Hints & Answers

I'm struggling with getting the player to jump!

This part is hard indeed, so we will go through it step by step:

First we must calculate the coordinates of the ground. We can do so using the height of the image (of the Actor) and the height of the World.

int groundLevel = getWorld().getHeight() - getImage().getHeight()/2;

We can now have a boolean which indicated whether the Actor is on the ground. From within the players act method, we use:

boolean onGround = getY() == groundLevel;

The next step is to know when to jump. We can add a new boolean value that belongs to the instance of the player, rather than a local variable in the act method. Let's call this boolean jumpTrigger.

We can use Greenfoot's setLocation(x, y) method to make the player jump:

setLocation(getX(), getY() + ySpeed);

Declaring ySpeed as an instance variable.

The following code brings this all together and adds a small delay when the Actor is at the apex of the jump.

int groundLevel = getWorld().getHeight() - getImage().getHeight()/2;
boolean onGround = (getY() == groundLevel);

if (!onGround) {
    if (ySpeed == 0 && apexTimer > 0) {
        apexTimer--;
        return;
    }
    ySpeed++;
    setLocation(getX(), getY() + ySpeed);
    if (getY() >= groundLevel) {
        setLocation(getX(), groundLevel); // set on ground
        Greenfoot.getKey(); // clears any key pressed during jump
    }
} else {
    if (jumpTrigger) {
        jumpTrigger = false;
        ySpeed = -18;
        setLocation(getX(), getY() + ySpeed);
        apexTimer = 6;
    }
}

How to trigger a jump?

We can use the Engduino's button as a trigger, and Greenfoot's addedToWorld method to set up the button callback.

For example:

protected void addedToWorld(World world) {
    engduino = ((MyWorld) getWorld()).getEngduino();

    engduino.addButtonHandler(new ButtonHandler() {
        @Override
        public void handle() {
            jumpTrigger = true;
        }
    });
}

Obstacles (Rocks) are going through the player!

You must destroy the Rocks when they collide with the player.

There is a very useful tutorial here on collision detection: https://www.greenfoot.org/doc/tut-3

In our case, we will use the Greenfoot getOneIntersectingObject method.

In the act method:

Rock collided = (Rock) getOneIntersectingObject(Rock.class);

We are casting the result of the getOneIntersectingObject method call to be of type Rock. This is due to generics but this is beyond the scope of this course. However there is plenty of interesting material online about this.

If we create a method in Rock that destroys itself, we can call this from another object.

In Rock:

public void destroy() {
    getWorld().removeObject(this);
}

We can then call this new method back where we found intersecting objects.

if(collided != null) {
    collided.destroy();
}