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use enum...

Java Perspective: Number of ways:
you can use synchronization - actually that is the first thing that should come to mind whenever encountering questions involving multithreaded environment plus single access.
The thing is, synchronization is needed only once since it is expensive in its nature.

you can make the getInstance() method ( or whatever method name you are using to return the object reference) as synchronized.

initialize object statically.static initailization is taken care by JVM..

Google double locking synchronization

check out Question 15 here : javabeat.net/2009/02/design-patterns-interview-questions/

Use SingletonHandler approach for lazily loading singleton instance.

public class Singleton {

	private Singleton() {
		
	}
	
	public static class SingletonHolder {
		public static final Singleton INSTANCE = new Singleton();
	}
}

See this example:

public class Singleton
{
private Singleton instance;
private Singleton(){}

public Singleton getInstance()
{
if(instance==null)
{
synchronized(Singleton.this)
{
instance = new Singleton();
}
}
return instance;
}
}