CareerCup

M: ....? ( I have no clue for last question, any idea?)

The only way is to redesign the application, keeping in mind the concurrency as a requirement.

if you create the instance static you don't need to synchronize getInstance()

To synchronize without using synchronized method...

public class Singleton {
public final static Singleton instance = new Singleton();

private Singleton() {}

}

instead of synchronize the method you may synchronize a block of code to make it more efficient

It says we can have private inner class.

http://www.builderau.com.au/program/java/soa/Using-the-Singleton-pattern-in-Java/0,339024620,339280992,00.htm

Double-Checked Locking is what they are looking for :).

Use volatile based implementation for better performance

the singleton object can be declared as a global variable and initialized by the main thread. and used by other thread.

it really depends on the actual program, i think. there might be other operations on the singleton object that requires synchronization.

en.wikipedia.org/wiki/Singleton_pattern

ibm.com/developerworks/java/library/j-jtp11234/