Constantly checking if a bullet has touched a node

Question

I have a very simple program where you can use your W, A, S, D and space keys to shoot. All of the shooting and moving animations work, but I'm not exactly sure how I'd implement a system in which the program is constantly checking if the bullets have touched a node, such as a circle.

I was thinking I could have an ArrayList to store all the bullets and then use a TimerTask to check if the bullets have touched a node; but I feel like that would slow down the program, and also the bullet could pass them in the time the TimerTask is waiting to go again.

Any suggestions would help.

Code: Pastebin

import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.Pane;
import javafx.scene.shape.Circle;
import javafx.animation.*;
import javafx.util.Duration;
public class functionalShooter extends Application {
    private String currentDirection = "W";
    public static void main(String[] args){ launch(args);   }
    @Override public void start(Stage stage) throws Exception{
        Pane root = new Pane();
        Scene scene = new Scene(root, 600, 400);
        stage.setScene(scene);
        stage.setResizable(false);
        stage.show();

        Circle player = new Circle(10);
        player.setLayoutX(300);
        player.setLayoutY(200);

        Circle other = new Circle(100, 100, 10);

        root.getChildren().addAll(player, other);

        //key events
        scene.setOnKeyPressed(event -> {
            switch(event.getCode()){
                case A:
                    player.setLayoutX(player.getLayoutX()-10);
                    currentDirection = "A";
                    break;
                case D:
                    player.setLayoutX(player.getLayoutX()+10);
                    currentDirection = "D";
                    break;
                case W:
                    player.setLayoutY(player.getLayoutY()-10);
                    currentDirection = "W";
                    break;
                case S:
                    player.setLayoutY(player.getLayoutY()+10);
                    currentDirection = "S";
                    break;
                case SPACE:
                    if (currentDirection.equals("D")){
                        Circle newCircle = new Circle(player.getLayoutX()+10, player.getLayoutY(), 5);
                        root.getChildren().add(newCircle);
                        shoot(newCircle);
                    }
                    else if (currentDirection.equals("A")){
                        Circle newCircle = new Circle(player.getLayoutX()-10, player.getLayoutY(), 5);
                        root.getChildren().add(newCircle);
                        shoot(newCircle);
                    }
                    else if (currentDirection.equals("S")){
                        Circle newCircle = new Circle(player.getLayoutX(), player.getLayoutY()+10, 5);
                        root.getChildren().add(newCircle);
                        shoot(newCircle);
                    }
                    else {
                        Circle newCircle = new Circle(player.getLayoutX(), player.getLayoutY()-10, 5);
                        root.getChildren().add(newCircle);
                        shoot(newCircle);
                    }
                    break;
            }
        });
    }

    private void shoot(Circle bullet){
        Timeline timeline = new Timeline();
        if (currentDirection.equals("D")){
            KeyValue start = new KeyValue(bullet.translateXProperty(), 0);
            KeyValue end = new KeyValue(bullet.translateXProperty(), 800);
            KeyFrame startF = new KeyFrame(Duration.ZERO, start);
            KeyFrame endF = new KeyFrame(Duration.seconds(10), end);
            timeline.getKeyFrames().addAll(startF, endF);
        }
        else if (currentDirection.equals("A")){
            KeyValue start = new KeyValue(bullet.translateXProperty(), 0);
            KeyValue end = new KeyValue(bullet.translateXProperty(), -800);
            KeyFrame startF = new KeyFrame(Duration.ZERO, start);
            KeyFrame endF = new KeyFrame(Duration.seconds(10), end);
            timeline.getKeyFrames().addAll(startF, endF);
        }
        else if (currentDirection.equals("S")){
            KeyValue start = new KeyValue(bullet.translateYProperty(), 0);
            KeyValue end = new KeyValue(bullet.translateYProperty(), 800);
            KeyFrame startF = new KeyFrame(Duration.ZERO, start);
            KeyFrame endF = new KeyFrame(Duration.seconds(10), end);
            timeline.getKeyFrames().addAll(startF, endF);
        }
        else{
            KeyValue start = new KeyValue(bullet.translateYProperty(), 0);
            KeyValue end = new KeyValue(bullet.translateYProperty(), -800);
            KeyFrame startF = new KeyFrame(Duration.ZERO, start);
            KeyFrame endF = new KeyFrame(Duration.seconds(10), end);
            timeline.getKeyFrames().addAll(startF, endF);
        }
        timeline.setAutoReverse(false);
        timeline.setCycleCount(1);
        timeline.play();
    }
}

Answer 1

What you need is something that will check for collision every time you draw a frame in your game. A timer would run in a separate thread which could work, but would be much harder to synchronize with the game.

Answer 2

In general, when you must "constantly observe" something in an OO language, the best solution is to use the Observer pattern.

Answer 3

You can check if the bullet intersects other using Shape.intersect() in a custom Interpolator supplied to each relevant KeyValue. The fragment below adds an Interpolator to shoot(), but you'll need an identical one for each direction. The implementation is linear, simply returning t unchanged, but you might also look at this parabolic interpolator. I also made other a class variable, accessible to shoot(). I put a dozen bullets into the air with no perceptible delay. Note that you don't need a start value: "one will be synthesized using the target values that are current at the time" the animation is played.

private Circle other = new Circle(100, 100, 10);
…
else {
    KeyValue end = new KeyValue(bullet.translateYProperty(), -800, new Interpolator() {
        @Override
        protected double curve(double t) {
            if (!Shape.intersect(bullet, other).getBoundsInLocal().isEmpty()) {
                System.out.println("Intersection");
            }
            return t;
        }
    });
    KeyFrame endF = new KeyFrame(Duration.seconds(10), end);
    timeline.getKeyFrames().addAll(endF);
}