Check null value question?

[Elahe Shafie]

Hi Friends,
I want to check if bug is null (bug is object) but I catch the error exception:
Exception occured
Message: $exceptionmsg
String: java.lang.NullPointerException
Could you please correct me what part I am wrong in syntax?
Many thanks,
ELahe

...
   bug = model.getBug(nBugNumber );
      
      /* -- 3 ---- generate appropriate response---  */
      if (bug.equals(""))
      {
        ctx.put("bug", null );
      }
      else
        ctx.put("bug", bug ); 
...

[Elahe Shafie]

Sorry I want to check if bug is empty...

[Bear Bibeault]

Your code:

if (bug.equals(""))

is not a check for null. What you are comparing bug against in this case is an empty String instance.
To check an object reference for null you'd use:

if (bug == null)

hth,
bear

[Bear Bibeault]

OK, so you say you want to check that bug is "empty". Well, what is bug? How is its equals() method defined?
In your code, you are checking bug against an emtpy String instance. Is that the check you want to make?
I guess we'd need a bit more information on what you expect "check if bug is empty" to mean in the context of what exactly bug is.
bear

[Jim Yingst]

Assuming you do indeed need to check to see if bug is an empty string (""), the problem seems to be that you first need to check for null, before you can use .equals() or any other instance method. There are a lot of ways to achieve this:

    if (bug == null)
      ctx.put("bug", null);
    else if (bug.equals(""))
      ctx.put("bug", null);
    else
      ctx.put("bug", bug); 

or

    if (bug == null | bug.equals(""))
      ctx.put("bug", null);
    else
      ctx.put("bug", bug); 

or

    if (bug != null && bug.equals(""))
      bug = null;
    ctx.put("bug", bug);

or

    if ("".equals(bug))
      bug = null;
    ctx.put("bug", bug);

or

    ctx.put("bug", ("".equals(bug)) ? null : bug);

But if you find it necessary to do this sort of thing more than once in your code, I'd favor making a utility method to handle the sitation, since a named method will be simpler to use elsewhere:

  /** 
   * If input string is empty (""), replace it with null;
   * otherwise return input unchanged.
   */
  private static String fixEmptyString(String input) {
    return "".equals(input) ? null : input;
  }
 
  ...
  
  // later:
 
    ctx.put("bug", fixEmptyString(bug));
 
  ...

[ June 10, 2002: Message edited by: Jim Yingst ]

[Ilja Preuss]

Originally posted by Jim Yingst:

    if (bug == null | bug.equals(""))
      ctx.put("bug", null);
    else
      ctx.put("bug", bug); 

That wouldn't work - you need to use the "shortcut-or" operator, so that the second operand doesn't get evaluated when the first is true:

    if (bug == null || bug.equals(""))
      ctx.put("bug", null);
    else
      ctx.put("bug", bug); 

[Elahe Shafie]

Dear friends,
To answer your question, bug is instance an object that calls Bug and I try to check if bug is empty then pass null to my template.
So I tried this way as you suggested but do I need to have this line as well?
bug = null;
thanks,
Elahe

 if ("".equals(bug)) 
      {
        bug = null;
        ctx.put("bug", null );
      }
      else
        ctx.put("bug", bug ); 

[Jim Yingst]

That wouldn't work - you need to use the "shortcut-or" operator
Thanks - typo. That's what I get for listing so many different versions. :roll:
So I tried this way as you suggested but do I need to have this line as well?
bug = null;
Not unless you do something else with bug afterwards. I was assuming it was a local variable that you would forget about afterwards, but if that's not the case, you'll need to decide for yourself whether you want the value to be "" or null later on.

[Elahe Shafie]

Got it and thanks every one.