How to compare a property of an object in a list with another property of a similar object in different list using java Sreams using Java 11

Question

Please consider 2 Classes:

Class Object1 {
  int id;
  String test;
  ...
}

Class Object2 {
  int id;
  Boolean flag;
  ...
}

Object1 and Object2 are not equal.

Now, please consider I have a list of Object1 and Object2 as follows:

List<Object1>
Set<Object2>

Problem statement: I need to write a filter condition in the Java streams where

Object1.getId() == Object2.getId().

Can you please guide me here.

Edit: I want to filter List.

Do you want to compare elements pairwise or check for inclusion anywhere in the second collection? I want to check for inclusion anywhere in the second collection.

Answer 1

You can use Java 8 Lambda.

1. Convert set to stream.
2. Use stream filter
3. Use containsKey to compare
4. Collect it to List.

Object1 List and Object2 Set

List<Object1> object1List = ... 
Set<Object2> object2Set = ... 

Filter Object1

Map<Integer, Object2> object2Map = object2Set.stream()
    .collect(Collectors.toMap(Object2::getId, Function.identity()));

List<Object1> result = object1List.stream()
    .filter(a -> object2Map.containsKey(a.getId()))
    .collect(Collectors.toList());