Re: question about assigning null to a reference object

ChrisW <c.c.wood@gmail.com> wrote:


It depends. This is actually fairly complicated. If the declarations
are for fields, then they are ALMOST the same. If the declarations are
for local variables, then they are not the same.

==============

For the case of fields:

Fields are always initialized to a "default" value, which is one of
null, 0, or false depending on the type, when an object is first
created. After this default initialization, the constructors are called
in order, from java.lang.Object all the way down the inheritance chain.

If a field is explcitly initialized with the "= null" bit, then that
assignment is placed into the constructor for the class. So, at least
logically speaking, a value of null is assigned to tempCelsius twice:
once at the instant of the object's creation, and once when this
particular constructor is called. If you don't add the "= null" bit,
then the variable is only assigned the value once. This can make a
difference, but if you're relatively new to the language then you should
just avoid situations where it matters, by refusing to EVER call any of
the following from a constructor:

   1. any non-final and non-private method
   2. any method that calls a method of type #1
   3. any method that calls a method of type #2
   4. any method that calls a method of type #3
   ... and so on

===============

For the case of local variables:

Actually, when local variables are involved, a declaration without an
initialization declares a variable which is not definitely assigned. A
variable that is not definitely assigned doesn't have any kind of value
at all; it is a compile-time error to do anything that reads its value,
until you definitely assign it. So the following code will fail to
compile, for example:

    public static void main(String[] args)
    {
        String a;
        System.out.println(a);
    }

The following code will also fail to compile, because there are some
situations in which a doesn't have a value:

    public static void main(String[] args)
    {
        String a;
        if (args.length > 1) a = "some value";
        System.out.println(a);
    }

But the following does compile.

    public static void main(String[] args)
    {
        String a;
        if (args.length > 1) a = "some value";
        else a = "another value";
        System.out.println(a);
    }

The following code will also (perhaps surprisingly) fail to compile,
because the compiler isn't expected to be smart enough to realize that
the if statements are exhaustive:

    public static void main(String[] args)
    {
        String a;
        if (args.length > 1) a = "some value";
        if (args.length <= 1) a = "another value";
        System.out.println(a);
    }

So not giving a value to a local variable is definitely not the same
thing as giving it a value of null. Instead, you are asking the
compiler to ensure (according to some specific over-simplified
reasoning, the details of which make up the entirety of chapter 16 of
the Java Language Specification) that you initialize the variable before
you use it.

This has specific implications. When there is no sensible "default"
value for a local variable in Java, and you expect to give it a value
later, it is best to not initialize it at all. This is very distasteful
to many programmers coming from C and C++, where uninitialized local
variables just cause undefined behavior and other nastiness; many of
them instinctively initialize all variables at their declaration. This
is very bad form in Java. By doing this, you are taking away the
compiler's ability to point out that you've made a mistake.


No, "int x;" certainly is valid. It follows the same definite
assignment rules for local variables, and it acts exactly the same as a
field declaration (except its default value is 0 instead of null).
There's nothing special here with JTextField. These things are true of
any type.


This is not about the language. This is a quirk of the Swing and AWT
APIs (and most other GUI frameworks in most other languages, as well).
There's a specific thread that you should use to do all GUI stuff.
Please ask in a new, and appropriately titled, thread for more details.
It doesn't help anyone when a single message addresses several wildly
different topics.

--
Chris Smith