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<title>Restrictions on subclasses of JavaScriptObject</title>
<p>Subclasses of JavaScriptObject represent a view of a JavaScript object
from Java. Such classes must conform to a number of restrictions so
that the compiler can implement them. This page lists those restrictions.
<p>In the following, "JSO class" means any subclass of
<code>JavaScriptObject</code>. Rationales are written <em>like
<li> All instance methods on JSO classes must be one of: explicitly
final, a member of a final class, or private. <em>Methods of JSO classes
cannot be overridden, because calls to such methods could require
dynamic dispatch.</em>
<li> JSO classes cannot implement interfaces that define
methods. <em>This prevents virtual calls that would arise by
upcasting to the interface and then calling through the interface.
The programmer should instead use a wrapper, for example using
<code>Comparator</code> instead of implementing
<li> No instance methods on JSO classes may override another
method. <em>This catches accidents where JSO itself did not finalize
some method from its superclass.</em>
<li> JSO classes cannot have instance fields. <em>The fields would
have no place to live in web mode. Programmers should instead make
an explicit wrapper class and put the fields there.</em>
<li> Nested JSO classes must be static. <em>The implicit
<code>this</code> fields of a non-static inner class has the same
problems as an explicit field.</em>
<li> "new" operations cannot be used with JSO classes. <em>This
avoids ever being able to try to instantiate JSO objects using the
new keyword. New JSO instances can only come from JSNI, as in
previous versions of GWT.</em>
<li> Every JSO class must have precisely one constructor, and it must
be protected, empty, and no-argument.