Roy T. Fielding wrote:
I am sorry, but that is factually incorrect. Please look it up in any
book on programming languages. Null (or its synonyms, like undef in perl)
is a language feature for indicating that a name (pointer, object,
reference, etc.) does not exist.
Since we are in the Java world so let's be precise.
In Java `null` is a type and the reference and reference itself is a value.
[quote "Java Language Specification Second Edition"]
There is also a special null type, the type of the expression null, which has no
name. Because the null type has no name, it is impossible to declare a variable
of the null type or to cast to the null type. The null reference is the only
possible value of an expression of null type.
[/quote]
Null is never an integer,
even though C will convert null to 0 within an int context (by design).
[quote "Java Language Specification Second Edition"]
The null reference can always be cast to any reference type.
[/quote]
Consider a method:
public void setAge(Integer age) - where I receive an age as null reference.
And a call:
setAge((Integer)null) - where I pass null reference as Integer.
Within a data model, there are no language references. There is no
value of null within hierarchical data structures like JCR. There is
merely the presence or absence of names. That is why setting a
property value to null causes the property to be deleted, just like
it does in perl. The only equivalent to null within JCR would be
a reference property with an empty string value, which is generally
considered to be an error.
We are discussing "Content Repository for Java" not for Perl or C so it is
understandable that we will refer to Java Language Specification for guidelines.
And when I set a variable value to null Java will not delete the variable
neither make it not referenceable.
If we will discuss "Content Repository for Perl" I will not bring this issue up,
but for Java it is unexpected behavior, and as you can see, many participants
from this list agree with it.
All of the examples you have given are just unwise design.
Could you please provide a `wise` design for optional date field that should be
stored in JCR. So we can use it to ease our pain.
When you read XML that
contains default attribute values, the reader of the XML is responsible
for filling in the default for its data model when an attribute is
absent. If you are using null to indicate data values then your
system is going to encounter a great deal of errors due to language
side-effects regarding the special meaning of null. Null is special
specifically so that it won't be mistaken for any normal data value,
thereby allowing the language to indicate NPE instead of accidentally
dereferencing some arbitrary memory location.
Somehow I am missing why would my application "encounter a great deal of errors"
Here is real-life use case from my application. I am parsing XML that came from
UI:
<node name="contact" type="chd:document" uuid="62b16ab8-...">
<property name="name" type="String" multiple="false">
<value>John tester</value>
</property>
<property name="age" type="Integer" multiple="false">
<value/>
</property>
</node>
So my ContentHandler will do:
Node node = root.addNode("contact", "chd:document");
node.setProperty("name", valueFactory.createValue((String)"John tester"));
node.setProperty("age", valueFactory.createValue((Integer)null));
So I don't see "great deal of errors" here.
--
Ivan Latysh
[EMAIL PROTECTED]
