[FIXED] How to use mock object to replace the original object in internal code section?

Issue

I met this problem when I tried to execute some JUnit mockito tests.

To make it easy to understand my question, I will provide an example below:

Class A {
   public String test(String para1) {
       //...do whatever stuff
       return para1;
   }
}

Class B {
   public void run() {
       A a = new A();
       String result = a.test("test");
       System.out.println(result);
   }
}

when(mockA.test(anyString()).thenReturn("mockResult");

A mockA = mock(A.class);

//Instead of doing mockA.test(), I do the following:
B b = new B();
b.run();

The question is, how can I replace the "a" object in B's run() method with the "mockA" object? This way I can start the code execution from b.run() and also utilize the mock object inside of the code execution process.

Any help would be greatly appreciated! :P

Solution

There are couple of options instead of creating new instance of A inside of run:

1. Pass instance of A in the constructor, like

   class B {
       private A a;
       B(A a) {
           this.a = a;
       }
       void run() {
           a.test("something");
       }
   }
   

   So your test code will change to

   B b = new B(mockA);
   b.run();
   

2. Create setter method:

   class B {
       private A a;
       void setA(A a) {
           this.a = a;
       }
       void run() {
           a.test("something");
       }
   }
   

   So your test code will change to

   B b = new B();
   b.setA(mockA);
   b.run();
   

Usually second method is preferred.

Answered By - artie
Answer Checked By - Clifford M. (JavaFixing Volunteer)