Java – EventHandler in JavaFX a class or interface

javajavafx

I have picked a basic example of printing "Hello World" on screen when the mouse is clicked The code goes like this.

package sample;

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

/**
 *
 * @author gauravp
 */
public class Sample extends Application {

    /**
     * @param args the command line arguments
     */
    Button btn = new Button("ok");
    //Label l = new Label("Done");

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) {

        primaryStage.setTitle("First Stage");


        //Created anonymous inner class EventHandler<ActionEvent>
        btn.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent event) {

                    System.out.print("Hello World !!");


            }
        });

        StackPane root = new StackPane();
        root.getChildren().add(btn);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
    }
}

In documentation it is mentioned that EventHandler is an interface , but how come the interface be instantiated…

"new EventHandler<ActionEvent>()"

In a lot of confusion….please reply if you have any idea.. Here is the link
for the EventHandler interface :
http://docs.oracle.com/javafx/2.0/api/javafx/event/EventHandler.html

Best Answer

The syntax

    new EventHandler<ActionEvent>() {
        @Override // <- notice the annotation, it overrides from the interface.
        public void handle(ActionEvent event) {
                System.out.print("Hello World !!");
        }
    }

creates an "anonymous inner class" that implements EventHandler, and defines the handle method. If you inspect the classes generated when you compile your project, you will probably find a class file named Sample$1 (or similar) which is the class generated for this code.

You can read up on inner (anonymous) classes here: http://docs.oracle.com/javase/tutorial/java/javaOO/innerclasses.html

To answer your question: EventHandler is an interface, and this code doesn't actually create an instance of it, but an instance of the newly declared anonymous class.

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Update:

Some 10 years later perhaps the best way to test a private method, or any inaccessible member, is via @Jailbreak from the Manifold framework.
@Jailbreak Foo foo = new Foo();
// Direct, *type-safe* access to *all* foo's members
foo.privateMethod(x, y, z);
foo.privateField = value;
This way your code remains type-safe and readable. No design compromises, no overexposing methods and fields for the sake of tests.

If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.

Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);

And for fields:

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

Notes:
1. TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for getDeclaredField.
2. The setAccessible(true) is required to play around with privates.

Java inner class and static nested class

From the Java Tutorial:

Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes.

Static nested classes are accessed using the enclosing class name:

OuterClass.StaticNestedClass

For example, to create an object for the static nested class, use this syntax:

OuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass();

Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:

class OuterClass {
    ...
    class InnerClass {
        ...
    }
}

An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance.

To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:

OuterClass outerObject = new OuterClass()
OuterClass.InnerClass innerObject = outerObject.new InnerClass();

see: Java Tutorial - Nested Classes

For completeness note that there is also such a thing as an inner class without an enclosing instance:

class A {
  int t() { return 1; }
  static A a =  new A() { int t() { return 2; } };
}

Here, new A() { ... } is an inner class defined in a static context and does not have an enclosing instance.

Best Answer

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Java inner class and static nested class

Related Topic