how to replace mock function with new funciton in java

Question

I am write UT with Mockito, and I want replace my mock function(which DB select operation)

class DataBaseSelect {
    List<Long> selectDataFromDB(Long key){
         List<Long>  result  = db.select(key);
         return result;
    }
}

with new function (mock db select using map) I wrote in Test class;

class MapSelect {
    List<Long> selectDataFromMap(Long key){
         List<Long> result = map.get(key);
         return result;
    }
}

and I want return the right value with right key input

I try to do this using ArgumentCaptor as below, but it did not work as I want

ArgumentCaptor<Long> argumentCaptor = ArgumentCaptor.forClass(Long.class);
Mockito.when(dataBaseSelect.selectDataFromDB(argumentCaptor.capture())).thenReturn(MapSelect.selectDataFromMap(argumentCaptor.getValue()));
//some thing else here , 

I want when call dataBaseSelect.selectDataFromDB, it will be mock and then return result from MapSelect.selectDataFromMap and argument is passed from dataBaseSelect.selectDataFromDB

Answer 1

If you need substitute your own implementation of method, you can do something like that:

// create method substitution
Answer<List<Long>> answer = invocation -> mapSelect.selectDataFromMap((Long) invocation.getArguments()[0]);

// Mock method
Mockito.doAnswer(answer).when(dataBaseSelect).selectDataFromDB(anyLong());

Your solution is incorrect, cause captors are intended for method call verification (how many times method was called), example:

// method calling: mockedObject.someMethod(new Person("John"));
//...
ArgumentCaptor<Person> capture = ArgumentCaptor.forClass(Person.class);
verify(mockedObject).someMethod(capture.capture());
Person expected = new Person("John");
assertEquals("John", capture.getValue().getName());

in this example expected that 'someMethod' was called one time and return person "John"