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The Singleton’s purpose is to control object creation, limiting the number of objects to only one. Since there is only one Singleton instance, any instance fields of a Singleton will occur only once per class, just like static fields. Singletons often control access to resources, such as database connections or sockets.
For example, if you have a license for only one connection for your database or your JDBC driver has trouble with multithreading, the Singleton makes sure that only one connection is made or that only one thread can access the connection at a time

Implementing Singletons

The easiest implementation consists of a private constructor and a field to hold its result, and a static accessor method with a name like getInstance().
The private field can be assigned from within a static initializer block or, more simply, using an initializer. The getInstance( ) method (which must be public) then simply returns this instance −

 

// File Name: Singleton.java
public class Singleton {
private static Singleton singleton = new Singleton( );

/* A private Constructor prevents any other
* class from instantiating.
*/
private Singleton(){ }

/* Static ‘instance’ method */
public static Singleton getInstance( ) {
return singleton;
}
/* Other methods protected by singleton-ness */
protected static void demoMethod( ) {
System.out.println(“demoMethod for singleton”);
}
}

Here is the main program file, where we will create a singleton object:

// File Name: SingletonDemo.java
public class SingletonDemo {
public static void main(String[] args) {
Singleton tmp = Singleton.getInstance( );
tmp.demoMethod( );
}
}

This will produce the following result −

demoMethod for singleton

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 Parsing

We will take a  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 the tail, barking, eating. An object is an instance of a class.
• Class – A class can be defined as a template/blueprint that describes the behavior/state that the 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 a behavior.
If we consider a dog, then its state is – name, breed, color, and the behavior 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 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 blueprint from which individual objects are created.
Following is a sample of a class.

public class Dog{
String breed;
int ageC
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.
  

When we consider a Java program, it can be defined as a collection of objects that communicate via invoking each other’s methods. Let us now briefly look into what do class, object, methods, and instance variables mean.

• Object – Objects have states and behaviors. Example: A dog has states – color, name, breed as well as behavior such as wagging their tail, barking, eating. An object is an instance of a class.
• Class – A class can be defined as a template/blueprint that describes the behavior/state that the object of its type supports.
•  Methods – A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
•  Instance Variables – Each object has its unique set of instance variables. An object’s state is created by the values assigned to these instance variables

The 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

Java Variables

Following are the types of variables in Java:

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

To write your Java programs, you will need a text editor. There are even more sophisticated IDEs available in the market. But for now, you can consider one of the following:

•  Notepad: On Windows machine, you can use any simple text editor like Notepad
  (Recommended for this tutorial), TextPad.
•  Netbeans: A Java IDE that is open-source and free, which can be downloaded
  from http://www.netbeans.org/index.html.
•  Eclipse: A Java IDE developed by the eclipse open-source community and can be
  downloaded from http://www.eclipse.org/.