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<title>Restrictions on subclasses of JavaScriptObject</title>
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<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>
<p>In the following, "JSO class" means any subclass of <code>JavaScriptObject</code>.
Rationales are written <em>like this</em>.</p>
<ol>
  <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>
  <li>An interface type may be implemented by at most one JSO
  subtype. <em>This ensures that polymorphic dispatch via a
  "SingleJsoImpl" interface can be statically resolved to exactly one
  implementing JSO subtype.</em>
  <ol>
    <li>A <code>JavaScriptObject</code> that implements a
    "SingleJsoImpl" interface may be further extended. The subclasses
    may implement additional "SingleJsoImpl" interfaces. <em>The
    methods on a JSO must be effectively final, so each "SingleJsoImpl"
    method still has a 1:1 mapping to a method defined within a JSO
    subtype.</em></li>
    <li>It is valid for any number of any non-<code>JavaScriptObject</code>
    types to implement a "SingleJsoImpl" interface. <em>There is a
    slight runtime dispatch penalty when a "SingleJsoImpl" interface is
    implemented by both JSO and non-JSO types.</em></li>
  </ol>
  </li>
  <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>
  <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>
  <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>
  <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>
  <li>JSNI methods may not refer to instance methods defined within
  a <code>JavaScriptObject</code>.</li>
  <li>Every JSO class must have precisely one constructor, and it
  must be protected, empty, and no-argument.</li>
</ol>
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