When to use a Constructor and when to use getInstance() method (static factory methods)?

Question

1. When and how should we use a constructor

   Foo bar = new Foo();
   

2. And when and how should we use getInstance() (static factory

Answer 1

Singletons are evil. The problems I've seen surrounding it are not about re-use or extendability of a system (although I could see how that could occur), more so that I can't count the number of times i've seen obscure bugs in a system that arise from singletons.

If you do need to use a singleton, ensure it's scope is extremely narrow, i.e. judiciously limit the number of other objects in your system that know about it.