creating an Object:
As mentioned previously, a class provides the blueprints for
objects. So basically an object is created from a class. In Java, the new key
word is used to create new objects.
There are three steps when creating an object from a class:
Declaration: A variable declaration with a variable name with an object type.
Instantiation: The 'new' key word is used to create the object.
Initialization: The 'new' keyword is followed by a call to a constructor. This call initializes the new object.
Example of creating an object is given below:
public class Puppy{
public Puppy(String
name){
// This
constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
public static void
main(String []args){
// Following
statement would create an object myPuppy
Puppy myPuppy =
new Puppy( "tommy" );
}
}
If we compile and run the above program, then it would
produce the following result:
Passed Name is :tommy
Accessing Instance Variables and Methods:
Instance variables and methods are accessed via created
objects. To access an instance variable the fully qualified path should be as follows:
/* First create an object */
ObjectReference = new Constructor();
/* Now call a variable as follows */
ObjectReference.variableName;
/* Now you can call a class method as follows */
ObjectReference.MethodName();
Example:
This example explains how to access instance variables and
methods of a class:
public class Puppy{
int puppyAge;
public Puppy(String
name){
// This
constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
public void setAge(
int age ){
puppyAge = age;
}
public int getAge(
){
System.out.println("Puppy's age is :" + puppyAge );
return
puppyAge;
}
public static void
main(String []args){
/* Object
creation */
Puppy myPuppy = new Puppy(
"tommy" );
/* Call class
method to set puppy's age */
myPuppy.setAge(
2 );
/* Call another
class method to get puppy's age */
myPuppy.getAge(
);
/* You can
access instance variable as follows as well */
System.out.println("Variable Value :" + myPuppy.puppyAge );
}
}
If we compile and run the above program, then it would
produce the following result:
Passed Name is :tommy
Puppy's age is :2
Variable Value :2
Source file declaration rules:
As the last part of this section let's now look into the
source file declaration rules. These rules are essential when declaring
classes, import statements and package statements in a source file.
There can be only
one public class per source file.
A source file can
have multiple non public classes.
The public class
name should be the name of the source file as well which should be appended by
.java at the end. For example : The class name is . public class Employee{}
Then the source file should be as Employee.java.
If the class is
defined inside a package, then the package statement should be the first
statement in the source file.
If import
statements are present then they must be written between the package statement
and the class declaration. If there are no package statements then the import
statement should be the first line in the source file.
Import and package
statements will imply to all the classes present in the source file. It is not
possible to declare different import and/or package statements to different
classes in the source file.
Classes have several access levels and there are different
types of classes; abstract classes, final classes, etc. I will be explaining
about all these in the access modifiers chapter.
Apart from the above mentioned types of classes, Java also
has some special classes called Inner classes and Anonymous classes.
Java Package:
In simple, it is a way of categorizing the classes and
interfaces. When developing applications in Java, hundreds of classes and
interfaces will be written, therefore categorizing these classes is a must as
well as makes life much easier.
Import statements:
In Java if a fully qualified name, which includes the
package and the class name, is given then the compiler can easily locate the
source code or classes. Import statement is a way of giving the proper location
for the compiler to find that particular class.
For example, the following line would ask compiler to load
all the classes available in directory java_installation/java/io :
import java.io.*;
A Simple Case Study:
For our case study, we will be creating two classes. They
are Employee and EmployeeTest.
First open notepad and add the following code. Remember this
is the Employee class and the class is a public class. Now, save this source
file with the name Employee.java.
The Employee class has four instance variables name, age,
designation and salary. The class has one explicitly defined constructor, which
takes a parameter.
import java.io.*;
public class Employee{
String name;
int age;
String designation;
double salary;
// This is the
constructor of the class Employee
public
Employee(String name){
this.name =
name;
}
// Assign the age
of the Employee to the variable age.
public void
empAge(int empAge){
age = empAge;
}
/* Assign the
designation to the variable designation.*/
public void
empDesignation(String empDesig){
designation =
empDesig;
}
/* Assign the
salary to the variable salary.*/
public void
empSalary(double empSalary){
salary =
empSalary;
}
/* Print the
Employee details */
public void
printEmployee(){
System.out.println("Name:"+ name );
System.out.println("Age:" + age );
System.out.println("Designation:" + designation );
System.out.println("Salary:" + salary);
}
}
As mentioned previously in this tutorial, processing starts
from the main method. Therefore in-order for us to run this Employee class
there should be main method and objects should be created. We will be creating
a separate class for these tasks.
Given below is the EmployeeTest class, which creates two
instances of the class Employee and invokes the methods for each object to
assign values for each variable.
Save the following code in EmployeeTest.java file
import java.io.*;
public class EmployeeTest{
public static void
main(String args[]){
/* Create two
objects using constructor */
Employee empOne
= new Employee("James Smith");
Employee empTwo
= new Employee("Mary Anne");
// Invoking
methods for each object created
empOne.empAge(26);
empOne.empDesignation("Senior Software Engineer");
empOne.empSalary(1000);
empOne.printEmployee();
empTwo.empAge(21);
empTwo.empDesignation("Software Engineer");
empTwo.empSalary(500);
empTwo.printEmployee();
}
}
Now, compile both the classes and then run EmployeeTest to
see the result as follows:
C :> javac Employee.java
C :> vi EmployeeTest.java
C :> javac
EmployeeTest.java
C :> java EmployeeTest
Name:James Smith
Age:26
Designation:Senior Software Engineer
Salary:1000.0
Name:Mary Anne
Age:21
Designation:Software Engineer
Salary:500.0
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