Java Object and Classes - Java

What is Java Object and Classes?

Java is an Object-Oriented Language. As a language that has the Object-Oriented feature, Java supports the resulting important 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. instance: A dog has states - color, name, breed in addition to behaviors – wagging the tail, barking, eating. An object is an instance of a class.
  • Class − a class may be defined as a template/blueprint that describes the behavior/state that the item of its type support.

Objects in Java

Let us now look deep into what are objects. If we consider the actual world, we will discover many items around us, cars, dogs, people, and so forth. a majority of these objects have a state and a behavior.

If we take into account a dog, then its state is - name, breed, color, and the behavior is - barking, wagging the tail, running.

If you compare the software program item with a real-world object, they've very comparable traits.

Software objects even have a state and a behavior. A software object's state is stored in fields and behavior is shown via methods.

So in software development, methods operate at the internal state of an object and the object-to-object conversation is done via methods.
Classes in Java
A class is a blueprint from which individual objects are created.

Following is a sample of a class.

Example

A class can comprise any of the subsequent variable types.

  • Local variables − Variables described inside methods, constructors or blocks are referred to as local variables. The variable could be declared and initialized within the method and the variable may be destroyed while the method has completed.
  • Instance variables − instance variables are variables inside a class however outside any method. these variables are initialized when the class is instantiated. instance variables can be accessed from inside any method, constructor or blocks of that particular class.
  • Class variables − class variables are variables declared within a class, outside any method, with the static keyword.

A class can have any variety of methods to access the value of various kinds of methods. In the above instance, barking (), hungry () and sleeping () are methods.

Following are some of the important topics that need to be mentioned when searching for lessons of the Java Language.

Constructors

When talk over classes, one of the best 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.

Following is an example of a constructor −

Example


Java also supports Singleton classes in which you will be capable of create only one instance of a class.

note − we've specific types of constructors. we're going to talk about constructors in element in the next chapters.

Creating an object

As cited previously, a class provides the blueprints for objects. So basically, an item is made out of a class. In Java, the brand new key-word is used to create new items.

There are 3 steps when developing an object from a class −

  • Declaration − A variable declaration with a variable name with an object kind.
  • Instantiation − the 'new' keyword is used to create the object.
  • Initialization − the 'new' keyword is followed by a call to a constructor. This call initializes the new object.

Following is an instance of making an Object –

Example


If we compile and run the above program, then it will produce the following result −

Output

Accessing Instance Variables and Methods

Instance variables and methods are accessed via created objects. To access an instance variable, following is the fully qualified path −

Example

This example explains how to access instance variables and methods of a class.

If we compile and run the above program, then it will produce the following result −

Output

Source File Declaration Rules

Let’s now look into the source file declaration rules. These rules are vital 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. We 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 words, 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 the compiler to load all the classes available in directory java_installation/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.

Example

As referred to previously on this tutorial, processing starts from the main method. Therefore, so as for us to run this employee class there should be a main method and objects should be created. We are able to be creating a separate class for those responsibilities.

Following 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.

Now, compile both the classes and then run EmployeeTest to see the result as follows −

Output

What is Next?

In the next session, we will discuss the basic data types in Java and how they can be used when developing Java applications.

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