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Learn Java - Java tutorial - Java classes and objects - Java examples - Java programs

Java is an object-oriented programming language. As a language that has an object-oriented function, it supports the following basic concepts:

In this lesson, we'll look at Java objects and classes , their concepts.

The class from which individual objects are created is indicated in green. An example of creating a class in Java is shown below:

public class Dog{
   String breed;
   int age;
   String color;

   void barking(){
   }
   
   void hungry(){
   }
   
   void sleeping(){
   }
}

A class can contain any of the following kinds of variables:

In Java, classes can have any number of methods for accessing the value of different kinds of methods. In the above example, barking (), hungry (), and sleeping () are methods. Further, some of the important topics that should be considered for understanding the meaning of classes and objects in a programming language are mentioned

An example of the constructor is shown below:

public class Puppy{
   public Puppy(){
   }

   public Puppy(String name){
      // So the constructor in Java looks like and it has one parameter, name.
   }
}

Note: in the following sections we will discuss in more detail if we have two different types of constructors.

The options for creating an object in the class are:

An example is given below:

public class Puppy{

   public Puppy(String name){
      // This is a constructor and has one parameter, name.
      System.out.println("Passed Name:" + name ); 
   }
   public static void main(String []args){
      // Create an object myPuppy.
      Puppy myPuppy = new Puppy( "Baguette" );
   }
}

If you compile and run the above program, it will produce the following result:

Passed Name: Baguette

Variables and methods are available through the created objects. To access the instance variable, the full path should look like this:

/ * First create an object * /
ObjectReference = new Constructor ();

/ * Now call the variable as follows * /
ObjectReference.variableName;

/ * Now you can call the class method * /
ObjectReference.MethodName ();

This example explains how to access the instance variables and class methods in Java:

public class Puppy {
   
   int puppyAge;

   public Puppy (String name) {
      // This is a constructor and it has one parameter, name.
      System.out.println ("Transmitted Name:" + name);
   }
   public void setAge (int age) {
       puppyAge = age;
   }

   public int getAge () {
       System.out.println ("Age of the puppy:" + puppyAge);
       return puppyAge;
   }
   public static void main (String [] args) {
      / * Create an object. * /
      Puppy myPuppy = new Puppy ("Baguette");

      / * Call the class method to set the age of the puppy. * /
      myPuppy.setAge (2);

      / * Call another class method to get the age of the puppy. * /
      myPuppy.getAge ();

      / * Get the instance variable of the class. * /
      System.out.println ("Value of the variable:" + myPuppy.puppyAge);
   }
}

If you compile and run the above program, it will produce the following result:

Passed Name: Baguette
Age of puppy: 2
Value of variable: 2

In the last part of this section, let's look at the declaration rules for the source file. These rules in Java are important when declaring classes, import statements, and package operators in the source file.

When developing applications, hundreds of classes and interfaces will be written, so the categorization of these classes is mandatory, and it also makes life much easier.

import java.io.*;

import java.io. *;
public class Employee {
   String name;
   int age;
   String designation;
   double salary;

   // This is the constructor of the Employee class.
   public Employee (String name) {
      this.name = name;
   }
   // Assign the age of the employee to the variable age.
   public void empAge (int empAge) {
      age = empAge;
   }
   / * Assignment of the variable designation. * /
   public void empDesignation (String empDesig) {
      designation = empDesig;
   }
   / * Assigning variable salary. * /
   public void empSalary (double empSalary) {
      salary = empSalary;
   }
   / * Output of detailed information. * /
   public void printEmployee () {
      System.out.println ("Name:" + name);
      System.out.println ("Age:" + age);
      System.out.println ("Name:" + designation);
      System.out.println ("Wages:" + salary);
   }
}

import java.io. *;
public class EmployeeTest {

   public static void main (String args []) {
      / * Create two objects using the constructor. * /
      Employee empOne = new Employee ("Oleg Olegov");
      Employee empTwo = new Employee ("Ivanov Ivanov");

      // Call the method for each created object.
      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 ();
   }
}

Currently, after compiling both the classes, run EmployeeTest and obtain the following result:

C:> javac Employee.java
C:> vi EmployeeTest.java
C:> javac EmployeeTest.java
C:> java EmployeeTest
First name: Oleg Olegov
Age: 26 years old
Title: Senior Software Engineer
Salary: 1000.0
First name: Ivan Ivanov
Age: 21 years old
Title: Software Engineer
Salary: 500.0