Objects and Classes in Java

Introduction to Java Classes:

A class is nothing but a blueprint or a template for creating different objects which defines its properties and behaviors. Java class objects exhibit the properties and behaviors defined by its class. A class can contain fields and methods to describe the behavior of an object.

Methods are nothing but members of a class that provide a service for an object or perform some business logic. Java fields and member functions names are case sensitive. Current states of a class’s corresponding object are stored in the object’s instance variables. Methods define the operations that can be performed in java programming.

A class has the following general syntax:

<class modifiers>class<class name>
<extends clause> <implements clause>{

// Dealing with Classes (Class body)
<field declarations (Static and Non-Static)>
<method declarations (Static and Non-Static)>
<Inner class declarations>
<nested interface declarations>
<constructor declarations>
<Static initializer blocks>
}

Below is an example showing the Objects and Classes of the Cuboid class that defines 3 fields namely length, breadth and height. Also the class contains a member function getVolume().

public class Cuboid {

 int length;
 int breadth;
 int height;
 public int getVolume() {
  return (length * breadth * height);
 }
}

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 describe 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, Bricks 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.

Syntax for the Object :

class_name object_name = new class_name();

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 kind of methods. In the above example, barking(), hungry() and sleeping() are methods.

Note: Method is not but is special type of function which define in the class.

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 Shepherd{
   public Shepherd(){
   }

   public Shepherd(String name){
      // This constructor has one parameter, name.
   }
}

Note: 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 Shepherd{

   public Shepherd(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 myShepherd
      Shepherd myShepherd = new Shepherd( "tommy" );
      Shepherd urShepherd = new Shepherd( "jimmy" );
   }
}

If we compile and run the above program then it would produce following result:
output:
Passed Name is :tommy
Passed Name is :jimmy



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 Shepherd{
   
   int shepAge;

   public Shepherd(String name){
      // This constructor has one parameter, name.
      System.out.println("Passed Name is :" + name ); 
   }
   public void setAge( int age ){
       shepAge= age;
   }

   public int getAge( ){
       System.out.println("Shepherd's age is :" + shepAge); 
       return shepAge;
   }
   public static void main(String []args){
      /* Object creation */
      Shepherd myShepherd = new Shepherd( "tommy" );

      /* Call class method to set Shepherd's age */
      myShepherd.setAge( 2 );

      /* Call another class method to get Shepherd's age */
      myShepherd.getAge( );

      /* You can access instance variable as follows as well */
      System.out.println("Variable Value :" + myShepherd.shepAge ); 
   }
}
If we compile and run the above program then it would produce following result:
output:
Passed Name is :tommy
Shepherd's age is :2
Variable Value :2

Source file declaration rules:

As the last part of this section lets us 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 following line would ask compiler to load all the classes available in directory java_installation/java/io :
import java.io.*;



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