java stream List<Object[]> to Map<Object, List<Object[]>>

Question

I have List<Object[]> inner join MySQL query, I need to create a map key id and value object.

The Code below works, but how to do it best with Streams?

Map<Object, List<Object[]>> convert(List<Object[]> objects) {
 Map<Object, List<Object[]>> objectListMap = objects.stream()
 .collect(Collectors.toMap(obj -> obj[0], obj -> {
 List<Object[]> list = new ArrayList<>();
 list.add(obj);
 return list;
 }, (obj1, obj2) -> {
 obj1.addAll(obj2);
 return obj1;
 }));
 return objectListMap;
}
Object[] objects structure:
objects[0] = person id
...
objects[10] = event id
...
objects[15] = event name

This query found all the person with the event visited, but the index in the objects array from 0 to 10 may be the same, 11-15 always change.

And I want to merge all object-lists that have the same id (objects[0]).

next for each value in Map> convert to POJO:

PersonWithEvent converToEntity(List<Object[]> objects) {
Optional< PersonWithEvent > personWithEvent = 
objects.stream()
.findFirst().map(obj -> {
 PersonWithEvent p = new PersonWithEvent();
 p.setId((Integer) obj[0]);
 ...
 return p;
});
personWithEvent.ifPresent(p -> {
 List<PersonEvent> personEvents = objects.stream()
 .map(obj -> {
 PersonEvent pe = new PersonEvent();
 pe.setName((String) obj[2]);
 ...
 return pe;
 })
 .collect(Collectors.toList());
 p.setPersonEvents(personEvents);
 });
 return personWithEvent.get();

And Is it possible to do it for one stream?

• Zabuzard: Can you explain the desired structure a bit? It's not that easy to tell from your code. How do you plan to convert the List<Object[]>? What are the keys and what the values for the map? What is the merge function supposed to do?
• Zabuzard: Am I correct that each Object[] array has an id at index 0 and you want to map this id to the rest whole array like objects[0] => objects. And additionally you want to merge all object-lists that have the same id (objects[0]).
• tsarenkotxt: yes, And I want to merge all object-lists that have the same id (objects[0]).

Answer 1

It seems you need to group by element at index zero of an array Object

Collectors.groupingBy groups by the key

Map<Object, List<Object[]>> map = objects.stream()
 .collect(Collectors.groupingBy(o -> o[0]));

with null and empty checks

Map<Object, List<Object[]>> map = objects.stream()
 .filter(o -> o != null && o.length != 0)
 .collect(Collectors.groupingBy(o -> o[0]));

Example

List<Object[]> objects = new ArrayList<>();
objects.add(new Object[] { 0, 1, 2, 3, 4, 5 });
objects.add(new Object[] { 1, 1, 2, 3, 4, 5 });
objects.add(new Object[] { 2, 1, 2, 3, 4, 5 });
objects.add(new Object[] { 0, 6, 7, 8, 9 });
Map<Object, List<Object[]>> map = objects.stream()
 .collect(Collectors.groupingBy(o -> o[0]));
System.out.println(map);

output is like

{
 0 = [[Ljava.lang.Object;@378bf509,
 [Ljava.lang.Object;@5fd0d5ae],
 1 = [[Ljava.lang.Object;@2d98a335],
 2 = [[Ljava.lang.Object;@16b98e56]
}

you can see 0 has two List<Object[]> values grouped by

• Zabuzard: Collectors#groupingBy is actually a nice and clean approach. Never worked with it before but it seems worth to learn.
• Zabuzard: You may consider using the filter .filter(Objects::nonNull) for removing null values. Just in case you didn't know that method.
• Saravana: @Zabuza yes but we need to filter empty arrays as well right here
• Saravana: @alex please vote and accept this answer if it answers your question

Answer 2

I would make the whole procedure more explicit and clean. Using a mixed-style with lots of comments and good variable names.

This should help a lot when it comes down to reading, understanding and maintaining your code.

Map<Object, List<Object[]>> convert(List<Object[]> entities) {
 Map<Object, List<Object[]>> idToEntitesWithSameId = new HashMap<>();
 entities.forEach(entity -> {
 // Create an entry for each entity,
 // merge with entities of same id
 Object id = entity[0];
 // Get current value or empty list
 List<Object[]> entitiesWithId = idToEntitesWithSameId
 .computeIfAbsent(id, ArrayList::new);
 // Add own entity, merge with other
 // entities of same id
 Arrays.stream(entity).forEach(entitiesWithId::add);
 });
 return idToEntitesWithSameId;
}