distro-source/core/src/doc/helpInfo/jsoRestrictions.html - gwt - Git at Google

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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>In the following,  "JSO class" means any subclass of
 <code>JavaScriptObject</code>.  Rationales are written <em>like
 this</em>.

 <ol>

   <li> All instance methods on JSO classes must be explicitly marked
   final.  <em>This enforces on the developer the restriction of no
   polymorphism, because all final instance methods can be dispatched
   statically.</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
   <code>Comparable</code>.</em>

   <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.

 </ol>

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 </html>
	<html>
	<head>
	<meta http-equiv="content-type" content="text/html; charset=UTF-8">
	<title>Restrictions on subclasses of JavaScriptObject</title>
	</head>
	<body>

	<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
	this</em>.

	<ol>

	<li> All instance methods on JSO classes must be explicitly marked
	final. <em>This enforces on the developer the restriction of no
	polymorphism, because all final instance methods can be dispatched
	statically.</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
	<code>Comparable</code>.</em>

	<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.

	</ol>

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