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1What is the diff btw a null reference and a reference to an empty object?
What do u mean by empty object? -
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1Is null an object?
Answer:
Absolutely not.
By that, I mean (null instanceof Object) is
false. Some other things you should know about null:
You can't call a method on null: X.m() is an error when
x is null and m is a non-static method.
(When m is a static method it is fine,
because it is the class of x that matters; the value is ignored.)
There is only one null, not one for each class. Thus,
((String) null == (Hashtable) null), for example.
It is ok to pass null as an argument to a method, as long as the
method is expecting it. Some methods do; some do not. So, for
example, System.out.println(null) is ok, but
string.compareTo(null) is not. For methods you write, your javadoc
comments should say whether null is ok, unless it is obvious.
In JDK 1.1 to 1.1.5, passing null as the literal argument
to a constructor of an anonymous inner class (e.g., new
SomeClass(null) { ...} caused a compiler error. It's ok to pass
an expression whose value is null, or to pass a coerced null, like
new SomeClass((String) null) { ...}
There are at least three different meanings that null is commonly used
to
express:
Uninitialized. A variable or slot that hasn't yet been assigned its
real value.
Non-existant/not applicable. For example, terminal nodes in a
binary tree might be represented by a regular node with null child
pointers.
Empty. For example, you might use null to represent the empty
tree. Note that this is subtly different from the previous case,
although some people make the mistake of confusing the two cases. The
difference is whether null is an acceptable tree node, or whether it
is a signal to not treat the value as a tree node. Compare the
following three implementations of binary tree nodes with an in-order print method:
Source:www.javafaq.nu -
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