Hello friends !Today we learn the concept of Inheritance , Interfaces, and objects and classes in Java, constructor, creating an object, accessing instance variables and methods, source file declaration rules.
Inheritance !!
In java classes can be derived from classes. Basically if you need to create a new class , then it is possible to derive your new class from the already existing class.
This concept allows you to reuse the fields and methods of the existing class with out having to rewrite the code in a new class. In this scenario the existing class is called the super class and the derived class is called the subclass.
Interfaces !!
In Java language an interface can be defined as a contract between objects on how to communicate with each other. Interfaces play a vital role when it comes to the concept of inheritance.
An interface defines the methods, a deriving class (subclass) should use. But the implementation of the methods is totally up to the subclass.
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
Objects in Java:
Software objects 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.
Classes in Java:
A class is a blue print from which individual objects are created.
A sample of a class is given below:
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()
methods. In the above example, barking(), hungry() and sleeping()
are variables.
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:
class Puppy{
public puppy(){
}
public puppy(String name){
// This constructor has one parameter, name.
}
}
Java also supports Singleton Classes where you would be able to create only one instance of a class.
Creating an Object:
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 o a constructor. This call initializes the new object.
Example of creating an object is given below:
|
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:
class Puppy{
int puppyAge;
public Puppy(String name){
// This constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
public setAge( int age ){
puppyAge = age;
}
public 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 );
}
}
Output :
Passed Name is :tommy
Puppy's age is :2
Variable Value :2
Source file declaration rules:
Rules followed 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.
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