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 methods)

   Foo bar = Foo.getInstance(); 

What is the difference between these two? I have always used a constructor, but when should I use getInstance() instead?

Answer 1

Everybody seems to focus on singletons while I think that the question is actually about constructor vs static factory methods.

This is actually Item 1: Consider static factory methods instead of constructors of Effective Java by Joshua Bloch:

Item 1: Consider static factory methods instead of constructors

The normal way for a class to allow a client to obtain an instance of itself is to provide a public constructor. There is another technique that should be a part of every programmer’s toolkit. A class can provide a public static factory method, which is simply a static method that returns an instance of the class. Here’s a simple example from Boolean (the boxed primitive class for the primitive type boolean). This method translates a boolean primitive value into a Boolean object reference:

public static Boolean valueOf(boolean b) {     return b ? Boolean.TRUE : Boolean.FALSE; } 

Note that a static factory method is not the same as the Factory Method pattern from Design Patterns [Gamma95, p. 107]. The static factory method described in this item has no direct equivalent in Design Patterns.

A class can provide its clients with static factory methods instead of, or in addition to, constructors. Providing a static factory method instead of a public constructor has both advantages and disadvantages.

Advantages (quoting the book):

• One advantage of static factory methods is that, unlike constructors, they have names.
• A second advantage of static factory methods is that, unlike constructors, they are not required to create a new object each time they’re invoked.
• A third advantage of static factory methods is that, unlike constructors, they can return an object of any subtype of their return type.
• A fourth advantage of static factory methods is that they reduce the verbosity of creating parameterized type instances.

Disadvantages (still quoting the book):

• The main disadvantage of providing only static factory methods is that classes without public or protected constructors cannot be subclassed.
• A second disadvantage of static factory methods is that they are not readily distinguishable from other static methods.