subdivide() {
        var quarterLength = this.boundaryAABB.halfLength / 2;
        this.nw = new QuadTree(new AABB(this.boundaryAABB.x - quarterLength,
                                this.boundaryAABB.y - quarterLength,quarterLength));
        this.ne = new QuadTree(new AABB(this.boundaryAABB.x + quarterLength,
                                this.boundaryAABB.y - quarterLength,quarterLength));
        this.sw = new QuadTree(new AABB(this.boundaryAABB.x - quarterLength,
                                this.boundaryAABB.y + quarterLength,quarterLength));
        this.se = new QuadTree(new AABB(this.boundaryAABB.x + quarterLength,
                                this.boundaryAABB.y + quarterLength,quarterLength));
    }