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To help any syntax checkers know that they should avoid parsing the JavaScript code, you wrap it as a comment by using a modified standard comment, which starts with the characters /*- (a forward slash, an asterisk, and a dash) and ends with -*/ (a dash, an asterisk, and a forward slash) (2). It’s also important not to forget the trailing semicolon at the end of the definition; otherwise your code won’t compile!

Crossing the boundary from Java to JavaScript can come in two forms: writing JavaScript code in the Java application that performs some dedicated functionality, or writing JavaScript in the Java application that calls functionality in a Java- Script library already loaded into the web browser.

If you provide a return type, rather than just a void, then the JavaScript must return an object of the correct type. (Be aware that JSNI code can’t create new Java objects; it can manipulate ones passed in or create new JavaScript objects, but not new Java objects.) REMEMBER It isn’t possible to create new Java objects in a JSNI code block. Return types must either be primitive types, manipulated Java objects that have been passed in as input parameters, or references to newly created Java- Script objects (a JavaScriptObject type). It’s important to understand how the objects you provide as parameters are handled across the Java-to-JavaScript boundary

ensure that a clearly defined and repeatable mapping exists between Java typed objects and JavaScript untyped objects, which is what GWT provides.

Primitive Java numeric types, such as byte, short, char, int, long, float, or double, become simple objects in JavaScript whose value is that of the original object. For example, if you have a Java char, char keyPress = ‘a’, then it will become the JavaScript variable var k = ‘a’. Similarly, the Java int, int val = 10, becomes the JavaScript variable var v = 10. A Java String is translated across into JavaScript as a simple variable to which the original text is assigned. Therefore the Java object String name = “GWT In Action” becomes the JavaScript variable var s = “GWT In Action”. Finally the simple Java boolean, becomes another simple JavaScript variable; the Java boolean b = true becomes the JavaScript variable var b = true.

Moving on to more complicated Java objects that can be passed across the boundary, you have the Java array, a Java object, and a new object to GWT called the JavaScriptObject. We’ll discuss the last object more in the next section; for now, you should think of it as just a reference to a JavaScript object created somewhere else that can be passed around the Java code and on to other JavaScript methods—but you can’t look into its contents from the GWT Java code. This may sound strange, but we’ll explain in more detail later in the section on this object. Passing an array across the Java-to-JavaScript boundary is treated in a similar opaque way; you know you have an array object, but you can’t look into its contents. This means that if the JavaScript code needs to use values in the array, then you should move them out of the array before you cross the boundary: either into separate parameters or into another type of user-defined Java object. You can manage Java objects in JavaScript through the JSNI interface, as we’ll discuss next. You can also pass your own defined Java objects across the Java-to-JavaScript  boundary; GWT provides a special syntax allowing you to access the fields and methods of that Java object. This will take a little bit of explaining, so please, stick with us.

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