This document contains brief knowledge about the concept of "Singleton Class in Java". This is the best, I can come up with. So, please go through it and if I have missed anything related to topic. Let me know. Thanks.

1. By: RAHUL SHARMA

2. What is Singleton class..?
If we can create only one object of java class then that
java class is said to be Singleton class.
OR
Singleton class is a class that can have only one object at
a time.
for e.g. Runtime
BusinessDelegate

3. Why use Singleton class…
If there are several similar requirement and we
have create only one object and reuse the same object for
every similar requirement.
The purpose of singleton is to control object
creation by keeping private constructor.
The main advantage of singleton class is
improvement of performance and memory utilization.

4. e.g.
Runtime r1 =
Runtime.getRuntime();
Runtime r2 =
Runtime.getRuntime();
.
.
.
.
.
Runtime rn =
Runtime.getRuntime();
Object
r1 r2 rn
Note : We can create singleton class objects by
using factory methods. As we can see that for n
number of references only one object has been
created.

5. Can we create our own
singleton classes..?
Yes, we can create our own singleton classes. To create
the user-defined singleton classes we have to use
private constructor, private static variable and public
factory method.
There are two approaches to create singleton class

6. Approach 1 :
Step 1: Create a Singleton
class
class Test1
{
private static Test1 t = new
Test1();
private Test1()
{
System.out.println("Approach
1 executed successfully.");
}
public static Test1 getTest()
{
return t;
}
}
Step 2: Access the object of
Singleton class
class Test2
{
public static void main(String[]
args)
{
Test1 t = Test1.getTest();
}
}
Output : Approach 1 executed
successfully.

7. Approach 2 :
Step 1: Create a Singleton
class
class Test1
{
private static Test1 t = null;
private Test1()
{
System.out.println("Appro
ach 2 executed successfully.");
}
public static Test1 getTest()
{
if(t == null)
{
t = new Test1();
}
return t;
}
}
Step 2: Access the object of
Singleton class
class Test2
{
public static void main(String[]
args)
{
Test1 t = Test1.getTest();
}
}
Output : Approach 2 executed
successfully.

8. Note :
1. In Approach-1(A1) object get created at the
beginning. If we are not using the object in that case
memory wastage occur.
2. Approach-2(A2) is recommended in comparison of
A1 because in A2 object get created only when it
requested to do so and after first request for every
new request it returns the existing object that is
created at first request.

9. Thanks