面试题与答案

Google Web Toolkit includes a library that emulates a subset of the Java run-time library. The list below shows the set of JRE packages, types and methods that GWT can translate automatically. Note that in some cases, only a subset of methods is supported for a given type.

java.lang
java.lang.annotation
java.math
java.io
java.sql
java.util
java.util.logging

Create a class that should extends Composite class of GWT.
Inside the constructor you can write you logic to create a widget and call the initWidget method().
Then you can use this class in anywhere in the application and add this widget in any panels.

private static class OptionalTextBox extends Composite implements
ClickHandler {
private TextBox textBox = new TextBox();
private CheckBox checkBox = new CheckBox();
--------
initWidget(panel);
-------
}

To split your code, simply insert calls to the method GWT.runAsync at the places where you want the program to be able to pause for downloading more code. These locations are called split points. For this you should use runAsync.

public class Hello implements EntryPoint {
public void onModuleLoad() {
Button b = new Button("Click me", new ClickHandler() {
public void onClick(ClickEvent event) {
GWT.runAsync(new RunAsyncCallback() {
public void onFailure(Throwable caught) {
Window.alert("Code download failed");
}

public void onSuccess() {
Window.alert("Hello, AJAX");
}
});
}
});

RootPanel.get().add(b);
}
}

When you want to display dynamic data or a range of data, not just a static list, then we should use AsyncDataProvider.

In GWT we can use ClientBundle to load all the images and CSS files in one single file and use it anywhere in the application. It provides flexible way to get the resources.

1. It creates a single image from the collection of images at compile time to reduce the image size.
2. Enable more aggressive caching headers for program resources. It will go to server for every-time to get the images.
3.Eliminate mismatches between physical file names and constants in Java code by performing consistency checks during the compile.

To use RequestBuilder we need to inherit HTTP Module in module.xml file
<inherits name="com.google.gwt.http.HTTP"/>

A user-defined class is serializable if all of the following apply:

It is assignable to IsSerializable or Serializable, either because it directly implements one of these interfaces or because it derives from a superclass that does
All non-final, non-transient instance fields are themselves serializable, and
As of GWT 1.5, it must have a default (zero argument) constructor (with any access modifier) or no constructor at all.

One key difference though is that , for security reasons, all Serializable classes must be included in a serialization policy, which is generated at compile time, while IsSerializable classes do not have that requirement.

If your interest is purely in GWT, and you don't e.g. share your model classes between the web application and another application, I suggest you have your model classes/DTOs implement IsSerializable.

Generators allow the GWT coder to generate Java code at compile time and have it then be compiled along with the rest of the project into JavaScript.

This is an event that is triggered when the user becomes happy.

Define a new event class. You can add arbitrary metadata in the event class. For simplicity, we will not include any here though.

public class HappyEvent extends GwtEvent {
...
}
Define a new handler and marker interface for the event class.

interface HappyHandler extends EventHandler {
public void onHappiness(HappyEvent event);
}

interface HasHappyEvents {
public HandlerRegistration addHappyHandler(HappyHandler handler);
}
Add a unique event type

class HappyEvent extends AbstractEvent{
public static AbstractEvent.Key KEY = new AbstractEvent.Key(){...}

public GwtEvent.Key getKey(){
return KEY;
}
...
}
Wire up the handler's fire method

class HappyEvent extends GwtEvent {
static Key KEY = new Key(){
protected void fire(HappyHandler handler, HappyEvent event) {
handler.onHappiness(event);
};
...
}