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Chapter 14
Java packages for LiveConnect

The LiveConnect facility allows your JavaScript application to work with Java objects and for those Java objects to work with JavaScript objects.

LiveConnect provides two Java applet API packages for communicating with JavaScript. These packages are netscape.javascript and netscape.plugin.

The netscape.javascript applet package is available both on the client and on the server and has the following classes:

The netscape.plugin applet API package can be used only on the client. It has the following class:

The following sections describe these classes and list their constructors and methods.

netscape.javascript.JSObject

The public final class JSObject extends Object.

java.lang.Object
   |
   +----netscape.javascript.JSObject
JSObject allows Java to manipulate objects that are defined in JavaScript. Values passed from Java to JavaScript are converted as follows:

  • JSObject is converted to the original JavaScript object.

  • Any other Java object is converted to a JavaScript wrapper, which can be used to access methods and fields of the Java object. Converting this wrapper to a string will call the toString method on the original object, converting to a number will call the floatValue method if possible and fail otherwise. Converting to a boolean will try to call the booleanValue method in the same way.

  • Java arrays are wrapped with a JavaScript object that understands array.length and array[index].

  • A Java boolean is converted to a JavaScript boolean.

  • Java byte, char, short, int, long, float, and double are converted to JavaScript numbers.
Values passed from JavaScript to Java are converted as follows:

  • Objects that are wrappers around Java objects are unwrapped.

  • Other objects are wrapped with a JSObject.

  • Strings, numbers, and booleans are converted to String, Float, and Boolean objects respectively.
This means that all JavaScript values show up as some kind of java.lang.Object in Java. In order to make much use of them, you will have to cast them to the appropriate subclass of Object, as shown in the following examples:

(String) window.getMember("name")
(JSObject) window.getMember("document")
Note If you call a Java method from JavaScript, this conversion happens automatically--you can pass in "int" argument and it works.

Methods and static methods

The netscape.javascript.JSObject class has the following methods:

Table 14.1 Methods for the JSObject class

MethodDescription
call
Calls a JavaScript method

eval
Evaluates a JavaScript expression

getMember
Retrieves a named member of a JavaScript object

getSlot
Retrieves an indexed member of a JavaScript object

removeMember
Removes a named member of a JavaScript object

setMember
Sets a named member of a JavaScript object

setSlot
Sets an indexed member of a JavaScript object

toString
Converts a JSObject to a string

The netscape.javascript.JSObject class has the following static methods:

Table 14.2 Static methods for the JSObject class

MethodDescription
getWindow
Gets a JSObject for the window containing the given applet

The following sections show the declaration and usage of these methods.

call

Method. Calls a JavaScript method. Equivalent to "this.methodName(args[0], args[1], ...)" in JavaScript.

Declaration

public Object call(String methodName,
   Object args[])

eval

Method. Evaluates a JavaScript expression. The expression is a string of JavaScript source code which will be evaluated in the context given by "this".

Declaration

public Object eval(String s)

getMember

Method. Retrieves a named member of a JavaScript object. Equivalent to "this.name" in JavaScript.

Declaration

public Object getMember(String name)

getSlot

Method. Retrieves an indexed member of a JavaScript object. Equivalent to "this[index]" in JavaScript.

Declaration

public Object getSlot(int index)

getWindow

Static method. Returns a JSObject for the window containing the given applet. This method is available only on the client.

Declaration

public static JSObject getWindow(Applet applet)

removeMember

Method. Removes a named member of a JavaScript object.

Declaration

public void removeMember(String name)

setMember

Method. Sets a named member of a JavaScript object. Equivalent to "this.name = value" in JavaScript.

Declaration

public void setMember(String name,
   Object value)

setSlot

Method. Sets an indexed member of a JavaScript object. Equivalent to "this[index] = value" in JavaScript.

Declaration

public void setSlot(int index,
   Object value)

toString

Method. Converts a JSObject to a String.

Overrides: toString in class Object

Declaration

public String toString()
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