Question

Please advise me the difference between two ways of declaration of java constructor

  public class A{

    private static A instance = new A();
    public static A getInstance() { return instance;
        }
    public static void main(String[] args) {
          A a= A.getInstance();
        }
 }

AND

    public class B{
    public B(){};


     public static void main(String[] args) {
     B b= new B();
     }
}

Thanks

Answer 1

• Class A is supposed to be a Singleton, where you can only have one instance of A. You retrieve that single instance by calling getInstance();

In software engineering, the singleton
pattern is a design pattern used to
implement the mathematical concept of
a singleton, by restricting the
instantiation of a class to one
object. This is useful when exactly
one object is needed to coordinate
actions across the system.

There are a few ways to go about this depending on your requirements:

public class A{
    private static A instance = new A();
    private A(){} // private constructor
    public static A getInstance() {return instance;}
}

or not creating the instance until the first call

public class A{
    private static A instance = null;
    private A(){} // private constructor
    public static A getInstance() {
        if(instance == null){
           instance = new A(); // create the one instance.
        }
        return instance;
    }
}

• Class B is a class with a no-parameter constructor. You can create as many B instances as you want by calling new B();