number of items in the second list that aren't in the first list in java

Question

Let's say my first list is:

[{tenancyNumber:777,no:1}, {tenancyNumber:888,no:2}, {tenancyNumber:999,no:3}]

And my second List is

[{tenancyNumber:444,no:4}, {tenancyNumber:999,no:5}, {tenancyNumber:666,no:6}]

So, if we compare by name in the two lists there are 2 objects in the second list which are not in the first list. How to find out this in Java?

This is what i tried:

final Long newAccounts = endDateData.stream()
  .map(TenancyHistory::getTenancyNumber)
  .filter(startDateData.stream().map(TenancyHistory::getTenancyNumber)::equals)
  .count();

which is throwing an error

[PredicateIncompatibleType] Predicate will always evaluate to false because types Stream and Long are incompatible

Answer 1

You can first create a lookup Set of tenancy numbers for easing in the filter while you iterate on the second list.

Set<Long> tenancies = startDateData.stream()
                                   .map(TenancyHistory::getTenancyNumber)
                                   .collect(Collectors.toSet());

Now the attempt you made can be fixed as;

final Long newAccounts = endDateData.stream()
                                    .map(TenancyHistory::getTenancyNumber)
                                    .filter(num -> !tenancies.contains(num)) // here
                                    .count();

---

To help you understand the error message that you were facing, consider the code

(end/start)DateData.stream().map(TenancyHistory::getTenancyNumber)

this would return a Stream<Long> , now what you ended up performing in your filter stage was to compare Stream<Long> to a Long value of tenancy number from another Stream as

startDateData.stream().map(TenancyHistory::getTenancyNumber)::equals

and hence the error message read this would evaluate always to false . Such that resulting value of your would always be 0 .