Question

I have two lists with different objects in them.

List<Object1> list1;
List<Object2> list2;

I want to check if element from list1 exists in list2, based on specific attribute (Object1 and Object2 have (among others), one mutual attribute (with type Long), named attributeSame).

right now, I do it like this:

boolean found = false;
for(Object1 object1 : list1){
   for(Object2 object2: list2){
       if(object1.getAttributeSame() == object2.getAttributeSame()){
           found = true;
           //also do something
       }
    }
    if(!found){
        //do something
    }
    found = false;
}

But I think there is a better and faster way to do this 🙂
Can someone propose it?

Thanks!

Answer 1

If you just need to test basic equality, this can be done with the basic JDK without modifying the input lists in the one line

!Collections.disjoint(list1, list2);

If you need to test a specific property, that’s harder. I would recommend, by default,

list1.stream()
   .map(Object1::getProperty)
   .anyMatch(
     list2.stream()
       .map(Object2::getProperty)
       .collect(toSet())
       ::contains)

…which collects the distinct values in list2 and tests each value in list1 for presence.