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

The uses for getInstance methods:

• If you want to control/restrict construction e.g. Singleton
• implement Factory pattern, e.g. DriverManager.getConnection
• When you want to provide better name as to how the instance is constructed (constructors must have the same name as the class name), checkout the NumberFormat factory methods getCurrencyInstance, getIntegerInstance and others as examples of this.

But most of the time your object will be a simple POJO and usage of public constructors is most practical and obvious solution.

U1: getInstance From Another Class

To return an instance of a different class:

public class FooFactory {
    public static Foo getInstance() {
        return new Foo();
    }
}

NumberFormat.getInstance methods do this as they actually return instances of DecimalFormat.

U2: Singleton Problems

The singleton pattern restricts many of the benefits of object oriented programming. Singletons generally have private constructors, therefore you cannot extend them. As you will be accessing it via its getInstance method and not referencing any interface, you will not be able to swap it out for another implementation.