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