How to obtain an Intersection of a List of complex custom Objects and a List of its String attributes and in Java using Stream API

Question

Here's my domain classes Location and Address:

public class Location {
    GPS gps;
    Address address;
    // etc.
}

public class Address {
    String street;
    String number;
    // etc.
}

Let's say I have a List of String's

List<String> houseNumbers

and a List of Locations

List<Location> locations

Now, I want to have only those Locations where the Location.address.number matches with any of the Strings in the houseNumbers list.

I've the following, which produces a compilation error: "expecting a Predicate <? super java.lang.String> instead of a String".

My attempt:

List<Location> filteredLocations = locations.stream()
    .anyMatch(location -> 
        housenumbers.stream().anyMatch(location.address.number)
    );

But how do I make sure to compare to every item in the houseNumbers List?

The following snippet did the trick:

List<Location> filteredLocations = 
 locations.stream().filter(
   location -> housenumbers.contains(location.address.number)
).collect(Collectors.toList());

***But how do I make sure to compare to every item in the List houseNumbers?*** - Use `filter` instead of `anyMatch` . — Arvind Kumar Avinash, Dec 09 '22 at 15:25
With filter it is still complaining about the Predicate. locations.stream().filter( location -> housenumbers.stream().anyMatch(location.address.number) ) — midi, Dec 09 '22 at 15:28
When posting questions, please put in minimal code to demonstrate the issue. That includes all classes and data structures required with input and expected output. It should be in the form of a [mre]. And you should also take the [tour]. — WJS, Dec 09 '22 at 15:28

Alexander Ivanchenko · Accepted Answer · 2022-12-09T15:46:14.003

Dump the data from the houseNumbers into a HashSet a then in the stream check the house number of each Location against the Set.

For that, you would need to apply:

filter() operation to discard the locations, which has a house number that is not present in the list;
toList() as a terminal operation to collect the result into a list (or collect() with Collectors.toList() specified as an argument if you're using JDK 15 or earlier).

List<String> houseNumbers = // initializing the list
Set<String> numbers = new HashSet<>(houseNumbers);

List<Location> locations = // initializing the list
    
List<Location> filteredLocations = locations.stream()
    .filter(location -> numbers.contains(location.getAddress().getNumber()))
    .toList(); // for Java 16+ or collect(Collectors.toList())

*Sidenote: in Java, we use access modifiers to encapsulate the data properly withing the class. And getter methods serve a mean of accessing the data. Don't keep the fields public (or package-private). If you're doing a lot of JavaScript you might be accustomed to this location.address.number, but in Java it's not appropriate.

How to obtain an Intersection of a List of complex custom Objects and a List of its String attributes and in Java using Stream API

1 Answers1