Java Beginner's Guide[3] - Classes & Objects

Hi Everyone! In this tutorial, we will look into the concepts - Classes and Objects.

Classes in Java

A class is a blueprint from which individual objects are created. A class describes the behavior/state that the object of its type support.

Following is a example for a class.

public class Person {
String breed; //variables(state)
int age;
String color;

void barking() { } //methods(behaviour)
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 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 number of methods to access the value of various kinds of methods. In the above example, barking(), hungry() and sleeping() are methods.

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. Objects are instances of Classes. All these objects have a state and a behavior.

If you compare the software object with a real-world object, they have very similar characteristics.
Software objects also 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 on the internal state of an object and the object-to-object communication is done via methods.

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

public class Person{

public Person() { }

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

}

Note − We have two different types of constructors. We are going to discuss constructors in detail in the another tutorial (Java - Object oriented programming)

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 keyword 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' 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 example of creating an object −