can we return null?

[sarada chellus]

Hi all,

I am reviewing the code and found that method returns null in the below code.
Please think as per code review.
Is it valid to return null as it may lead others to take care of Nullpointer Exception?

public Image getImage(Object item) {
try {
if (item instanceof IMarker) {
return getBreakpointImage((IBreakpoint) bp);
}

if(item instanceof IUDTThread){
return getUDTThreadImage((IUDTThread)item);
}
} catch (CoreException e) {
UDTDebugUIPlugin.logError(e);
}
return null;
}

[Rob Spoor]

Whether it is valid to return null is up to your business logic. Sometimes you want to return null indicating something is wrong, sometimes you want to throw an exception.

For instance, java.util.Map's get method returns null if there is no key-value mapping for the specified key. java.util.List's get method on the other hand throws an IndexOutOfBoundsException if the index is invalid.


Should you choose to allow a null return value, you should document that this is a valid return value. Then programmers that call your method know that they should check if the return value is null:

Image image = myObject.getImage(someObject);
if (image == null)
{
    // handle a null return value
}
else
{
    // handle a non-null return value
}

[Martijn Verburg]

Hi there,

Yes it can be considered a valid return technique, it just needs to be clearly documented in the Javadoc for that method e.g. "Returns x if one exists else null".

[Nitin Pathak]

As rightly pointed out by Rob, there are circumstances (rather language specifications) which need to be followed during implementation. Since, the pre-existing return statements return the appropriate values local to 'if' loops, 'return null' is added as per the java specifications - considering a situation if your control never enters the 'if' condition; since the return type of your method is Image (a subclass of the Almighty Object).
[ December 17, 2008: Message edited by: Nitin Pathak ]

[Rob Spoor]

But as I also said, you can choose to throw an exception instead:

public Image getImage(Object item)
{
    // I'm ignoring exceptions for simplicity
    if (item instanceof IMarker)
    {
        return getBreakpointImage((IBreakpoint)bp); 
    }
    if (item instanceof IUDTThread)
    {
        return getUDTThreadImage((IUDTThread)item);
    } 
    throw new IllegalArgumentException(item + " is invalid");
}

In the end, the choice is up to you.

[Sunil Kumar]

Agreeing with Rob, I have another point in terms of code review. Methods should always have a single return statement i.e. create a reference, point it to whatever value you want as per your logic (including null) and finally return the reference at the end.

[Sunil Kumar]

Again that is my point of view. Others may differ

[Yegor Bugayenko]

It's an old topic but still relevant. See this blog post: http://www.yegor256.com/2014/05/13/why-null-is-bad.html It claims that using (returning or accepting) NULL is a terrible practice