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Bryan Romero
Asked: 2020-11-12 07:17:01 +0800 CST 2020-11-12 07:17:01 +0800 CST

Method to extract data from a Set in Java

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I am creating a method to extract values ​​from a Client Set , the values ​​in question are the IDs of all clients present in the Set, the ID is obtained by calling getClientID() .

My method:

public String[] extractIDsFromClientSet(Set<Client> clients) {
    Client[] arrayClients = (Client[]) clients.toArray();
    String[] clientIDs = new String[clients.size()];

    for (int i = 0; i < arrayClients.length; i++) {
        clientIDs[i] = arrayClients[i].getClientID();
    }
    return clientIDs;
}

When using my method I get the following exception:

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Lmodels.Client;
at view.userPanes.UserView.extractIDsFromClientSet(UserView.java:128)
at view.userPanes.UserView.setFieldData(UserView.java:82)

How do I correct my method?

java

1 Answers

  1. Best Answer

    As Jack the Ripper said, let's go by parts:

    toArray()

    (Client[]) clients.toArray();

    Object[] Set.toArray()returns a Object[]. Although all the elements of Object[]are instances of Client, that is something the JVM does not control so the casting fails . This method was before Java 5 so you couldn't change the interface to use generics as that would cause errors in existing code.

    The solution is to use the generics version of the method, which is T[] Set.toArray(T[] a). You pass it a Client[](even if it is of length 0) and it will return an instance of Client[]. If you already initially pass it an array of a suitable size, it will return that same array.

    So

    clients.toArray(new Client[0]);

    or even

    clients.toArray(new Client[clients.size()]);

    Of course, another option is to redefine the array to be Object[]and cast on thefor

    Object[] arrayClients = (Object[]) clients.toArray();
...
for (int i = 0; i < arrayClients.length; i++) {
    clientIDs[i] = ((Client) arrayClients[i]).getClientID();
}

    iterable

    But the best part is that, like many collections, Setit already implements Iterable, so you don't need the cast to [] but can instead use an enhanced for .

    int i = 0;
for (Client client : clients) {
   clientsIDs[i] = client.getClientID();
   i++;
}

    or the classic version (although I recommend the enhanced for ):

    for(Iterator<Cliente> it = clients.iterator(); int i = 0; it.hasNext(); i++) {
    clientsIDs[i] = it.next().getClientID();
 }
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