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Editorial Team
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Asked: 2026-05-23T18:57:24+00:00

I’m unclear on the rules for how types are converted between Javascript and Java

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I’m unclear on the rules for how types are converted between Javascript and Java when using (Mozilla) Rhino.

There’s some specifics in the documentation about String:

It’s important to keep in mind that Java strings and JavaScript
strings are not the same […]Rhino provides some help in reducing the
differences between the two types. First, you can pass a JavaScript
string to a Java method that requires a Java string and Rhino will
perform the conversion. We actually saw this feature in action on the
call to the java.lang.String constructor in the preceding example.
Rhino also makes the JavaScript methods available to Java strings if
the java.lang.String class doesn’t already define them

But what about others? If I pass a javascript Number to a Java method expecting int, double (or Integer or Double) will it get converted? What about long/Long? (which won’t fit in a Double and so won’t fit in a JS number?

What about Java methods returning these values?

Then there’s Boolean/boolean. Are the JS constants true and false converted to and from the appropriate Java value? I’ve seen code like

java.lang.Boolean.TRUE.booleanValue()

used from JS, so at least some people think it isn’t.

I have looked at the Mozilla Rhino documentation but do point out if I’ve missed something obvious.

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1 Answer

  1. Editorial Team

    Here’s how it converts JavaScript types to Java types: http://www-archive.mozilla.org/js/liveconnect/lc3_method_overloading.html#InvocationConversion.

    Try it:

    $ java -cp js.jar org.mozilla.javascript.tools.shell.Main

js> new java.lang.Integer(12345)
12345
js> new java.lang.Integer(12345) == 12345
true

js> new java.lang.Double(12345.12345)
12345.12345

js> new java.lang.Long(9223372036854775807)                 
js: Cannot convert 9223372036854776000 to java.lang.Long
js> 9223372036854775807
9223372036854776000
js> new java.lang.Long("9223372036854775807")
9223372036854775807
js> new java.lang.Long("-9223372036854775808")
-9223372036854775808

js> new java.lang.Boolean(true)
true
js> new java.lang.Boolean(true) == true
true
js> new java.lang.Boolean(true) == false
false
js> java.lang.Boolean.TRUE.booleanValue() == true
true
js> java.lang.Boolean.FALSE.booleanValue() == false
true

    UPD

    Unfortunately I can’t find any docs about JavaScript-from-Java type mapping either. But the tutorial shows that JavaScript objects are inserted into and retrieved from context as Java Objects that actually can be Doubles, Booleans, Functions (for JavaScript functions; also implements Scriptable) or Scriptables (for objects).

    Using this code snippet it’s possibly to get JavaScript-Java type mapping reference:

    https://gist.github.com/1089320#file_java_script_java_type_mapping.textile

    As for LiveConnect compatibility. If you are referring to this footnote:

    The ability to call Java from JavaScript was first implemented as
    part of a Netscape browser technology called LiveConnect. However,
    since that technology also encompassed communication with browser
    plugins, and since the way of calling JavaScript from Java in Rhino is
    entirely different, that term won’t be used in this paper.

    I think it’s about using JavaScript from Java is different from LiveConnect specification. Using Java from JavaScript should be the same.

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