Re: Checking for null parameter

From:
Lew <conrad@lewscanon.com.invalid>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 11 Jun 2008 09:07:54 -0400
Message-ID:
<sJWdnS4cDd08UNLVnZ2dnUVZ_t6dnZ2d@comcast.com>
pek wrote:

OK... Let me take you back to what was the meaning of this whole
discussion because I believe that the thing is going out of hand. The
discussion was about CHECKING a null reference, not HANDLING. Here is
an example:

public class ChatExample {
  private Connection conn;

  public ChatExample() {
    // I intentionally forget to instantiate the conn
  }

  public void connect() throws ConnectionException {
    try {
      conn.connect();
    } catch (ConnectionExcetpion e) {
      // Delegate the exception
      throw new ConnectionException("Connection failed in Chat",e);
    }
  }

  public sendMessage(String message) throws ConnectionException {
    try {
      conn.sendMessage(message);
    } catch (ConnectionException e) {
      // Delegate the exception
      throw new ConnectionException("Couldn't send message in
Chat",e);
    }
  }

  class Connection {
    public void connect() throws ConnectionException {
      // Do socket connection etc. etc.
    }
    public void sendMessage(String message) throws ConnectionException
{
      // Send the message using sockets etc. etc.
    }
  }
}

public class Main {
  public static void Main() {
    // Catch and log any exceptions that wheren't planned without
crashing
    try {
      startProgram();
    } catch (NullPointerException e) {
      // Log it and inform user
    } catch (Exception e) {
      // Same here (probably combining with the above exception
wouldn't have any difference
    }
  }

  private void startProgram {
    // Do anything
    // At some point you want to send a message
    ChatExcample chat = new ChatExample();
    chat.sendMessage("Hello world");
  }
}

OK, now, did you notice that I forgot to instantiate the conn
attribute? That is a bug, which means that the user can NEVER send a
message simply because it is a NPE and it will stay that way no matter
what.


For this sort of thing, use 'assert':

  public class ChatExample {

    private final Connection conn; // final added

    public ChatExample() {
      // I intentionally forget to instantiate the conn

      assert conn != null;
    }

    public void connect() throws ConnectionException {
      assert conn != null;

      try {
        conn.connect();
      } catch (ConnectionExcetpion e) {
        // Delegate the exception
        throw new ConnectionException("Connection failed in Chat",e);
      }
    }

    public sendMessage(String message) throws ConnectionException {
      assert conn != null;

      try {
        conn.sendMessage(message);
      } catch (ConnectionException e) {
        // Delegate the exception
        throw new ConnectionException("Couldn't send message in
  Chat",e);
      }
    }

We assume some appropriate mechanism for invoking conn.close(), not shown here.

Now it is impossible to throw NPE. Problem solved.

As for rethrowing the ConnectionException, that is controversial. At some
point you should recover to correct program state instead of propagating the
exception. Also, in real life of course one would log the ConnectionException.

--
Lew

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