Question

I have two sets of objets and I want to get the intersection of the two sets. The objects in the sets look like this

@BeanInfo
class User {

  @JsonProperty
  @BeanProperty
  var name:String = ""

  @JsonProperty
  @BeanProperty
  var id:Long = 0

  override def toString = name

  override def equals(other: Any)= other match {
      case other:User => other.id == this.id
      case _ => false
   }

}

In another class I get the sets of users and want to see the intersection.

val myFriends = friendService.getFriends("me")
val friendsFriends = friendService.getFriends("otheruser")
println(myFriends & friendsFriends) 

The above code does not work and prints

Set()

However if I manually loop over the sets using foreach I get the desired result

var matchedFriends:scala.collection.mutable.Set[User] = new HashSet[User]()    
myFriends.foreach(myFriend => {
  friendsFriends.foreach(myFriend => {
      if(myFriend == myFriend){
        matchedFriends.add(myFriend)
      }
  })
})
println(matchedFriends)

the above code prints

Set(Matt, Cass, Joe, Erin)

This works just fine

val set1 = Set(1, 2, 3, 4)
val set2 = Set(4,5,6,7,1)

println(set1 & set2)

The above prints

Set(1, 4)

Do the set operations & &- etc.. only work on primitive objects ?
Do I have to do something additional to my user object for this to work ?

Answer 1

I’m not 100% positive about this, but I think your issue is caused by having implemented a custom equals without a corresponding custom hashCode. I’m sort of surprised your hash sets are working at all, actually…

Your manual loop through the elements of each set works fine, of course, because you don’t call hashCode at all 🙂