How to mock constructor of subclass in Junit?

Classes structures look likes below:

class A {
 protected String a;
 protected String b;

 A(String b1) {
    new A("A111", b1);
 }

 A(String a1, String b1) {
   a = a1;
   b = b1;
 }
}

class B extends A {

  B(String b) {
    super("A", b);
  }
}

I need to write JUnit test case and needs to mock constructor for class A so that whenever object for class B needs to create then mock constructor for class A should get invoke and returns object from mock constructor.

I tried following :

       new MockUp<A>() {

            @Mock
            public void $init(String b1) {
                new A("A11111111", b1);
            }
        };

But object created in mocked constructor hasn't been returned.