All the ways to pass 'blocks of code' to methods in java.

samples.java

package javafxapplication1;

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.VBox;
import javafx.stage.Stage;

public class JavaFXApplication1 extends Application
{
    /*
    Here are all the ways to assign a handler to a button via the 
        setOnAction() method. They are listed in order from
        most verbose to least. Every way does the exact same thing.
    */
    
    @Override
    public void start(Stage primaryStage)
    {
        VBox root = new VBox();
        
        /*
        Implement the EventHandler and then pass an instance
            of that class to the button.
        */
        Button button1 = new Button("Button 1");
        class EventHandlerImpl implements EventHandler<ActionEvent>
        {
            @Override
            public void handle(ActionEvent event)
            {
                System.out.println("Hello from button 1!");
            }
        }
        EventHandlerImpl handler = new EventHandlerImpl();
        button1.setOnAction(handler);
        root.getChildren().add(button1);
        
        /*
        Create an anonymous class that implements the EventHandler
            and pass that to the button.
        */
        Button button2 = new Button("Button 2");
        EventHandler<ActionEvent> anonEventHandler = new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event)
            {
                System.out.println("Hello from button 2!");
            }
        };
        button2.setOnAction(anonEventHandler);
        root.getChildren().add(button2);
        
        /*
        Create the anonymous class and pass it straight into the button's
            setOnAction() method. This is what the default project template does.
        */
        Button button3 = new Button("Button 3");
        button3.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event)
            {
                System.out.println("Hello from button 3!");
            }
        });
        root.getChildren().add(button3);
        
        /*
        Use the closest thing java has to method poiters. The jvm makes
            the method passed in act like an instance of the EventHandler
            interface. As long as the interface only has a single method,
            and that method signature matches the signature of the method
            you pass into the setOnAction() method, it will work.
        */
        Button button4 = new Button("Button 4");
        button4.setOnAction(this::onClick);
        root.getChildren().add(button4);
        
        /*
        Use a lambda (multiple lines) expression. The stuff in the parentheses is the arguments
            of the handle() method. Their types are inferred so they are not
            declared like they are everywhere else in java. This method only
            works if the same conditions mentioned in button 4 are met.
        */
        Button button5 = new Button("Button 5");
        button5.setOnAction((event) -> {
            System.out.println("Hello from button 5!");
        });
        root.getChildren().add(button5);
        
        /*
        Same as button 5, but if only a single line is executed, the curly
            braces can be omitted.
        */
        Button button6 = new Button("Button 6");
        button6.setOnAction((event) -> System.out.println("Hello from button 6!"));
        root.getChildren().add(button6);
        
        /*
        Same as button 6, but if there is only one parameter in the method
            signature, the parentheses around the argument can be omitted.
            Note that if there are no parameters in the method signature,
            An empty set of parenthese () are required.
        */
        Button button7 = new Button("Button 7");
        button7.setOnAction(event -> System.out.println("Hello from button 7!"));
        root.getChildren().add(button7);
        
        
        Scene scene = new Scene(root, 300, 250);
        
        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args)
    {
        launch(args);
    }
    
    private void onClick(ActionEvent e)
    {
        System.out.println("Hello from button 4!");
    }
}