Intersection between two HashSets in Java 8 - collections

Question

I have an object as following :
public Class MyObjDTO {
private Long id;
private Boolean checked;
//getter and setters
#Override
public final int hashCode() {
Long id = getId();
return (id == null ? super.hashCode() : id.hashCode());
}
#Override
public boolean equals(final Object obj) {
if (this == obj)
return true;
if (!(obj instanceof MyObjDTO))
return false;
Long id = getId();
Long objId = ((MyObjDTO) obj).getId();
if (id.equals(objId)) {
return true;
} else {
return false;
}
}
}
And I have two hash sets containing some instances from this object :
HashSet oldSet = new HashSet();
oldSet.add(new MyObjDTO(1,true));
oldSet.add(new MyObjDTO(2,true));
oldSet.add(new MyObjDTO(3,false));
HashSet newSet = new HashSet();
newSet.add(new MyObjDTO(1,false));
newSet.add(new MyObjDTO(2,true));
newSet.add(new MyObjDTO(4,true));
So what I want to do here is to select objects that are in the newSet and not in the oldSet, in this case its : new MyObjDTO(4,true) which I did using this :
Stream<MyObjDTO> toInsert = newSet.stream().filter(e -> !oldSet.contains(e));
Then I want to select objects that are in the oldSet and not in the newSet, in this case its :new MyObjDTO(3,false) which I did using this :
Stream<MyObjDTO> toRemove = oldSet.stream().filter(e -> !newSet.contains(e));
The last step is that I want to select the objects that are in both newSet and oldSet but they have a different value for the attribute checked , in this case it's : new MyObjDTO(1,false).
What I tried is this :
Stream<MyObjDTO> toUpdate = oldSet.stream().filter(newSet::contains);
But this one will return both new MyObjDTO(1,false) and new MyObjDTO(2,true).
How can I solve this ?

Answer 1

One way is to first use a map and then adjust your filter condition:
Map<MyObjDTO, Boolean> map = newSet.stream()
.collect(Collectors.toMap(Function.identity(), MyObjDTO::getChecked));
Stream<MyObjDTO> toUpdate = oldSet.stream()
.filter(old -> newSet.contains(old) && old.getChecked() != map.get(old));

Answer 2

Firstly, your equals() and hashCode() methods violate their basic contract. As per the javadoc of hashCode():
If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
Your implementation of hashCode() does not follow this contract. Your first step should be to fix that.
Secondly, since Java 1.2 (nearly 20 years ago), java has provided the method removeAll() that does exactly what you want to do for the first part:
// Given these 2 sets:
HashSet<MyObjDTO> oldSet = new HashSet<>();
HashSet<MyObjDTO> newSet = new HashSet<>();
HashSet<MyObjDTO> onlyInNew = new HashSet<>(newSet);
onlyInNew.removeAll(oldSet);
// similar for onlyInOld
For the second part, you'll need to create a Map to find and get the object out:
Map<MyObjDTO, MyObjDTO> map = new HashMap<>O;
oldSet.forEach(o -> map.put(o, o);
HashSet<MyObjDTO> updated = new HashSet<>(newSet);
updated.removeIf(o -> oldSet.contains(o) && o.getChecked()() != map.get(o).getChecked());

Answer 3

In the last step, you rely on the equals() method of the DTO :
Stream<FonctionnaliteDTO> toUpdate = oldSet.stream().filter(newSet::contains);
The method uses only the id field to determinate object equality.
You don't want to do that.
You want to filter on a specific field : checked.
Besides, you should perform the operation on the result of the intersection of the two Sets.
Note that you should use simply Collection.retainAll() to compute the intersection between two collections:
Set<MyObjDTO> set = ...
Set<MyObjDTO> setTwo = ...
set.retainAll(setTwo);
Then you can filter objects that have both same id and checked value by using a double loop : for + iterator.
for (MyObjDTO dto : set){
for (Iterator<MyObjDTO> it = set.iterator(); it.hasNext();){
MyObjDTO otherDto = it.next();
if (otherDto.getId().equals(dto.getId()) &&
otherDto.getChecked() == dto.getChecked()){
it.remove();
}
}
}
You could do that with Stream but IHMO it could be less readable.