Object creation mocking mockito

Question

i have a class top and bottom. i want to unit test the atom class which create the object of bottom.

public class top {

  publiv top() {
     bottom b = new bottom("value");
  }
}

Unit test class:

public class topTest {
   @Test
   public void test(){ 
     top a = new top();
   }
}

while creating a object for top class in test, it will create a object for bottom class.

Here, i want to mock the bottom class object creation. Could anyone help me out on this.

Answer 1

For doing so you should consider to pass the bottom object as parameter. It is always a good practise to pass objects as parameters when an objects are depending on that object.

public class Top {
   private Bottom bottom;

   public Top(Bottom bottom) {
      this.bottom = bottom
   }
}

public class TopTest {
   @Test
   public void test(){ 
      Bottom bottom = mock(Bottom.class);
      Top top = new Top(bottom);
   }
}

Answer 2

You will need to move the logic creating the instance of bottom somewhere. Either in an overrideable method like:

public class top {

  public top() {
     bottom b = new bottom("value");
  }

  protected bottom createBottom() {
     return new bottom("value");
  }
}

And then you can override it in the test:

public class topTest {
   @Test
   public void test(){ 
     top a = new top() {
            protected bottom createBottom() {
               return Mockito.mock(bottom.class);
             }   
             };
   }
}

Or you create a BottomCreator class pass it to Top in construction time and use a mocked/fake version when you test it.

public class top {

      private final BottomCreator bottomCreator;

      public top(BottomCreator creator) {
           this.bottomCreator = creator;
      }

      public top() {
         bottom b = bottomCreator.newBottom();
      }

    }