Factory design pattern - Not use static methods because unit testing is a problem

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I know this question has been asked several times in stackoverflow but somehow still had some trouble figuring out a solution. The following example I think is a good case to have static methods

public class ConnectionFactory
{
     public static Connection createConnection(ConnectionType connectionType, String ipAddr, Integer port)
    {
           //Some error checking
         switch(connectionType)
         {   
             case TCP:
                  return createTcpConnection(ipAddr, port);
             case UDP:
                  return createUdpConnection(ipAddr, port);
             case RTP:
                  return createRtpConnection(ipAddr, port);
             case SCTP:
                  return createRtpConnection(ipAddr, port);
             default:
                  break;
         }
    }

    // TcpConnection, RtpConnection, SctpConnection and UdpConnection implement interface Connection
    public Connection createTcpConnection()
    {
        Connection connection = new TcpConnection();
         .....
         .....
         return connection;
    }

    public Connection createUdpConnection()
    {
        Connection connection = new UdpConnection();
        .....
        .....
        return connection;
    }

    ....
    ....
}

And suppose if I have a CommunicationService like the following

public class CommunicationService
{
    public void initConnectionPool(ConnectionType connectionType)
    {
        for(int i = 0; i < MAX_CONNECTIONS; i++)
             connectionList.add(ConnectionFactory.createConnection(connectionType, "domain.com", 40203));

        //Some more code here to do further processing
          ......
          ......
    }    

    //Some more methods
}

Like this, different communication services can create and maintain multiple type of connections.

I want to test initConnectionPool method and in a unit test environment, socket creation will definitely fail.

I can change the ConnectionFactory to a concrete class and mock it out. But isn't this case a good situation to create a class with static methods? I am not trying to maintain any state in ConnectionFactory. So, if using static methods where it may be appropriate may cause testing problems when do we use static methods? Or is it not appropriate to use static methods here?

EDIT: Solution I went with

public class CommunicationService
{
    public void initConnectionPool(ConnectionType connectionType)
    {
        for(int i = 0; i < MAX_CONNECTIONS; i++)
             connectionList.add(connectToHost(connectionType));

        //Some more code here to do further processing
          ......
          ......
    }    

    public Connection connectToHost(ConnectionType connectionType)
    {
        ConnectionFactory.createConnection(connectionType, "domain.com", 40203)
    }
    //Some more methods
}

In the test overrode connectToHost and returned a mock.

factory-pattern junit design-patterns java
9 Answers

Another common practice is to define your own helper class which can wrap calls static methods and constructors. It is quite useful if you need to mock some third-party libraries, that you can't rewrite.

public class CommunicationService {
    private final CommunicationServiceHelper helper;
    public CommunicationService() {
        this(new CommunicationServiceHelper());
    }

    @VisibleForTesting
    CommunicationService(CommunicationServiceHelper helper) {
        this.helper = helper;
    }

    public void initConnectionPool(ConnectionType connectionType)
    {
        for(int i = 0; i < MAX_CONNECTIONS; i++)
             connectionList.add(helper.createConnection(connectionType, "domain.com", 40203));

        //Some more code here to do further processing
          ......
          ......
    }    

    //Some more methods

    @VisibleForTesting
    static class CommunicationServiceHelper {
        public Connection createConnection(ConnectionType connectionType, String ipAddr, Integer port) {
            return ConnectionFactory.createConnection(connectionType, ipAddr, port);
        }
    }
}
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