calling a superclass constructor

kpachopoulos · 09-04-2006, 07:46 AM

Hi,
in the following code segment i get the error, which is marked in comments. What does it mean? I don't seem to find anything relevant

Code:

public class MulticastIncoming extends DatagramIncoming
{
    MulticastSocket msock;
    InetAddress multicastserverIP;    
    	
    public MulticastIncoming() throws SocketException, IOException, UnknownHostException
    {    	
    	super(msock); //CANNOT REFER TO AN INSTANCE FIELD MSOCK, WHILE EXPLICITLY INVOKING A       
                      //CONSTRUCTOR --> ?????? super(new MulticastSocket(PORT)) for example works OK
    	msock=new MulticastSocket(PORT);
    	multicastserverIP=InetAddress.getByName(SERVER);    	
	msock.joinGroup(multicastserverIP);		
    }
    ..........
}

Instead of calling the super() method, the following would compile without any errors. Does it have the same results with the previous constructor? If it is exactly the same, shouldn't the compiler throw some kind of error/warning here too?

Code:

public class MulticastIncoming extends DatagramIncoming
{
    MulticastSocket msock;
    InetAddress multicastserverIP;    
    	
    public MulticastIncoming() throws SocketException, IOException, UnknownHostException
    {    	
 
    	msock=new MulticastSocket(PORT);
    	multicastserverIP=InetAddress.getByName(SERVER);    	
	msock.joinGroup(multicastserverIP);		

        new DatagramIncoming(msock);
    }
    ..........
}

ntubski · 09-04-2006, 12:15 PM

The error message is saying that msock is uninitialized before the constructor is run. You can't use variables from the child class before you finish the constructor of the parent.

The second version is slightly different since it calls the default superclass constructor (ie the one without any arguments). Also calling

Code:

new DatagramIncoming(msock);

after the first bit of code won't have any effect: it will create a DatagramIncoming object then discard it.

I'm wondering if you need a seperate MulticastSocket variable in your MulticastIncoming class. If you already have a MulticastSocket variable called msock in the parent DatagramIncoming class then you can reference it from the child class. Assuming the constructor for DatagramIncoming sets the MulticastSocket variable, then you could do this:

Code:

...
public MulticastIncoming() throws SocketException, IOException, UnknownHostException
    {    	
    	super(new MulticastSocket(PORT));

    	multicastserverIP=InetAddress.getByName(SERVER);    	
	msock.joinGroup(multicastserverIP);		
    }
...

Of course I'm just guessing about your DatagramIncoming class.