Real Unit Tests using ApexMocks

MyController

public class MyController{
    private MyInterface myImplementation;
    public Integer valueOne{get;set;}
    public Integer valueTwo{get;set;}
    
    public MyController(MyInterface myImplementation){
        this.myImplementation = myImplementation;
    }
    
    public Integer calculatedTotalValue(){
        return myImplementation.calculateValues(valueOne, valueTwo);
    }
}

MyControllerTest

@isTest
public class MyControllerTest{
    public static testMethod void testCalculateValues(){
        fflib_ApexMocks mocks = new fflib_ApexMocks();
        MyInterface mockMyService = new MockMyService(mocks);
        mocks.startStubbing();
        mocks.when(mockMyService.calculateValues(5, 3)).thenReturn(8);
        mocks.stopStubbing();
        Test.startTest();
        MyController controller = new MyController(mockMyService);
        controller.valueOne = 5;
        controller.valueTwo = 3;
        Integer totalValue = controller.calculatedTotalValue();
        Test.stopTest();
        
        System.assertEquals(8, totalValue, 'The service should return 8');
    }
   private class MockMyService implements MyInterface{
      private fflib_ApexMocks mocks;
      public MockMyService(fflib_ApexMocks mocks){
         this.mocks = mocks;
      }
        public Integer calculateValues(Integer valueOne, Integer valueTwo){
            if (mocks.Stubbing){
                mocks.prepareMethodReturnValue(this, 'calculateValues', new List<Object> {valueOne, valueTwo});
                return null;
            }else{
                mocks.recordMethod(this, 'calculateValues', new List<Object> {valueOne, valueTwo});
                fflib_MethodReturnValue methodReturnValue = mocks.getMethodReturnValue(this, 'calculateValues', new List<Object> {valueOne, valueTwo});
                if (methodReturnValue != null){
                    if (methodReturnValue.ReturnValue instanceof Exception)
                    {
                        throw ((Exception) methodReturnValue.ReturnValue);
                    }
                    return (Integer) methodReturnValue.ReturnValue;
                }
            }
            return null;
        }
   }
}

MyInterface

public interface MyInterface{
   Integer calculateValues(Integer valueOne, Integer valueTwo);
}

MyService

public class MyService implements MyInterface{
    public Integer calculateValues(Integer valueOne, Integer valueTwo){
        return valueOne + valueTwo;
    }
}

All of this code is related to the following article: Real Unit Tests using ApexMocks

This code is further explained in: ApexMocks: How Does It Work?