In JavaFX, the FileChooser class provides a dialog that allows the user to select a file or multiple files. You can use it to enable the user to open or save files.

Here's an example of how to use a FileChooser to let the user choose a file to open:

import java.io.File;

import javafx.application.Application;
import javafx.stage.FileChooser;
import javafx.stage.Stage;

public class MyJavaFXApp extends Application {

    @Override
    public void start(Stage primaryStage) {
        // Create a FileChooser object
        FileChooser fileChooser = new FileChooser();
        
        // Set the title of the dialog
        fileChooser.setTitle("Open File");
        
        // Show the dialog and get the selected file
        File selectedFile = fileChooser.showOpenDialog(primaryStage);
        
        if (selectedFile != null) {
            // Open the selected file
            System.out.println("Selected file: " + selectedFile.getAbsolutePath());
        }
    }

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

In this example, we create a FileChooser object, set the title of the dialog to "Open File", and show the dialog using the showOpenDialog() method. If the user selects a file and clicks the "Open" button, the method returns the selected File object. We can then use this object to open the file.

Similarly, you can use the showSaveDialog() method to let the user choose a file to save.

You can also customize the behavior of the FileChooser dialog by setting its properties, such as the initial directory, the allowed file extensions, and whether to allow multiple file selection. For example, the following code sets the initial directory to the user's home directory, and only allows the user to select files with the ".txt" extension:

FileChooser fileChooser = new FileChooser();
fileChooser.setTitle("Save File");
fileChooser.setInitialDirectory(new File(System.getProperty("user.home")));
fileChooser.getExtensionFilters().add(new FileChooser.ExtensionFilter("Text Files", "*.txt"));
File selectedFile = fileChooser.showSaveDialog(primaryStage);