Re: Instantiation of a class
On Jul 11, 8:29 pm, Fred <albert.xtheunkno...@gmail.com> wrote:
Eric Sosman wrote:
On 7/11/2011 7:03 PM, Fred wrote:
I'm thinking about statements like String foo = new String("foo");
When is the constructor specified not the name of the type of
reference variable?
Aside: The example is sort of silly, because one seldom need=
s
two instances of the same String.
At the risk of answering a homework question, I can think of
two situations in which `x = new Y(...)' works even though x does
not have type Y. First, x's type may be that of one of Y's
superclasses, or of an interface implemented by Y.
Right - that's all I wanted to know.
This information is in the tutorials and other introductory material
for Java programmers, as well as the JLS.
Consider:
JLS =A74.12.2:
http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.12=
..2
"A variable of a class type T can hold a null reference or a reference
to an instance of class T or of any class that is a subclass of T. A
variable of an interface type can hold a null reference or a reference
to any instance of any class that implements the interface."
JLS =A712.5
http://java.sun.com/docs/books/jls/third_edition/html/execution.html#12.5
"A new class instance is explicitly created when evaluation of a class
instance creation expression (=A715.9) causes a class to be
instantiated."
JLS =A715.9
http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.9
in particular, JLS =A715.9.1
http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.9=
..1
"The type of the class instance creation expression is the class type
being instantiated."
JLS =A715.26.1
http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.2=
6.1
"A compile-time error occurs if the type of the right-hand operand
cannot be converted to the type of the variable by assignment
conversion (=A75.2)."
JLS =A75.2
http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.2
"Assignment conversion occurs when the value of an expression is
assigned (=A715.26) to a variable: the type of the expression must be
converted to the type of the variable. Assignment contexts allow the
use of one of the following:
- an identity conversion (=A75.1.1)
- a widening primitive conversion (=A75.1.2)
- a widening reference conversion (=A75.1.5)
- a boxing conversion (=A75.1.7) optionally followed by a widening
reference conversion
- an unboxing conversion (=A75.1.8) optionally followed by a widening
primitive conversion.
"If, after the conversions listed above have been applied, the
resulting type is a raw type (=A74.8), unchecked conversion (=A75.1.9) may
then be applied. It is a compile time error if the chain of
conversions contains two parameterized types that are not not in the
subtype relation."
See also the tutorial on "Polymorphism", arguably the most important
topic to understand for object-oriented programming:
http://download.oracle.com/javase/tutorial/java/IandI/polymorphism.html
You must grok polymorphism and the larger issue of inheritance to
succeed with Java.
--
Lew