NullPointerException in StringToCollectionConverter.convertToPresentation

Vaadin 6 / 7 / 8 Framework Data Binding
1

I am using StringToCollectionConverter, to convert between a List property and a TextField.
But the StringToCollectionConverter throw a NullPointerException, because my property is null.

In the source code of StringToCollectionConverter.convertToPresentation (ver 7.6.1):

    public String convertToPresentation(Collection value,
            Class<? extends String> targetType, Locale locale)
            throws Converter.ConversionException {
        ......
        for (Iterator<?> iterator = value.iterator(); iterator.hasNext();) {
            ......
        }
        ......
    }

There is no null check for value.

And also, In the source code of StringToCollectionConverter.convertToModel:

public Collection convertToModel(String value, Class<? extends Collection> targetType, Locale locale) throws Converter.ConversionException { int index = value.indexOf(delimiter); ...... } There is no null check for value, too.

Is this a bug?

2

Hi Ying,

this is not a bug - it’s good practice to not check for null, in order to then throw ConversionException or RuntimeException or yet another NullPointerException.
Quite obviously, for the two methods to work in a sensible fashion, the ‘value’ must be non-null; and the NullPointerException is signalling exactly that this precondition was violated.

If you want to use an empty collection or an empty String, why not use
List myEmptyList = new ArrayList();
or
String myEmptyString = “”;
for example?

Best,
Enver

3

Hi Enver,
Thanks for your answer.
I will set a default empty list value for my property, to avoid this NullPointerException.

But I still don’t understand:

The other Converters doing this check.
For example,

in method AbstractStringToNumberConverter.convertToPresentation(T value,
Class<? extends String> targetType, Locale locale)
throws ConversionException {
if (value == null) {
return null;
}
…
}

and in method AbstractStringToNumberConverter. convertToNumber(String value,
Class<? extends Number> targetType, Locale locale)
throws ConversionException {
if (value == null) {
return null;
}

…
}

both doing null value check.

And in StringToDateConverter, it also doing null value check.

Looks like StringToCollectionConverter is the only one which not doing null value check.

Why?

4

You are right in that this is somewhat inconsistent.
We never really specified what should happen in a Converter when it is asked to convert a NULL.

Now that you spotted that one converter behaves different from the others in a non-specified corner
case I invite you to write a ticket on this so we can take care of it: https://dev.vaadin.com/
Please mark it as ‘enhancement’.

Thanks for helping making Vaadin better,
–Enver

5

Hi Enver,
Thanks for your answer.

I created a ticket for this problem:

https://dev.vaadin.com/ticket/19481

Please check it. Thanks very much

6

Thank you!

We’re working on it.
https://dev.vaadin.com/review/#/c/12897/

Thank you again for pointing it out.

Best,
–Enver