Checking for null inputs

[Majid Khan]

Hi all,
How does one check for null values from input
what happens when I try this :
if (name != null) {}
else {};
what if name is 'null', is the Else ever executed?
please elaborate on this.
I get in the above case null pointer exception.
regards,
Majid

[Arun Boraiah]

Hi,
If the name is null then else part will get executed.

[William Barnes]

Is "name" a string?

[Francis Siu]

hi Majid
One method you can use try{} and catch {} box to check it

[Majid Khan]

Hi again,
I guess this does not seem as simple as it looks.
The qstn how can one can compare null input with null.
I want to avoid null ptr exception.
In this case I have converted name to a string ,
Even checking the original object gives null ptr exception.
I hope you can elaborate all cases.

thanks

[Thomas Paul]

I'm not sure what the problem is.
Let's say we have this:

String nullString = null;
if (nullString == null) {
    System.out.println("It's a null String");
} else {
    System.out.println("It's not a null String");
}

Or you can do this:

String nullString = null;
if (nullString != null && nullString.equals("a value")) {
    System.out.println("String equals \"a value\"");
} else {
    System.out.println("String doesn't equal \"a value\"");
}

Or you can do this:

String nullString = null;
if ("a value".equals(nullString)) {
    System.out.println("String equals \"a value\"");
} else {
    System.out.println("String doesn't equal \"a value\"");
}

Why not give us a bit of a better description of the problem you are having.

[Jim Yingst]

Majid - if you're getting a NullPointerException, look at the stack trace, and find out exactly which line number is throwing the exception. Study that line. If you can't figure out why it's a problem, post the code here (especially the code of the line that throws the exception) and we'll see what's going on.

[Mary Leners]

How about:

try
{
name = request.getParameter("NAME");
}
catch (nullPointerException npe)
{
// set name equal to "" or whatever you want to
// do when you encounter a null
}

[Thomas Paul]

Mary, your solution won't throw a NullPointerException (unless the request object is null but that should never happen). In any case, throwing an exception is EXPENSIVE on system resources and should never be done if it doesn't have to be.

[Layne Lund]

Majid, you really need to explain your problem in more detail. Post any error messages you get (compiler error or run-time exceptions). Also, post the relevent code to give this whole situation some context. In particular, post the method that is causing you problems. Also post declarations of any class-level variables. We will be much better able to help you from there.
Layne

[Majid Khan]

HI ALL ,
THKS for your inputs here is more of code
SessionDictionary sessionDictionary = p_formSession.getSessionDictionary();
name = sessionDictionary.getPropertyValue((String)getAttributeValue(2)).toString()
this is the error i get:
java.lang.NullPointerException
at tblreferenceinsert.evaluate(tblreferenceinsert.java:78)
line 78 is where i get name value
name =.........
any pointers?
this is the error i get :

[Jim Yingst]

NullPointerException always means that if you look at the line that's throwing the exception and you look for dots (.), and then you look at whatever is immediately to the left of a dot, then one of those expressions to the left of a dot is equal to null. So in this line
name = sessionDictionary.getPropertyValue((String)getAttributeValue(2)).toString()
either sessionDictionary is null, or getPropertyValue() returns null. You can rewrite this to add some debugging statemtents:

    System.out.println("sessionDictionary = " + sessionDictionary);
    String attValue = (String)getAttributeValue(2);
    System.out.println("attValue = " + attValue);
    Object propValue = sessionDictionary.getPropertyValue(attValue);
    System.out.println("propValue = " + propValue);
    name = sessionDictionary.getPropertyValue((String)getAttributeValue(2)).toString();
    System.out.println("name = " + name);

Something is null. Find out what, and then study why it's null, and think about what to do about it.

[Dirk Schreckmann]

Depending on which part of the exception stack trace is being considered, the culprit null reference could also be an argument to a method invoked on that line.

[Jim Yingst]

Right. To clarify, my previous statement assumed that we're looking at the very first line of the stack trace, which is the second line overall (after the message "java.lang.NullPointerException"). E.g.
java.lang.NullPointerException
at tblreferenceinsert.evaluate(tblreferenceinsert.java:78)
at tblreferenceinsert.someOtherMethod(tblreferenceinsert.java:1234)
at tblreferenceinsert.yetAnotherMethod(tblreferenceinsert.java:5678)
Here we only care about the first "at" line, which tells us the exception is being thrown from line 78. In this case the exception is not being thrown from some other method called by line 78; it's being thrown from 78. Which means something on that line is null. And it's something that has a dot immediately to the right - that's what causes the NullPointerException, a null followed by a dot.

[Thomas Paul]

name = sessionDictionary.getPropertyValue((String)getAttributeValue(2)).toString()
sessionDictionary might be null or the getPropertyValue() method is returning null.

[Jim Yingst]

That sounds familiar for some reason.

[Majid Khan]

Hi All,
Thanks for answers .
Also many thanks to Jim, his comments did help to sort the problem.
the session dictionary object throws null.
In this case will an if else statement work or will it return the same answer for both cases ?
I explicitly used multiple if statements.