How to mock constructor of subclass in Junit?

Question

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.

Answer 1

Ok, you were on the right path ... it takes a combination of @Mock, MockUp, Invocation and Deencapsulation ... You have to add an Invocation to your $init mock method, and then use Deencapsulation to poke around at A internals. Here's an example. I used your A and B classes, only cleaning them up a little bit and adding getters for ease-of-use. I deliberately did not add setters, so I could show off how to get around a lack of setters.

package com.example.dcsohl;

import org.junit.Test;

import mockit.Deencapsulation;
import mockit.Invocation;
import mockit.Mock;
import mockit.MockUp;

public class TestTest {

    public static class A {
        protected String a;
        protected String b;

        public A(String b1) {
            this("A111", b1);
        }

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

    public static class B extends A {
        public B(String b1) {
            super("A", b1);
        }

        public String getA() {
            return this.a;
        }

        public String getB(){
            return this.b;
        }
    }

    @Test
    public void test() {
        new MockUp<A>() {
            @Mock public void $init(Invocation inv, String a1, String b1) {
                A a = inv.getInvokedInstance();
                Deencapsulation.setField(a, "b", b1);
            }
        };

        B b = new B("foo");

        System.out.println(b.getA());
        System.out.println(b.getB());

    }

}

You'll note that, at the end, the printouts show that I successfully set the value of b but, having left a alone, it comes out as null.