How to filter collection based on property within double nested list using Java streams

Question

I am in the process of trying to better understand how to use Java streams. I have these classes:

public class Plan {

    List<Request> requestList;
}

public class Request {

    List<Identity> identityList;
    boolean isCancelled;

}

public class Identity {

    String idNumber;
}

I am trying to write a method that returns the plan that contains the non-cancelled request with a matching identity number.

This is what I have tried:

public static Plan findMatchingPlan(List<Plan> plans, String id) {
    List<Plan> filteredPlan = plans.stream()
            .filter(plan -> plan.getRequestList().stream()
                    .filter(request -> !request.isCancelled())
                    .filter(request -> request.getIdentityList().stream()
                        .filter(identity -> identity.getIdNumber().equals(id))))
            .collect(Collectors.toList());
}

This gives me an error:

java.util.stream.Stream<com.sandbox.Identity> cannot be converted to boolean

I sort of understand why there is an error. The nested filter returns a filter that cannot be evaluated as a boolean. The problem is, I don't know what I am missing.

Any help would be appreciated.

Make up your mind. Do you want a Plan (return type)( or a List<Plan> *(local variable). If just a single Plan, do you want first, any, or last match? — Andreas
– Andreas, Commented Apr 26, 2018 at 23:05
@Andreas first match of plan is ideal. My apologies for not cleaning this up. — Seephor
– Seephor, Commented Apr 26, 2018 at 23:13

Andreas · Accepted Answer · 2018-04-26 23:12:55Z

6

Assuming you want first matching Plan, it can be done like this, using lambda expressions:

public static Plan findMatchingPlan(List<Plan> plans, String id) {
    return plans.stream()
                .filter(plan -> plan.getRequestList()
                                    .stream()
                                    .filter(request -> ! request.isCancelled())
                                    .flatMap(request -> request.getIdentityList().stream())
                                    .anyMatch(identity -> identity.getIdNumber().equals(id)))
                .findFirst()
                .orElse(null);
}

Or like this, using method references, and finding any matching Plan:

public static Plan findMatchingPlan(List<Plan> plans, String id) {
    return plans.stream()
                .filter(plan -> plan.getRequestList()
                                    .stream()
                                    .filter(request -> ! request.isCancelled())
                                    .map(Request::getIdentityList)
                                    .flatMap(List::stream)
                                    .map(Identity::getIdNumber)
                                    .anyMatch(id::equals))
                .findAny()
                .orElse(null);
}

answered Apr 26, 2018 at 23:12

Andreas

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3 Comments

Seephor Over a year ago

ah, thank you. so was my issue that I was just filtering without specifying what to do with the filtered streams?

Ousmane D. Over a year ago

Would be nice to have the method return Optional<Plan> to avoid dealing with nullity as well as improving code documentation.

Andreas Over a year ago

@Seephor No, your problem was that filter() is not a terminal operation. In my solution, anyMatch and findAny are both terminal operations.

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