taylorialcom/ GUI


Declarative versus Imperative Syntax

Imperative Syntax

In English grammar, an imperative sentence is a sentence that gives instructions that expresses a command. With imperative programming you give step-by-step instructions to the compiler for what it should do.

The GUI layout created in EventHandlingFX uses an imperative syntax. For example,

    public void start(Stage primaryStage) {
        Pane root = new StackPane();
        label = new Label("Here is some text that can be manipulated with the button above.");
        Button clickMe = new Button("Click Me");
        root.getChildren().addAll(label, clickMe);

        primaryStage.setTitle("Simple GUI");
        primaryStage.setScene(new Scene(root, 400, 300));

Here we tell the compiler to:

  1. Create a StackPane object
  2. Create a Label object
  3. Create a Button object
  4. Attach the handleClickMe() method as a listener to the button
  5. Add the label and button to the stack pane
  6. Set the title for the window
  7. Set the scene for the stage to a newly created Scene object with the stack pane in it
  8. Show the stage

The compiler is required to generate code that completes the tasks described in exactly this order. This is a powerful way to tell the computer what I want to have happen. However, the above instructions provide more specificity than I actually care about. For example, the order of the first three steps do not really matter. In fact, the order of most of these steps is arbitrary. With an imperative programming approach, I must specify how to accomplish the desired task(s).

Declarative Syntax

In English grammar, a declarative sentence is a sentence in the form of a statement. Declarative sentences describe what is true. Declarative sentences are the most common form of sentence since they communicate what currently is efficiently.

Similarly, with declarative programming we write code that describes what we want but not necessarily how to get it.

JavaFX provides a tool for declaratively describing GUI layouts. For example, the GUI layout above could be described using JavaFX's FXML as shown below:

<StackPane fx:controller="ClickMeController" xmlns="http://javafx.com/javafx/17" xmlns:fx="http://javafx.com/fxml/1">
      <Label fx:id="message" text="Here is some text that can be manipulated with the button above." />
      <Button text="Click Me!" onAction="#handleClickMe" />

You should see we have declared that we want a StackPane that contains children within it: a Label and a Button. Looking more closely, we see that the label contains the text "Here is some text that can be manipulated with the button above." and the button has "Click Me!" on it.

The fx:controller attribute specifies the class containing controller code (code that describes how UI elements will respond to actions). The fx:id specifies the name of the instance variable in the controller class that is associated with the label. The onAction specifies the method in the controller class that will be called when the button is activated.