What is singleton class?

Answer Posted / test

In computer science the singleton design pattern is designed
to restrict instantiation of a class to one (or a few)
objects. This is useful when exactly one object is needed to
coordinate actions across the system. Sometimes it is
generalized to systems that operate more efficiently when
only one or a few objects exist.

The singleton pattern is implemented by creating a class
with a method that creates a new instance of the object if
one does not exist. If one does exist it returns a reference
to the object that already exists. To make sure that the
object cannot be instantiated any other way the constructor
is made either private or protected.

The singleton pattern must be carefully constructed in
multi-threaded applications. If two threads are to execute
the creation method at the same time when a singletondoes
not yet exist they both must check for an instance of the
singleton and then only one should create the new one.

The classic solution to this problem is to use mutual
exclusion on the class that indicates that the object is
being instantiated.

A Java programming language solution is as follows. It is
based on the Q&A link found below modified for
multi-threading however it is still vulnerable to the
double-checked locking anti-pattern also found below:

public class Singleton {
private static Singleton INSTANCE null;

// Private constructor suppresses
// default public constructor
private Singleton() {}

//synchronized creator to defend against multi-threading issues
//another if check here to avoid multiple instantiation
private synchronized static void createInstance() {
if (INSTANCE null) {
INSTANCE new Singleton();

public static Singleton getInstance() {
if (INSTANCE null) createInstance();
return INSTANCE;

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