Re: default constructor
josh wrote:
On 6 Dic, 13:16, Patricia Shanahan <p...@acm.org> wrote:
josh wrote:
Hi all, I've a dubt!
when my classes doesn't have constructor, as in origin all derived from
Object, my instance variable
are initizializated to 0 or false or null from the Object constructor?Nope. Regardless of what constructors and initializers are present, the
memory for the new object is created with each instance field
initialized to the default for its type.
who make this?
It is part of the language, just like the rules for the result of adding
two ints. See the JLS, "12.5 Creation of New Class Instances" at
http://java.sun.com/docs/books/jls/third_edition/html/execution.html#12.5
"Whenever a new class instance is created, memory space is allocated for
it with room for all the instance variables declared in the class type
and all the instance variables declared in each superclass of the class
type, including all the instance variables that may be hidden (?8.3). If
there is not sufficient space available to allocate memory for the
object, then creation of the class instance completes abruptly with an
OutOfMemoryError. Otherwise, all the instance variables in the new
object, including those declared in superclasses, are initialized to
their default values (?4.12.5)."
I've seen in Object.java but I haven't found the Object constructor,
why?Because Object also uses a default constructor. The default constructor
for Object has a completely empty body, with no explicit or implicit
superclass constructor call.
and than how they are initializated to that default values?
Just like any other language rule, the compiler is responsible for
making sure it happens. If the compiler is generating JVM bytecode, it
can depend on JVM behavior.
It is possible, and conventional, to choose encodings for the Java data
types that make the all-zeros bit pattern represent the default value
for each type. The required behavior can be achieved by bulk zeroing of
the memory for the new object.
Patricia