Java: filter 2 collections of different objects by an attribute value

Question

Given 2 Lists containing 2 kinds of different objects (as if you are getting a collection from an API to create a client or update if it exists):

public static void main(String[] args) {
        List<ClientA> clientsA = new ArrayList<>();
        List<ClientB> clientsB = new ArrayList<>();
        for (int i = 1; i <=5; i++) {
            clientsA.add(new ClientA("JohnA-" + i, "DoeA-" + i, "A-" + i));
            clientsB.add(new ClientB("JohnB-" + i, "DoeB-" + i, "B-" + i));
        }
    }

    @Getter
    @Setter
    @AllArgsConstructor
    static class ClientA {
        private String firstName;
        private String lastName;
        private String ssNumber;

    }

    @Getter
    @Setter
    @AllArgsConstructor
    static class ClientB {
        private String firstName;
        private String lastName;
        private String security;
    }

The purpose is to build a new list of ClientA objects:

if there is an entry in clientsA list with the same ssNumber value equal to the security value of the clients in ClientB list, update the found entry firstNameand lastName attributes;
otherwise, create a new ClientA object with the same attributes/values from the clientsB list, assigning firstName-> firstName, lastName -> lastName, securityNumber -> ssNumber.

I was going to use contains or retainAll methods, but it requires overriding equals and hashCode of the above classes what I can't do.

I expecting to have something like this:

public void process() {
        List<ClientA> clientsA = new ArrayList<>();
        List<ClientB> clientsB = new ArrayList<>();
        for (int i = 1; i <=5; i++) {
            clientsA.add(new ClientA("John-" + i, "Doe-" + i, "A-" + i));
            clientsB.add(new ClientB("JohnB-" + i, "DoeB-" + i, "B-" + i));
        }

        clientsA.add(new ClientA("Samantha", "Smith", "123456789"));
        clientsB.add(new ClientB("Michael", "Smith", "123456789"));

        findExistingEClientsA(clientsA, clientsB);
        findNewClientsB(clientsA, clientsB);
    }

    private void findNewClientsB(List<ClientA> clientsA, List<ClientB> clientsB) {
        Set resultSet = new HashSet();
        for (ClientA clientA : clientsA) {
            List<ClientB> collect = clientsB.stream().filter(c -> !c.getSecurity().equals(clientA.getSsNumber())).collect(Collectors.toList());
            resultSet.addAll(collect);
        }
        System.out.println("+++++++ New clients B +++++++");
        System.out.println(resultSet);
    }

    private void findExistingEClientsA(List<ClientA> clientsA, List<ClientB> clientsB) {
        Set resultSet = new HashSet();
        for (ClientA clientA : clientsA) {
            List<ClientB> collect = clientsB.stream().filter(c -> c.getSecurity().equals(clientA.getSsNumber())).collect(Collectors.toList());
            resultSet.addAll(collect);
        }

        System.out.println("++++++ existing clients B +++++++ ");
        System.out.println(resultSet);
    }

What returns the below result:

++++++ existing clients B +++++++ 
[ClientB{firstName='Michael', lastName='Smith', security='123456789'}]
+++++++ New clients B +++++++
[ClientB{firstName='JohnB-4', lastName='DoeB-4', security='B-4'}, ClientB{firstName='JohnB-2', lastName='DoeB-2', security='B-2'}, ClientB{firstName='JohnB-5', lastName='DoeB-5', security='B-5'}, ClientB{firstName='JohnB-3', lastName='DoeB-3', security='B-3'}, ClientB{firstName='JohnB-1', lastName='DoeB-1', security='B-1'}, ClientB{firstName='Michael', lastName='Smith', security='123456789'}]

Is it a good solution or there is a better one?

But still no success.

Why not just loop through the collection yourself?

Louis Wasserman
– Louis Wasserman

2021-03-19 21:16:37 +00:00
Commented Mar 19, 2021 at 21:16 — Louis Wasserman
– Louis Wasserman, Commented Mar 19, 2021 at 21:16
Just updated the post :).

belgoros
– belgoros

2021-03-19 21:19:56 +00:00
Commented Mar 19, 2021 at 21:19 — belgoros
– belgoros, Commented Mar 19, 2021 at 21:19

Nowhere Man · Accepted Answer · 2021-03-19 22:11:59Z

1

It seems that for both cases an instance of ClientA should be created from ClientB instances and collected into a new list, thus, the following constructor should be added to ClientA:

static class ClientA {
    public ClientA(ClientB b) {
        this(b.getFirstName(), b.getLastName(), b.getSecurity());
    }
}

So the conversion would be a simple remapping:

List<ClientA> newA = clientsB.stream().map(ClientA::new).collect(Collectors.toList());

If the "update" means that the change needs to be promoted to clientsA list, a map of ClientA by ssNumber can be built first:

Map<String, ClientA> map = clientsA.stream()
        .collect(Collectors.toMap(
            ClientA::getSsNumber, clientA -> clientA, 
            (c1, c2) -> c1  // select first clientB if duplicate entries are detected
        ));

Then a new list can be created like this:

List<ClientA> newA = new ArrayList<>();

clientsB.forEach(b -> {
    ClientA a = map.getOrDefault(b.getSecurity(), new ClientA(b));
    a.setFirstName(b.getFirstName());
    a.setLastName(b.getLastName());
    newA.add(a);
});

Or a helper method should be implemented (possibly added to ClientA) to copy the values from ClientB:

public static ClientA copyToA(ClientA a, ClientB b) {
    Objects.requireNonNull(a);
    Objects.requireNonNull(b);
    a.setFirstName(b.getFirstName());
    a.setLastName(b.getLastName());
    return a;
}

Then the new list of ClientA may be built in a more streamed way:

List<ClientA> newClientsA = clientsB.stream()
        .map(b -> map.containsKey(b.getSecurity()) 
            ? copyToA(map.get(b.getSecurity()), b)
            : new ClientA(b)
        )
        .collect(Collectors.toList());

edited Mar 19, 2021 at 22:11

answered Mar 19, 2021 at 21:50

Nowhere Man

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1 Comment

belgoros Over a year ago

Thank you Alex for the response. Yes, the update means: if there is a clientA with the same ssNumber as ClientB's security, update this ClientA instance by assigning the values of firstName, lastName from the corresponding ClientB instance.

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