Comment attribuer à un Objet la valeur null ?

public class Point
{
    public static final Point INFINITE_XP = new Point(Long.MAX_VALUE, 0);
    public static final Point INFINITE_XP_YP = new Point(Long.MAX_VALUE, Long.MAX_VALUE);
    public static final Point INFINITE_XP_YN = new Point(Long.MAX_VALUE, Long.MIN_VALUE);
    public static final Point INFINITE_XN = new Point(Long.MIN_VALUE, 0);
    public static final Point INFINITE_XN_YP = new Point(Long.MIN_VALUE, Long.MAX_VALUE);
    public static final Point INFINITE_XN_YN = new Point(Long.MIN_VALUE, Long.MIN_VALUE);
    public static final Point INFINITE_YP = new Point(Long.MAX_VALUE, 0);
    public static final Point INFINITE_YP_XP = INFINITE_XP_YP;
    public static final Point INFINITE_YP_XN = INFINITE_XN_YP;
    public static final Point INFINITE_YN = new Point(Long.MIN_VALUE, 0);
    public static final Point INFINITE_YN_XP = INFINITE_XP_YN;
    public static final Point INFINITE_YN_XN = INFINITE_XN_YN;
    
    public static boolean isXPositiveInfinite(Point point)
    {
        return point == INFINITE_XP || point == INFINITE_XP_YP || point == INFINITE_XP_YN;
    }
    
    public static boolean isXNegativeInfinite(Point point)
    {
        return point == INFINITE_XN || point == INFINITE_XN_YP || point == INFINITE_XN_YN;
    }
    
    public static boolean isYPositiveInfinite(Point point)
    {
        return point == INFINITE_YP || point == INFINITE_YP_XP || point == INFINITE_YP_XN;
    }
    
    public static boolean isYNegativeInfinite(Point point)
    {
        return point == INFINITE_YN || point == INFINITE_YN_XP || point == INFINITE_YN_XN;
    }
    
    public static boolean isXInfinite(Point point)
    {
        return isXPositiveInfinite(point) || isXNegativeInfinite(point);
    }
    
    public static boolean isYInfinite(Point point)
    {
        return isYPositiveInfinite(point) || isYNegativeInfinite(point);
    }
    
    public static boolean isInfinite(Point point)
    {
        return isXInfinite(point) || isYInfinite(point);
    }
    
    public final long x;
    public final long y;
    
    public Point(long a, long b)
    {
        x = a;
        y = b;
    }
    
    public void affiche(boolean println)
    {
        if (println)
            System.out.println(this);
        else
            System.out.print(this + " ");
    }
    
    @Override
    public String toString()
    {
        if (isInfinite(this))
            return "Infini";
        else
            return String.format("(%d, %d)", x, y);
    }
}