Merge function of a map collector to add in list in Java 8

Question

While creating a Map on a stream, I want to insert the values in the same object list for duplicate Keys.
I used Collectors.toMap(keyMapper, valueMapper, mergeFunction).
But I’m unable to write a correct merge function for the same keys.

Here is a working example code to show the exact problem.

Create a Java file named MapCollector and add the below code.

public class MapCollector {
 public static Map<ItemType, ItemRequest> breakRequestByItemType(final ItemRequest originalRequest) {
 return originalRequest
 .getItemStatuses()
 .stream()
 .filter(e->e.getType()!= null)
 .collect(Collectors.toMap(
 ItemStatus::getType,
 e->ItemRequest
 .builder()
 .prop1(originalRequest.prop1)
 .prop2(originalRequest.prop2)
 .itemStatuses(Arrays.asList(e))
 .build()
 //TODO: Need a merge function that will add this status to the list `itemStatuses` 
 ));
 }
}
@Data
@Builder
class ItemRequest{
 List<ItemStatus> itemStatuses;
 String prop1;
 String prop2;
}
enum ItemType{
 A,B,C
}
@AllArgsConstructor
@Getter
enum ItemStatus{
 A_STATUS_1("some desc1",ItemType.A),
 A_STATUS_2("some desc2",ItemType.A),
 B_STATUS_1("some desc3",ItemType.B),
 B_STATUS_2("some desc4",ItemType.B),
 C_STATUS_1("some desc5",ItemType.C);
 private final String desc;
 private final ItemType type;
}

Add the following dependencies in pom.xml for less verbose test code

 <dependencies>
 <dependency>
 <groupId>org.projectlombok</groupId>
 <artifactId>lombok</artifactId>
 <version>1.18.24</version>
 </dependency>
 <dependency>
 <groupId>org.junit.jupiter</groupId>
 <artifactId>junit-jupiter</artifactId>
 <version>5.8.2</version>
 <scope>test</scope>
 </dependency>
 <dependency>
 <groupId>org.assertj</groupId>
 <artifactId>assertj-core</artifactId>
 <version>3.21.0</version>
 </dependency>
 </dependencies>

This is the test class showing the expected & test values

public class MapCollectorTest {
 @Test
 void testIt(){
 final ItemRequest originalRequest = ItemRequest.builder()
 .itemStatuses(Arrays.asList(ItemStatus.A_STATUS_1,
 ItemStatus.C_STATUS_1,
 ItemStatus.B_STATUS_2,
 ItemStatus.A_STATUS_2))
 .prop1("abc")
 .prop2("bcd")
 .build();
 Map<ItemType, ItemRequest> expectedMap = new HashMap<ItemType, ItemRequest>();
 expectedMap.put(ItemType.A, ItemRequest
 .builder()
 .itemStatuses(Arrays.asList(ItemStatus.A_STATUS_1,ItemStatus.A_STATUS_2))
 .prop1("abc")
 .prop2("bcd")
 .build());
 expectedMap.put(ItemType.B, ItemRequest
 .builder()
 .itemStatuses(Arrays.asList(ItemStatus.B_STATUS_1))
 .prop1("abc")
 .prop2("bcd")
 .build());
 expectedMap.put(ItemType.C, ItemRequest
 .builder()
 .itemStatuses(Arrays.asList(ItemStatus.C_STATUS_1))
 .prop1("abc")
 .prop2("bcd")
 .build());
 Assertions.assertThat(MapCollector.breakRequestByItemType(originalRequest))
 .containsAllEntriesOf(expectedMap);
 }
}

Answer 1

//TODO: Need a merge function that will add this status to the list itemStatuses

A merge function of toMap is a BinaryOperator which expects two arguments which are Values associated with the same Key, which should be resolved into a single value returned by the merge function.

A quote from the documentation:

mergeFunction - a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)

You need to extract a list itemStatuses from both values and combine them together using Collection.addAll, then return a value whose list has been modified.

(left, right) -> {
 left.getItemStatuses().addAll(right.getItemStatuses());
 return left;
}