A singleton is an object that cannot be instantiated. At
first, that might seem counterintuitive - after all, we need
an instance of an object before we can use it. Well yes a
singleton can be created, but it can't be instantiated by
developers - meaning that the singleton class has control
over how it is created. The restriction on the singleton is
that there can be only one instance of a singleton created
by the Java Virtual Machine (JVM) - by prevent direct
instantiation we can ensure that developers don't create a
second copy.

We'll start with the class definition, for a SingletonObject
class. Next, we provide a default constructor that is marked
as private. No actual code needs to be written, but you're
free to add some initialization code if you'd like. 

public class SingletonObject
{
	private SingletonObject()
	{
		// no code req'd
	}
}

So far so good. But unless we add some further code,
there'll be absolutely no way to use the class. We want to
prevent direct instantiation, but we still need to allow a
way to get a reference to an instance of the singleton object.

singleton is a design pattern. in this per application only 
one object should exist.we implement it as follows:

public class abcd
{
private static abcd instance=null;
protected abcd()
{
//only to defeat instantiation;
}
public static abcd getInstance()
{
if(instance==null)
{
instance=new abcd();
}
return instance;
}