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Programming languages - Java

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Lesson Description


Lession - #82 Java - Objects & Classes


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```     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 Passing     In this chapter, we will look into the concepts - Classes and Objects. Objects in Java    Let us now to look deep into what are objects. If we consider the real world , we can find many objects around us, vehicles, dogs, people, and so on.All these objects have a state and a behaviour.    If we consider a dog, its state is - name, breed, color, and the way of behaving is - barking, wagging the tail, running.    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 through methods.    So in software development, methods work on the internal state of an object and the object-to-object communication is done through methods. Classes in Java    A class is a blueprint from which individual objects are created.    Following is a sample of a class. Example : ```plaintext 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 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.    Following are some of the important topics that need to be discussed when looking into classes of the Java Language. Constructors    A class is a blueprint from which individual objects are created.    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.    Following is an example of a constructor − Example : ```plaintext public class Puppy { public Puppy(>
{ } public Puppy(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 subsequent chapters. 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 − Example : ```plaintext 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" >
; } } ``` [Try it Here](https://onecompiler.com/java>
   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 − ```plaintext /* 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. ```plaintext public class Puppy { int puppyAge; public Puppy(String name>
{ // This constructor has one parameter, name. System.out.println("Name chosen 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 >
; } } ``` [Try it Here](https://onecompiler.com/java>
   If we compile and run the above program, then it will produce the following result − Output :

  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 −