Adds after / before mapping example (#7518)

mapstruct/src/test/java/com/baeldung/mapper/CarsMapperUnitTest.java

@@ -0,0 +1,42 @@
+package com.baeldung.mapper;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.jupiter.api.Test;
+import org.mapstruct.factory.Mappers;
+
+import com.baeldung.dto.CarDTO;
+import com.baeldung.dto.FuelType;
+import com.baeldung.entity.BioDieselCar;
+import com.baeldung.entity.Car;
+import com.baeldung.entity.ElectricCar;
+
+class CarsMapperUnitTest {
+
+    private CarsMapper sut = Mappers.getMapper(CarsMapper.class);
+
+    @Test
+    void testGivenSubTypeElectric_mapsModifiedFieldsToSuperTypeDto_whenBeforeAndAfterMappingMethodscarCalled() {
+        Car car = new ElectricCar();
+        car.setId(12);
+        car.setName("Tesla_Model_C");
+        
+        CarDTO carDto = sut.toCarDto(car);
+        
+        assertEquals("TESLA_MODEL_C", carDto.getName());
+        assertEquals(FuelType.ELECTRIC, carDto.getFuelType());
+    }
+    
+    @Test
+    void testGivenSubTypeBioDiesel_mapsModifiedFieldsToSuperTypeDto_whenBeforeAndAfterMappingMethodscarCalled() {
+        Car car = new BioDieselCar();
+        car.setId(11);
+        car.setName("Tesla_Model_X");
+        
+        CarDTO carDto = sut.toCarDto(car);
+        
+        assertEquals("TESLA_MODEL_X", carDto.getName());
+        assertEquals(FuelType.BIO_DIESEL, carDto.getFuelType());
+    }
+
+}