Re: class/getClass()

From:
Lew <lew@lewscanon.com>
Newsgroups:
comp.lang.java.programmer
Date:
Sun, 11 May 2008 13:15:43 -0400
Message-ID:
<VdadnckkXc5ftLrVnZ2dnUVZ_g2dnZ2d@comcast.com>
Mark Space wrote:

What others have said. ".class" is a psuedo field that the compiler
maintains for you. It's a bit like the ".this" keyword,


There is no reasonable way to interpret 'this' as a field of anything, unlike
'class'. Where the expression ".this" occurs is to resolve lexically
enclosing instances of an inner class; it isn't really a "field" or even
exactly a member of the enclosing class. It is the inner class that is the
member, not "this". (Similar reasoning applies to ".new".)

or .length for arrays. Classname.class gives the Class type. It's a literal.


Unlike array 'length', which is not a literal.
<http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.8>

object.getClass() is an actual function call that burns CPU cycles at
runtime. ".class" doesn't, it's resolved at compile time.


The JLS does not refer to 'class' as a field.
<http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.8.2>

In that regard it is indeed like 'this'; both are "primary expressions". This
is not true for array 'length'. 'length' actually is a field of the array
type, and is, in fact, defined as a 'public final int'.

<http://java.sun.com/docs/books/jls/third_edition/html/arrays.html#10.7>

The members of an array type are all of the following:
  * The public final field length,
    which contains the number of components of the array
    (length may be positive or zero).

....

An array thus has the same public fields and methods as the following class:

    class A<T> implements Cloneable, java.io.Serializable {
     public final int length = X;
     public T[] clone() {
     try {
     return (T[])super.clone(); // unchecked warning
     } catch (CloneNotSupportedException e) {
     throw new InternalError(e.getMessage());
     }
     }
    }
Note that the cast in the example above would generate an unchecked warning
(?5.1.9) if arrays were really implemented this way.


--
Lew

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