Re: Checking for null parameter

From:
pek <kimwlias@gmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 11 Jun 2008 06:26:28 -0700 (PDT)
Message-ID:
<8ad473d9-9452-47d6-a22e-ae2d7dddf4fb@8g2000hse.googlegroups.com>
On Jun 11, 4:07 pm, Lew <con...@lewscanon.com.invalid> wrote:

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 ConnectionExcepti=

on

{
      // 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 so=

me

point you should recover to correct program state instead of propagating t=

he

exception. Also, in real life of course one would log the ConnectionExc=

eption.

--
Lew


Also, by what I see, you changed your code from

if ( conn == null ) {
  handleIt();
} else {
  conn.connect();
}

to

assert conn != null

WHICH IS STILL CHECKING FOR NULL!

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