In a multi-threaded environment, if we have a non-synchronized method for returning instances, there are chances that the method can create more than one instance. Consider that we have 2 threads and both enter the condition for checking if the instance already exists. Both threads will find that the instance has not been created and hence both will create the class instances. This goes against the principle of the Singleton pattern. Hence, in a multi-threaded environment, it is recommended to use synchronized checks.
