How to return a string array of unknown size?

[D. McPherson]

Hi everyone,

My first post here, I'm pretty new to Java and have what I'm sure is a stupid question to ask.

I have a procedure call (srvIMP.registerClient) within a procedure (clientMessenger) that passes an object and a string. Within registerClient these are added to a HashMap collection as a key/value pair. What I would like is for registerClient to then return an array of all the keys in the collection as an array of strings - allowing them to be displayed within a listbox. However, since I don't know how many values the returned array will contain - how can I initialise an array within clientMessenger to receive the returned array?

My code is as follows (I've removed a lot of stuff that isn't pertinent to my question to try to simplify things a little):

 public clientMessenger(String _strCompanyName) throws RemoteException
    {
        strCompanyName = _strCompanyName;

       //register the new client with the server. Return an array of all registered clients
       //Need something below to accept the returned array of strings
       srvIMP.registerClient(this, strCompanyName);

}


public class CommunicateIMPL extends java.rmi.server.UnicastRemoteObject
            implements ICommunicate
{
    
    private String strCompanyName;
    private String strSent;
    
    IClient client;
    
    //Create a map collection to hold a list of the currently connected clients along with the 
    //company names associated with them
    Map<IClient, String> mp = new HashMap<IClient, String>();

    public CommunicateIMPL() throws RemoteException
    {
        // initialise instance variables
    }
    
    //Procedure used to add a client to the list of connected clients upon them connecting
    public String[] registerClient(IClient _client, String _strCompanyName){
        
 	strCompanyName = _strCompanyName;
 	client = _client;
        
 	//put the client instance and the clients company name into the mp collection
 	mp.put(client, strCompanyName);
 	String[] arCompanies = mp.keySet().toArray(new String[0]);
        
 	return arCompanies;
        
    }

//...
}

   

It is the line srvIMP.registerClient(this, strCompanyName) that is the problem - how can I initialise a string array e.g.

String[] myStringArray = new String[?];

when I don't know in advance how many strings are going to be returned?

Sorry if this is a ridiculously simple question, but I would be grateful for any help that you could provide.

Don.

[W. Joe Smith]

Are you dead set on an array, because it seems to me if you don't know how big it is going to be you could use an ArrayList, which allows you to dynamically add elements and not worry about declaring the size.

[Paul Clapham]

You don't have to know the size of the array which that method returns. It's already created, you just have to accept it:

String[] whatever = srvIMP.registerClient(this, strCompanyName);

[D. McPherson]

Thanks for the replies guys

An ArrayList is a possibility, I just hoped that I could avoid that step as an array could be used to directly populate the listbox.

  String[] whatever = srvIMP.registerClient(this, strCompanyName);  

I tried using the above, but got an error on compile of:

"incompatible types - found void but expected java.lang.String[]"

I'll have a go at using an ArrayList, and see how it goes.

Thanks again for the advice,

Don.

[Paul Clapham]

D. McPherson wrote:

  String[] whatever = srvIMP.registerClient(this, strCompanyName);  

I tried using the above, but got an error on compile of:

"incompatible types - found void but expected java.lang.String[]"

Then the method doesn't return anything. Why did you ask how to get the array of strings which it returns, when it doesn't actually return anything?

Trying some other class isn't going to help. The method doesn't return anything, so trying to assign its value to an ArrayList won't work either. You need to go back a step and figure out why you thought getting its return value was something you should do.

[D. McPherson]

Doh!

Found the problem, in the original code I posted I declared the HashMap as shown below:

  Map<IClient, String> mp = new HashMap<IClient, String>();  

I then populated it:

  mp.put(client, strCompanyName);  

i.e. Value and then Key

and of course the order of the parameters should be Key and then Value. The reason I wasn't getting any return values was that I was trying to add IClient objects into an array of strings

Once I got that sorted, everything else went fine and the code snippet that you posted Paul worked a treat. No wonder I couldn't get it to work before!

Cheers for your help guys,

Don.