java2script/SwingJS Notes

=========================

updated 5/4/2025 -- clarity about not supporting UI subclassing

updated 3/11/2023 -- adds support for Java Regex Matcher.start/end(groupID) and .start/end(groupName)

updated 12/31/2020 -- full support for 64-bit long

updated 12/6/2020 -- note about restrictions on long, including BitSet and Scanner

updated 3/21/2020 -- adds note about HashMap, Hashtable, and HashSet iterator ordering

updated 3/20/2020 -- adds note about interning, new String("xxx"), and "xxx"

updated 2/26/2020 -- adds Graphics.setClip issue

updated 12/22/19 -- additional issues

updated 11/03/19 -- adds information about File.exists() and points to src/javajs/async

updated 10/26/19 -- adds information about File.createTempFile()

updated 8/16/19 -- minor typos and added summary paragraph

updated 7/19/19 -- clarification that AWT and Swing classes are supported directly

updated 5/13/19 -- Mandarin U+79D8 reserved character; Missing Math methods; int and long

updated 5/10/19 -- adds a section on static issues in multi-(duplicate)-applet pages

updated 1/4/19 -- nio

updated 9/15/18 -- adds integer 1/0 == Infinity

updated 7/24/18 -- most classes replaced with https://github.com/frohoff/jdk8u-jdk

updated 6/5/17 -- reserved package name "window"

updated 3/11/17 -- myClass.getField

updated 3/7/17 -- overloading of JSplitPane.setDividerLocation

updated 3/2/17 -- more indication of classes not implemented (KeyListener)

---IMPORTANT CHARACTER SET NOTE---

It is critical that all development work in Java2Script be done in UTF-8. This means:

- making sure your Eclipse project is set up for UTF-8 (not the Eclipse default?)

- making sure your server can serve up UTF-8 by default for any browser-loaded files

- making sure you don't edit a Java2Script class file or one of the site .js files

using a non-UTF-8 editor. It may replace non-Latin characters with "?" or garbage.

- making sure that your web pages are delivered with proper headings indicating HTML5 and UTF-8

<!DOCTYPE html>

<html>

<head>

<meta charset="utf-8">

Note that the DOCTYPE tag is critical for some browsers to switch into HTML5 mode. (MSIE?)

In particular, the Mandarin character 秘 (mi; "secret") is used extensively throughout

the SwingJS class files to distinguish j2s-specific fields and methods that must not

ever be shadowed or overridden by subclasses. For example, we see in java.lang.Thread.java:

public static JSThread 秘thisThread;

----------------------------------

=============================================================================

SwingJS and OpenJDK 8+

=============================================================================

SwingJS implements a wide range of the Java language in JavaScript. The base

version for this implementation is OpenJDK8. some classes are implemented using

older source code, and there are some missing methods. For the most part, this is

no real problem. You can add or modify any java class just be adding it as source

in your project. Or (preferably) you can contact me, and I can get it into the

distribution. Or (even more preferably) you can do that via a patch submission.

=================

DESIGN PHILOSOPHY

=================

The java2script/SwingJS design goal is to recreate a recognizable, easily debuggable

equivalent in JavaScript for as much of Java as practical. This means, for example,

that one can call in JavaScript

new java.util.Hashtable()

and for all practical purposes it will appear that Java is running.

The goal of java2script/SwingJS is NOT to reproduce Java byte code processing in a

browser. We leave that task and its own issues to others. Here, instead, we have a

working JavaScript version of the Java classes along with runtime assistance in the

j2sClazz.js library. This design has several advantages:

1) It leads to much smaller downloads, since the class loader can dynamically load

code at the class level.

2) It allow the browser to use its own optimizations and features, not to ignore those.

This leads to huge performance gains and in many cases much simpler coding.

3) It allows for in-browser debugging and analysis.

4) It allows for code switching between Java and JavaScript. Working Java code

can be annotated (@j2sNative, @j2sAlias, @j2sIgnore) in a fashion that

allows the code to run slightly differently in a Java than a JavaScript environment.

For example:

int delayMS = /** @j2sNative 10 ||*/2;

will read "var delayMS = 10 || 2;" (i.e. 10) in JavaScript but read by the Java

compiler as "int delayMS = 2".

5) Just generally, it allows for a much more integrated environment. JavaScript on

the page can call into any SwingJS program, and, likewise, any SwingJS code can

access anything on the page.

Method and Field Disambiguation

-------------------------------

This is no problem. SwingJS has no problem with the overloading of methods, for example:

public void print(int b);

public void print(float b);

JavaScript does not allow overloading of methods, and the common practice in

Java of naming a field the same as a method -- isAllowed and isAllowed() -- is

not possible in JavaScript. As a result, SwingJS implements "fully-qualified"

method names using "$" parameter type separation. Thus, these methods in SwingJS

will be referred to as print$I and print$F. The rules for this encoding are

relatively simple:

1. The seven primitive types in Java are encoded $I (int), $L (long), $F (float),

$D (double), $B (byte) $Z (boolean), and $H (short).

2. String and Object are encoded as $S and $O, respectively.

3. "java_lang_" is dropped for all other classes in the java.lang package (as in Java).

For example: $StringBuffer, not $java_lang_StringBuffer

4. All other classes are encoded as

"$" + Class.getName().replace(".","_")

For example, in Java we see:

public void equals(Object o) {...}

Whereas in SwingJS we have:

Clazz.newMeth(C$, 'equals$O', function (o) {...}

And

this.getContentPane().add(bar, "North");

becomes

this.getContentPane$().add$java_awt_Component$O(bar, "North");

5. Arrays are indicated with appended "A" for each level. So

setDataVector(Object[][] dataVector, Object[] columnIdentifiers)

becomes

setDataVector$OAA$OA(dataVector, columnIdentifiers)

(It is recognized that this design does introduce a bit of ambiguity, in that

in principal there could be user class named XA and X in the same package,

and methods a(X[]) and a(XA) in the same class that cannot be distinguished.

The benefit of this simple system, however, triumphed over the unlikelyhood

of that scenario.) The transpiler could be set to flag this possibility.

6. Constructors are prepended with "c$". So

public JLabel(String text) {...}

becomes:

Clazz.newMeth(C$, 'c$$S', function (text) {...});

Field disambiguation involves prepending.

In Java, a class and its subclass can both have the same field name, such as

boolean visible;

When this happens, it is called "shadowing", and though not recommended, Java allows

it. The Java2Script transpiler will prepend such shadowing fields with "$" so that the

subclass instance has both "visible" (for use in its methods inherited from its

superclass) and "$visible" (for its own methods). Thus, we might see in Java:

this.visible = super.visible;

while in SwingJS we will see:

this.$visible=this.visible;

since JavaScript does not have the "super" keyword.

Parameterless methods such as toString() are appended with "$" to become toString$().

The one exception to this rule is private methods, which are saved in (truly) private

array in the class (and are not accessible by reflection). Private parameterless

methods retain their simple Java name, since they cannot conflict with field names.

This renaming of methods has a few consequences, which are discussed more fully below.

See particularly the section on "qualified field and method names", where it is described

how you can use packages or classes or interfaces with ".api.js" in them to represent JavaScript

objects for which all method names are to be left unqualified, and how individual methods

can have more than one name using @j2sAlias.

The swingjs.api.js package in particular contains a number of useful interfaces that

you can import into your project for JavaScript-specific capabilities.

Applet vs. Application

----------------------

One of the very cool aspects of SwingJS is that it doesn't particularly matter if a browser-based

Java app is an "applet" or an "application". We don't need JNLP (Java Network Launch Protocol)

because now we can just start up any Java application in a browser just as easily as any applet.

The associative array that passes information to the SwingJS applet (information that formerly

might have been part of the APPLET tag, such as width, height, and codebase, always referred to

in our writing as "the Info array") allows the option to specify the JApplet/Applet "code"

class or the application "main" class. Either one will run just fine.

Performance

-----------

Obviously, there are limitations. One is performance, but we have seen reproducible

performance at 1/6 - 1/3 the speed of Java. Achieving this performance may require

some refactoring of the Java to make it more efficient in both Java and JavaScript.

"for" loops need to be more carefully crafted; use of "new" and "instanceof" need to be

minimized in critical areas. Note that method overloading -- that is, the same method name

with different parameters, such as read(int) and read(byte) -- is no longer any problem.

Threads

-------

Although there is only a single thread in JavaScript, meaning Thread.wait(), Thread.sleep(int) and

Thread.notify() cannot be reproduced, we have found that this is not a serious limitation.

For example, javax.swing.Timer() works perfectly in JavaScript. All it means is that threads

that use sleep(int) or notify() must be refactored to allow Timer-like callbacks. That is,

they must allow full exit and re-entry of Thread.run(), not the typical while/sleep motif.

The key is to create a state-based run() that can be exited and re-entered in JavaScript.

The javajs.async package can be added to any Java program to provide Java+JavaScript asynchronous

classes, including AsyncColorChooser, AsyncDialog, AsyncFileChooser, and AsyncSwingWorker. All

of these classes work just as well in Java as in JavaScript. There is no need to run them

only when in JavaScript.

Static fields

-------------

Final static primitive "constant" fields (String, boolean, int, etc.) such as

static final int TEST = 3;

static final String MY_STRING = "my " + "string";

are converted to their primitive form automatically by the Eclipse Java compiler

and do not appear in the JavaScript by their names.

Other static fields are properties of their class and can be used as expected.

Note, however, that SwingJS runs all "Java" code on a page in a common "jvm"

(like older versions of Java). So, like the older Java schema, the JavaScript

equivalents of both applets and applications will share all of their static

fields and methods. This includes java.lang.System.

Basically, SwingJS implementations of Java run in a browser page-based sandbox

instead of an applet-specific one.

In general, this is no problem. But if we are to implement pages with

multiple applets present, we must be sure to only have static references

that are "final" or specifically meant to be shared in a JavaScript

environment only (since they will not be shared in Java).

A simple solution, if static non-constant references are needed, is to attach the

field to Thread.currentThread.threadGroup(), which is an applet-specific reference.

Be sure, if you do this, that you use explicit setters and getters:

For example,

private static String myvar;

...

public void setMyVar(String x) {

ThreadGroup g = Thread.currentThread().threadGroup();

/**

* @j2sNative g._myvar = x;

*

*/

{

myvar = x;

}

}

public String getMyVar() {

ThreadGroup g = Thread.currentThread().threadGroup();

/**

* @j2sNative return g._myvar || null;

*

*/

{

return myvar;

}

}

in Java will get and set x the same in JavaScript and in Java.

A convenient way to do this in general is to supply a singleton class with

explicitly private-only constructors and then refer to it in Java and in JavaScript

instead of using static field, referring to myclass.getIntance().xxx instead of

myclass.xxx in Java (and JavaScript).

This was done extensively in the Jalview project. See jalview.bin.Instance.

Helper Packages -- swingjs/ and javajs/

---------------------------------------

The SwingJS library is the swingjs/ package. There are interfaces that may be of assistance

in swingjs/api, but other than that, it is not recommended that developers access classes in

this package. The "public" nature of their methods is really an internal necessity. Most access

to this package in working Java should be via the swingjs.api.JSUtilI interface.

In addition to swingjs/, though, there are several useful classes in the javajs/ package

that could be very useful. This package is a stand-alone package that can be

cloned in any Java project that also would be great to have in any JavaScript project

-- SwingJS-related or not. Functionality ranges from reading and writing various file

formats, including PDF, BMP, PNG, GIF, JPG, JSON, ZIP, and CompoundDocument formats.

A variety of highly efficient three- and four-dimensional point, vector, matrix, and

quaternion classes are included, as they were developed for JSmol and inherited from that

project.

Of particular interest should be javajs/async/, which includes

javajs.async.Async

javajs.async.AsyncColorChooser

javajs.async.AsyncDialog

javajs.async.AsyncFileChooser

See javajs.async.Async JavaDoc comments for a full description of

these useful classes.

Modal Dialogs

-------------

Although true modal dialogs are not possible with only one thread, a functional equivalent --

asynchronous modal dialogs -- is relatively easy to set up. All the JOptionPane dialogs will

return PropertyChangeEvents to signal that they have been disposed of and containing the results.

See below and classes in the javajs.async package.

Native calls

------------

Native calls in Java are calls to operating system methods that are not in Java. JavaScript

has no access to these, of course, and they must all be replaced by JavaScript equivalents.

Fortunately, they are not common, and those that are present in Java (for example, in calculating

checksums in ZIP file creation) are at a low enough level that most developers do not utilize them

or do not even have access to them. All native calls in Java classes have been replaced by

Java or JavaScript equivalents.

Swing GUI Peers and UIClasses

-----------------------------

SwingJS does not support custom UI classes that subclass UI classes in javax/swing/plaf

or com/sun/javax/swing/plaf.

One of the biggest adaptations introduced in SwingJS is in the area of the graphical

user interface. Basically, what we have is a Java Swing "LookAndFeel" customized for HTML.

The issue here is complex but workable. In Java there are two background concepts -- the

Component "peer" (one per "heavy-weight" component, such as a Frame) and the

component "uiClass" (one per component, such as BasicButtonUI or BasicTextFieldUI).

Peers are native objects of the operating system. These are the virtual buttons and text areas

that the user is interacting with at a very base level. They are chunks of low-level code that

paint the screen to give the illusion that you really are pressing a button or typing text

to the screen. Their events are being passed on to Java or the browser by the operating system.

UI classes provide a consistent "look and feel" for these native objects, rendering them onto

the native window canvas and handling all user-generated events. They paint the borders,

the backgrounds, the highlights, of every control you see in Java. There is one-to-one

correspondence of Swing classes and UI classes. Setting the Look and Feel for a project amounts

to selecting the directory from which to draw these UI classes. Java's UI class interfaces can

be found in the javax.swing.plaf ("platform look and feel") package. Individual look and feel

implementations are found in sun.plaf.basic, sun.plaf.metal, and other such specialized packages.

Early on in the development of SwingJS, we decided not to fully reproduce the painfully detailed

bit-by-bit painting of controls as is done in Java. Instead, we felt it was wiser to utilize the standard

HTML5 UI capabilities as much as possible, using DIV, and INPUT especially, with extensive use

of CSS and sometimes jQuery (menus, and sliders, for example). Thus, we have created a new

set of UIs -- the "HTML5 Look and Feel". These classes can be found in swingjs.plaf. Besides being

more adaptable, this approach allows far more versatility to SwingJS developers, allowing them

to modify the GUI to suit their needs if desired.

In SwingJS, since we have no access to native peers except through the browser DOM,

it seemed logical to merge the peer and UI idea. So instead of having one peer per heavy-weight control and

one UI class instance for each control type, we just have one UI class instance per control, and

that UI class instance is what is being referred to when a "peer" is notified.

In some ways this is a throw back to when all of Swing's components were subclasses of

specific AWT components such as Button and List. These "heavy-weight components" all had their

own individual native peers and thus automatically took on the look and feel provided by the OS.

Later Swing versions implemented full look and feel for all peers, leaving only JDialog, JFrame,

and a few other classes to have native peers. But in SwingJS we have again a 1:1 map of component

and UI class/peer instance.

The origin of most issues (read "bugs") in relation to the GUI will probably be found in the

swingjs.plaf JSxxxxUI.java code.

Swing-only Components -- no longer an issue

-------------------------------------------

Swing was introduced into Java well after the Java Abstract Window Toolkit (AWT) was well

established. As such, its designers chose to allow AWT controls such as Button and List to be used

alongside their Swing counterparts JButton and JList. Reading the code, it is clear that this

design choice posed a huge headache for Swing class developers.

For SwingJS, we decided from the beginning NOT to allow this mixed-mode programming and

instead to require that all components be Swing components. However, this is not really an

issue. We have reconfigured the class relationships a bit. In SwingJS, all AWT components

are now subclasses of javax.swing.JComponent. While this might seem error prone, so far we have

found no problem with this arrangement. It's a little surprising to me that the original developers

of Swing did not think of this.

The a2s Adapter Package

-----------------------

Originally, we thought that we would restrict ourselves to JApplets only. That is, only

Swing-based applets. But as we worked, we discovered that there are a lot of great

applets out there that are pre-Swing pure-AWT java.applet.Applet applets. Our problem was

that we also wanted it to be possible to quickly adapt these applets to JavaScript as well.

The solution turned out to be simple: Write a package (a2s) that recreates the interface for

non-Swing components as subclasses of Swing components. Thus, a2s.Button subclasses javax.swing.JButton

but also accepts all of the methods of java.awt.Button. This works amazingly well, with a few

special adaptations to the core javax.swing to be "AWT-aware."

Then, to tie it all togeter, all AWT components such as java.awt.Button now subclass their respective

a2s components, which in turn subclass JComponents. So no changes in code are necessary. We have

successfully transpiled over 500 applets using this strategy.

Working with Files

==================

Simple String file names are not optimal for passing information about

files read by SwingJS applications. That is because just peeking at a file

in SwingJS will load its entire byte[] data.

Optimally, all work with files should either use Path or File objects exclusively.

These objects, after a file is read or checked for existence, will already

contain the file byte[] data. The string name can be used alone, since SwingJS will

cache the files itself and not reload them -- just as the browser normally does.

SwingJS uses the following criteria to determine if File.exists() returns true:

(1) if this File object has been used directly to read data, or

(2) if reading data using this File object is successful.

Note that you cannot check to see if a file exists before input or if it

was actually written or if it already exists prior to writing in SwingJS.

Thus, you should check each use of file.exists() carefully, and if necessary, provide a J2sNative

block that gives an appropriate "OK" message, for example:

(/** @j2sNative 1 ? false : */ outputfile.exits())

or

(/** @j2sNative 1 ? true : */ inputfile.exits())

Temporary files can be created in SwingJS. SwingJS will maintain a pseudo-filesystem for files

created with File.createTempFile(). This is useful in that closure of writing to a temporary file

does not generate a pseudo-download to the user's machine. Temporary files will be placed in the

"/TEMP/" directory, as seen from the running Java program. Any file written to this directory will

simply be stored in memory; files written to any other directory, when closed, will appear to the

user as a download, often involving a "What do you want to do with this file" dialog.

See below for details relating to each of the subjects below:

UNIMPLEMENTED CLASSES BY DESIGN

===============================

The SwingJS implementation of the following classes are present

in a way that gracefully bypasses their functionality:

accessibility

security

serialization

TODO LIST FOR UNIMPLEMENTED CLASSES

===================================

none as of 2020.12.31. Source code for classes and methods missing

from Java 8 or from Java 9+ can be inserted by any developer along

with their running code source, and they should run.

MINOR ISSUES--required some rewriting/refactoring by Bob and Udo

================================================================

Thread.currentThread() == dispatchThread

MINOR ISSUES--requiring some rewriting/refactoring outside of SwingJS

=====================================================================

See below for a full discussion.

primitive type restrictions - int, long, and float

HashMap, Hashtable, and HashSet iterator ordering

interning, new String("xxx") vs "xxx"

Names with "$" and "_"

ArrayIndexOutOfBounds

java.awt.Color

native methods

javax.swing.JFileDialog

key focus

LookAndFeel and UI Classes

System.exit(0) does not stop all processes

list cell renderers must be JComponents

myClass.getField not implemented

"window" and other reserved JavaScript names

reserved field and method names

qualified field and method names

Component.getGraphics(), Graphics.dispose()

Graphics.setClip()

MAJOR ISSUES--for Bob and Udo within SwingJS

============================================

fonts

OS-dependent classes

AWT component peers

some aspects of reflection

MAJOR ISSUES--to be resolved by implementers

============================================

fonts

threads

modal dialogs

image loading

no format internationalization

Graphics2D: missing winding rules

text-related field implementation

Formatter/Regex limitations

========================================================================

DISCUSS

=======

Table row/col sorter needs checking after removal of java.text.Collator references

==========================================================================

//////////////////////////////////////////////////////////////////////////////

UNIMPLEMENTED CLASSES

=====================

accessibility

-------------

All Accessibility handling has been commented out to save the download footprint.

This removes the need for sun.misc.SharedSecrets as well.

Nothing says we could not implement accessibility. We just didn't.

security

--------

All JavaScript security is handled by the browser natively.

Thus, Java security checking is no longer necessary, and

java.security.AccessController has been simplified to work without

native security checking.

Note that private methods in a class are REALLY private.

serialization

-------------

All serialization has been removed. It was never very useful for Swing anyway,

because one needs exactly the same Java version to save and restore serialized objects.

keyboard accelerators and mnemonics

-----------------------------------

This work was completed in the spring of 2019. Note that in a browser, some

key strokes, particularly CTRL-keys, are not available. Bummer.

MINOR ISSUES--required some rewriting/refactoring by Bob and Udo

================================================================

Thread.currentThread() == dispatchThread

----------------------------------------

changed to JSToolkit.isDispatchThread()

MINOR ISSUES--requiring some rewriting/refactoring outside of SwingJS

=====================================================================

primitive restrictions - int, long, and float

---------------------------------------------

int

For performance reasons, int addition and multiplication do not by default overflow to

negative values. Instead, they just get bigger. Java code that relies on overflow to

negative values should be surrounded by ()|0 -- an OR with integer 0:

int bigI, bigJ;

...

bigI = (bigI + bigJ)|0;

bigI = (bigI + 1)|0; //instead of bigI++

Thus, in Java, the following is true:

2000000000 + 2000000000 == -294967296

But in SwingJS, that will be 4000000000. So, for example, the following

strategy will fail in SwingJS:

int newLength = lineBuf.length * 2;

if (newLength < 0) {

newLength = Integer.MAX_VALUE;

}

This is because, generally, "-1" in JavaScript is not 0xFFFFFFFF. The simple ()|0 takes

caes of this:

int newLength = (lineBuf.length * 2)|0;

if (newLength < 0) {

newLength = Integer.MAX_VALUE;

}

JavaScript does process bitwise operators & | ^ ~ properly for int values. There is no issue using

these operations.

Note that int 1/0 in Java throws "java.lang.ArithmeticException: / by zero", but in JavaScript is just Infinity.

Importantly, the JavaScript Int32Array does behave properly. From the Firefox developer console:

>> x = new Int32Array(1)

<- Int32Array(1) [ 0 ]

>> x[0] = 4000000000

<- 4000000000

>> x[0]

<- -294967296

Notice that, perhaps unexpectedly, the following two constructs produce

different results in JavaScript:

x = new Int32Array(1);

b = x[0] = 4000000000;

(b will be 4000000000)

and

x = new Int32Array(1);

x[0] = 4000000000;

b = x[0];

(b will be -294967296)

SwingJS leverages array typing to handle all byte and short arithmetic so as

to ensure that any byte or short operation in JavaScript does give the same

result in Java.

long

Java's 64-bit long type is fully supported, starting with java2script 3.3.1 (2020.12.31)

The transpiler handles all conversions to and from long appropriately. See the discussion

at https://github.com/BobHanson/java2script/issues/202 for how this is done.

float

SwingJS does not distinguish between float and double. Everything is double.

HashMap, Hashtable, and HashSet iterator ordering

-------------------------------------------------

In Java, iterators for HashMap, Hashtable, and HashSet do not guarantee any particular order.

From the HashMap documentation for Java 8:

This class makes no guarantees as to the order of the map; in particular, it does not

guarantee that the order will remain constant over time.

Likewise, for HashSet (because it is simply a convenience method for HashMap<Object,PRESENT>:

[HashSet] makes no guarantees as to the iteration order of the set.

JavaScript's Map object is different. It is basically a LinkedHashMap, so it guarantees iteration

in order of object addition.

Starting with java2script 3.2.9.v1, these classes use the JavaScript Map object rather than hash codes

whenever all keys are strictly of JavaScript typeof "string". If any key is introduced that is not a string, the

implementation falls back to using hash codes, the same as Java.

The result is significantly faster performance (3-12 x faster) than originally, and up to 3 x faster

performance in JavaScript than in Java itself. That is not a misprint. Faster than Java.

The JavaScript Map implementation is implemented UNLESS the constructor used is the one that

specifies both initial capacity and load factor in their constructor. Thus,

new Hashtable()

new HashMap()

new HashMap(16)

new HashSet()

all use the JavaScript Map. But

new Hashtable(11, 0.75f)

new HashMap(16, 0.75f)

new HashSet(16, 0.75f)

do not.

This design allows for opting out of the JavaScript Map use in order to retain the exact behavior of

iterators in JavaScript as in Java.

interning, new String("xxx") vs "xxx"

-------------------------------------

Note that the following are true in JavaScript:

typeof new String("xxxx") == "object"

typeof "xxxx" == "string"

var s = "x";typeof ("xxx" + s) == "string"

There is no equivalence to this behavior in Java, where a String is a String is a String.

Be aware that SwingJS does not always create a JavaScript String object using JavaScript's

new String(...) constructor. It only does this for Java new String("xxxx") or new String(new String()).

In all other cases, new String(...) (in Java) results in a simple "xxxx" string in JavaScript.

That is, it will be JavaScript typeof "string", not typeof "object".

The reason for this design is that several classes in the Java core use toString()

methods that return new String(), and those classes that do that would cause a JavaScript error

if implicitly stringified if new String() returned a JavaScript String object.

This is fine in JavaScript:

test1 = function() { return { toString:function(){ return "OK" } } }

"testing" + new test1()

>> "testingOK"

But for whatever reason in JavaScript:

test2 = function() { return { toString:function(){ return new String("OK") } } }

"testing" + new test2()

>> Uncaught TypeError: Cannot convert object to primitive value

The lesson here is never to use

return new String("...");

in a Java toString() method. In Java it will be fine; in JavaScript it will also be fine as long as

that method is never called in JavaScript implicitly in the context of string concatenation.

A note about interning. Consider the following six Java constructions, where we have a == "x";

"xxx"

"xx" + "x"

new String("xxx").intern()

new String("xxx")

"xx" + a.toString()

"xx" + a

All six of these will return java.lang.String for .getClass().getName().

However, the first three are String literals, while the last three are String objects.

Thus:

"xxx" == "xxx"

"xxx" == "xx" + "x"

"xxx" == new String("xxx").intern()

but none of the other three are equivalent to "xxx" or each other:

"xxx" != new String("xxx")

"xxx" != "xx" + a.toString()

"xxx" != "xx" + a

new String("xxx") != new String("xxx")

"xx" + a != new String("xxx")

etc.

As in Java, in SwingJS, all of the following Java assertions pass as true:

assert("xxx" == "xx" + "x");

assert("xxx" == ("xx" + a).intern());

assert("xxx" === new String("xxx").intern());

and both of these do as well:

assert(new String("xxx") != "xxx");

assert(new String("xxx") != new String("xxx"));

But the following two fail to assert true in SwingJS:

assert("xxx" != "xx" + a);

assert("xxx" != "xx" + a.toString());

because, in JavaScript, both of these right-side expressions evaluate to a simple "interned" string.

In Java, however, these assertions are true because Java implicitly "boxes" String

concatentaion as a String object, not a literal.

Most of us know not to generally use == with Strings unless they are explicitly interned.

Where this problem may arise, though, is in IdentityHashMap, which compares objects using

System.identityHashCode(), which is not the same for different objects or their string literal equivalents.

My recommendation, if you need to use IdentityHashMap with strings is to always use an explicit String.intern()

for any keys -- unless you really want to keep every string as separate keys even if they are the same sequence,

in which case, use new String(). This will work in Java and in JavaScript.

Be aware when working with strings that come from SwingJS and are being used by other JavaScript modules

that those that are String objects will return "object" for the JavaScript typeof operator, not "string".

The easy way to ensure this is no problem is to concatenate strings with "" to force immediate interning:

var x = aJavaObject.getString() + "";

unless you are certain that the string is being returned is a raw JavaScript string.

Names with "$" and "_"

----------------------

For the most part, this should be no problem.

Note that the use of $ and _ in Java field names has always been discouraged:

[https://docs.oracle.com/javase/tutorial/java/nutsandbolts/variables.html]

You may find some situations where auto-generated names will contain the dollar sign,

but your variable names should always avoid using it. A similar convention

exists for the underscore character; while it's technically legal to begin your

variable's name with "_", this practice is discouraged.

Some impacts of transpiling method names with full qualification:

1) SwingJS will introduce fields that start with $ or _. These will not conflict

if the above convention is followed.

2) Fields that have the same Java name as a method are not an issue.

3) Fields that have a Java name with $ that matches a transpiled method name,

such as toString$, will need to be refactored in Java to not have that name collision.

4) Fields in a subclass that have the same name as private fields in a superclass

represent a name collision, because the superclass method needs to call its private

field even if invoked from a subclass. The solution was to modify the subclass field

name using one or more prepended $.

5) Use of Class.getDeclaredMethods() reflection will return Method objects having the transpiled

name, not the Java name. This could require some j2sNative adjustment

to strip the $... parameters from the name if that is needed.

6) Use of Method.getParameterTypes() should work fine, provided class names

do not contain "_". This is because the transpiler converts "." to "_" when

creating the fully qualified JavaScript name.

ArrayIndexOutOfBounds

---------------------

You cannot implicitly throw an ArrayIndexOutOfBoundsException in JavaScript.

JavaScript will simply return "undefined", not throw an Exception. So:

boolean notAGoodIdeaIsOutOfBounds(String[] sa, int i) {

try {

return (sa[i] == sa[i]);

} catch (ArrayIndexOutOfBoundsException e) {

return false;

}

}

will work in Java but not in JavaScript. Code should not depend upon this sort

of trap anyway, if you ask me.

Throwable vs Error vs Exception

-------------------------------

True JavaScript errors are trapped as Throwable, whereas you can still trap

Error and Exception as well. So if you want to be sure to catch any JavaScript

error, use try{}catch (Throwable t){}, not try{}catch (Exception e){}.

java.awt.Color

--------------

ColorSpace: only "support" CS_sRGB.

TODO -- any volunteers??

javax.swing.JFileDialog

-----------------------

HTML5 cannot expose a file reading directory structure. But you certainly

can still do file reading and writing. It just works a little differently.

It's a simple modification:

b = new JButton("FileOpenDialog");

b.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

JFileChooser fc = new JFileChooser();

Test_Dialog.this.onDialogReturn(fc.showOpenDialog(Test_Dialog.this));

// Java will wait until the dialog is closed, then enter the onDialogReturn method.

// JavaScript will exit with NaN immediately, and then call back with its actual value

// asynchronously.

}

});

public void onDialogReturn(int value) {

if (value != Math.floor(value))

return; // in JavaScript, this will be NaN, indicating the dialog has been opened

// If we are here, the dialog has closed, in both Java and JavaScript.

System.out.println("int value is " + value);

}

@Override

public void propertyChange(PropertyChangeEvent event) {

Object val = event.getNewValue();

String name = event.getPropertyName();

System.out.println(name);

switch (event.getSource().getClass().getName()) {

case "javax.swing.JOptionPane":

switch (name) {

case "inputValue":

onDialogReturn(val);

return;

case "value":

if (val instanceof Integer)

onDialogReturn(((Integer) val).intValue());

else

onDialogReturn(val);

return;

}

break;

case "javax.swing.ColorChooserDialog":

switch (name) {

case "SelectedColor":

onDialogReturn(val);

return;

}

break;

case "javax.swing.JFileChooser":

switch (name) {

case "SelectedFile":

File file = (File) val;

byte[] array = (val == null ? null : /** @j2sNative file.秘bytes || */

null);

onDialogReturn("fileName is '" + file.getName() + "'\n\n" + new String(array));

return;

}

break;

}

System.out.println(

event.getSource().getClass().getName() + " " + event.getPropertyName() + ": " + event.getNewValue());

}

Developers are encouraged to create a separate class that handles general calls to JFileDialog.

An example class can be found in the SwingJS distribution as

/sources/net.sf.j2s.java.core/src/javajs/async/AsyncFileChooser.java.

javax.swing.JOptionPane dialogs

-------------------------------

For a full discussion of modal dialogs, see the javajs.asyc.AsyncDialog.java discussion.

For this action to work, the parent component must implement

propertyChangeListener, and any call to JOptionPanel should allow for

an asynchronous response, meaning that there is no actionable code following the