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Java - Objects and Classes

• 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 describes 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, 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.

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

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 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:

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 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" );
   }
}

If we compile and run the above program, then it would produce the following result:

Passed Name is :tommy

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 */
Teleconference.MethodName();

Example:

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

public class Puppy{
   
   int puppyAge;

   public Puppy(String name){
      // This constructor has one parameter, name.
      System.out.println("Passed Name 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 ); 
   }
}

If we compile and run the above program, then it would produce the following result:

Passed Name is :tommy
Puppy's age is :2
Variable Value :2

First Java Program:

Let us look at a simple code that would print the words Hello World.

public class MyFirstJavaProgram {

   /* This is my first java program.  
    * This will print 'Hello World' as the output
    */

    public static void main(String []args) {
       System.out.println("Hello World"); // prints Hello World
    }
} 

Let's look at how to save the file, compile and run the program. Please follow the steps given below:

• Open notepad and add the code as above.
• Save the file as: MyFirstJavaProgram.java.
• Open a command prompt window and go o the directory where you saved the class. Assume it's C:\.
• Type ' javac MyFirstJavaProgram.java ' and press enter to compile your code. If there are no errors in your code, the command prompt will take you to the next line (Assumption : The path variable is set).
• Now, type ' java MyFirstJavaProgram ' to run your program.
• You will be able to see ' Hello World ' printed on the window.

C : > javac MyFirstJavaProgram.java
C : > java MyFirstJavaProgram 
Hello World

Basic Syntax:

About Java programs, it is very important to keep in mind the following points.

• Case Sensitivity - Java is case sensitive, which means identifier Hello and hello would have different meaning in Java.
• Class Names - For all class names the first letter should be in Upper Case. 
  
  If several words are used to form a name of the class, each inner word's first letter should be in Upper Case.
  
  Example class MyFirstJavaClass
• Method Names - All method names should start with a Lower Case letter. 
  
  If several words are used to form the name of the method, then each inner word's first letter should be in Upper Case.
  
  Example public void myMethodName()
• Program File Name - Name of the program file should exactly match the class name. 
  
  When saving the file, you should save it using the class name (Remember Java is case sensitive) and append 'java' to the end of the name (if the file name and the class name do not match your program will not compile).
  
  Example : Assume 'MyFirstJavaProgram' is the class name. Then the file should be saved as 'MyFirstJavaProgram.java'
• public static void main(String args[]) - Java program processing starts from the main() method which is a mandatory part of every Java program..

Java Identifiers:

• All Java components require names. Names used for classes, variables and methods are called identifiers.
  In Java, there are several points to remember about identifiers. They are as follows:
• All identifiers should begin with a letter (A to Z or a to z), currency character ($) or an underscore (_).
• After the first character identifiers can have any combination of characters.
• A key word cannot be used as an identifier.
• Most importantly identifiers are case sensitive.
• Examples of legal identifiers: age, $salary, _value, __1_value
• Examples of illegal identifiers: 123abc, -salary

Java Modifiers:

Like other languages, it is possible to modify classes, methods, etc., by using modifiers. There are two categories of modifiers:

• Access Modifiers: default, public , protected, private
• Non-access Modifiers: final, abstract, strictfp

We will be looking into more details about modifiers in the next section.

Java Variables:

We would see following type of variables in Java:

• Local Variables
• Class Variables (Static Variables)
• Instance Variables (Non-static variables)

Java Arrays:

Arrays are objects that store multiple variables of the same type. However, an array itself is an object on the heap. We will look into how to declare, construct and initialize in the upcoming chapters.

Java Enums:

Enums were introduced in java 5.0. Enums restrict a variable to have one of only a few predefined values. The values in this enumerated list are called enums.

With the use of enums it is possible to reduce the number of bugs in your code.

For example, if we consider an application for a fresh juice shop, it would be possible to restrict the glass size to small, medium and large. This would make sure that it would not allow anyone to order any size other than the small, medium or large.

Example:

class FreshJuice {

   enum FreshJuiceSize{ SMALL, MEDIUM, LARGE }
   FreshJuiceSize size;
}

public class FreshJuiceTest {

   public static void main(String args[]){
      FreshJuice juice = new FreshJuice();
      juice.size = FreshJuice. FreshJuiceSize.MEDIUM ;
      System.out.println("Size: " + juice.size);
   }
}

Above example will produce the following result:

Size: MEDIUM

Note: enums can be declared as their own or inside a class. Methods, variables, constructors can be defined inside enums as well.

Java Keywords:

The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.

abstract	assert	boolean	break
byte	case	catch	char
class	const	continue	default
do	double	else	enum
extends	final	finally	float
for	goto	if	implements
import	instanceof	int	interface
long	native	new	package
private	protected	public	return
short	static	strictfp	super
switch	synchronized	this	throw
throws	transient	try	void
volatile	while

Comments in Java

Java supports single-line and multi-line comments very similar to c and c++. All characters available inside any comment are ignored by Java compiler.

public class MyFirstJavaProgram{

   /* This is my first java program.
    * This will print 'Hello World' as the output
    * This is an example of multi-line comments.
    */

    public static void main(String []args){
       // This is an example of single line comment
       /* This is also an example of single line comment. */
       System.out.println("Hello World"); 
    }
} 

Using Blank Lines:

A line containing only whitespace, possibly with a comment, is known as a blank line, and Java totally ignores it.

Inheritance:

In Java, classes can be derived from classes. Basically if you need to create a new class and here is already a class that has some of the code you require, then it is possible to derive your new class from the already existing code.

This concept allows you to reuse the fields and methods of the existing class without having to rewrite the code in a new class. In this scenario the existing class is called the superclass 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.