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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. |
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| <p>In the following, "JSO class" means any subclass of |
| <code>JavaScriptObject</code>. Rationales are written <em>like |
| this</em>. |
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| <ol> |
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| <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> |
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| <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> |
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| <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> |
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| <li> Every JSO class must have precisely one constructor, and it must |
| be protected, empty, and no-argument. |
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