Vector2D

public static final class Vector2D {
	private double x;
	private double y;

	public Vector2D( final double x,
	                 final double y ) {
		this.x = x;
		this.y = y;
	}

	public Vector2D add( final Vector2D other ) {
		return new Vector2D(
				x + other.x,
				y + other.y
		);
	}

	public void addAccumulate( final Vector2D other ) {
		x = x + other.x;
		y = y + other.y;
	}

	public double dot( final Vector2D other ) {
		return x * other.x + y * other.y;
	}

	public double length() {
		return Math.sqrt( this.dot( this ) );
	}
}