Add interaction through collision detection

A standard feature of a game engine is collision detection. It is an algorithmically complex calculation that determines if any two entities occupy the same part of the screen. If so, they have collided.

Yaeger differentiates between entities that need to be notified about a collision (a Collided), and those that do not need to be notified (a Collider). Think of this as a train and a fly. If they collide, the train doesn't even notice it; the fly does (and dies).

With this approach, it is possible to minimize the number of entities that need to be checked for collisions every GWU, and it also enables a good Object-Oriented approach to place the responsibility of handling a collision on the right entity.

Add collision detection for Hanny and the swordfish

The swordfish is a dangerous foe and each time Hanny collides with him, she will lose a life point. At the start of the game Hanny has ten of those and when she reaches zero, she dies, and it is Game Over.

There are several algorithms for collision detection but Yaeger only supports the simplest implementation, which is based on the Bounding Box of an entity. This method is called Axis Aligned Bounding Box (AABB) collision detection and is implemented through the interfaces Collided and Collider.

Edit Add the correct interface to Hanny and the swordfish. You do not yet need to implement the event handler, but for testing purposes you should add a System.out.println("Collision!");

Run Start the game and test if the collision has been detected. To get more insight into these collisions, it is possible to run Yaeger with the commandline argument --showBB, which makes all bounding boxes visible.

You might have noticed that because Yaeger uses the Bounding Box to check for collisions, the collision detection is not as accurate as you might like it to be. This can be solved by using the notion of a hitbox, a shape that defines the area that is being checked during a collision detection cycle.

We will first finish implementing what happens after a collision. In the next chapter we will rework the swordfish to a version where only the sword causes a collision.

Let Hanny respawn after a collision with the swordfish

Because Hanny is the one who needs to know if she has collided with the swordfish, she will be the one who implements Collided. We are going to use the event handler to let Hanny respawn at a different location, using her setAnchorLocation() method.

Edit Use the following event handler to let Hanny respawn at a random location:

@Override
public void onCollision(List<Collider> collidingObject){
    setAnchorLocation(
        new Coordinate2D(new Random().nextInt((int)(getSceneWidth() 
        - getWidth())),
        new Random().nextInt((int)(getSceneHeight() - getHeight())))
    );
}

Notice that we have access to the SceneWidth and SceneHeight and that we subtract, respectively, the width and height of Hanny to ensure that Hanny respawns within the scene.

Add health points and subtract one on a collision

The next step should be fairly simple, since we will use only features we have already seen.

Edit Create a new static TextEntity called HealthText with the constructor and method shown below. Add it to the package com.github.hanyaeger.tutorial.entities.text.

public HealthText(Coordinate2D initialLocation){
    super(initialLocation);

    setFont(Font.font("Roboto",FontWeight.NORMAL, 30));
    setFill(Color.DARKBLUE);
}

public void setHealthText(int health){
    setText("Health: " + health);
}

Edit Add this entity to GameLevel, by using the setupEntities() method, but also pass the instance to the constructor of Hanny. This way, Hanny has access to the HealthText entity and can call the method setHealthText(int) whenever her health changes.

Edit Give Hanny a private instance field called health of type int and initialize it to 10. Also bind the constructor parameter HealthText to an instance field. After this change, the constructor and instance fields of Hanny should look like:

private HealthText healthText;
private int health = 10;

public Hanny(Coordinate2D location, HealthText healthText){
    super("sprites/hanny.png", location, new Size(20,40), 2);

    this.healthText = healthText;
    healthText.setHealthText(health);

    setGravityConstant(0.005);
    setFrictionConstant(0.04);
}

The last step is to integrate the health into the event handler of Hanny.

Edit Change the event handler to ensure that the health is decreased, and the healthText changed:

@Override
public void onCollision(List<Collider> collidingObject){
    setAnchorLocation(new Coordinate2D(
        new Random().nextInt((int)(getSceneWidth()-getWidth())),
        new Random().nextInt((int)(getSceneHeight()-getHeight())))
    );

    health--;
    healthText.setHealthText(health);
}

Add a Game Over-scene for when health reaches zero

When health reaches 0 Hanny dies, and the player should see a new scene containing the text Game Over, with below it the clickable text Play again. We have seen all of Yaeger's features that are required for this, so it should be clear how to implement this.

Edit Add a Game Over scene with a Play Again button. Clicking the Play Again button should load the Game Level Scene.

Edit Change the event handler in Hanny in such a way that when the health reaches zero, the Game Over scene is loaded.

Add a quit game button to the game over scene

Edit Add a second button to the Game Over scene. Clicking this button should quit Yaeger. The class YaegerGame provides a method to quit the game, so use the JavaDoc to figure out which one it is.