Google Web Toolkit (GWT): Casting an Element retrieved with DOM.getElementById()
By Thomas Steiner. Written on . Technical
Google Web Toolkit is just great! Being able to use Eclipse in order to develop an Ajax application for many (including me) is already argument enough. While I am not really into strong typing, for me as a mainly PHP and JavaScript programmer, sometimes it is still good to be reminded that strong typing exists. Let's have a look at an interesting thing that can happen in an Ajax app written with GWT, assuming that we wanted to retrieve a checkbox input element:
Element checkbox = DOM.getElementById("someId");
In JavaScript this looks like this:var checkbox = document.getElementById("someId");
Notice the small difference? In Java we are forced to give the checkbox a type, in JavaScript we just take whatever comes back. But what if you know that the Element actually is a checkbox? In this case you need to cast the Element to an InputElement:InputElement checkbox = DOM.getElementById("someId").cast();
It took me some time to figure this out, so I thought I'd share this with others. Maybe the solution was too obvious, anyway, suddenly the InputElement becomes something useful you can work with, e.g. check the isChecked() method:InputElement checkbox = DOM.getElementById("someId").cast();
if (!checkbox.isChecked()) {
??checkbox.setChecked(true);
}
if (!checkbox.isChecked()) {
??checkbox.setChecked(true);
}
If you are looking for a good GWT book, I found GWT in Action: Easy Ajax with the Google Web Toolkit relatively helpful when I started with my first GWT applications.
Image from amazon.de