CanSharePosition With - double dispatch

Piece class
-----------

    /**
     * Check whether this piece can share its position
     * with another piece.
     *
     * @param piece
     *          The other piece to check.
     *
     * @return  False if the other piece is not effective.
     *          | if (piece == null)
     *          |   result == false
     */
    public abstract boolean canSharePositionWith(Piece piece);



Robot class
--------------
/**
     * @return  False if the other piece is not effective.
     *          | if (piece == null)
     *          |   result == false
     * @return  If the other piece is a robot, true if and only if
     *          the other piece equals this robot.
     *          | else if (piece instanceof Robot)
     *          |   result == (piece == this)
     * @return  True if the other piece is an item.
     *          | else if (piece instanceof Item)
     *          |   result == true
     * @return  Otherwise, the call is dispatched to the other piece.
     *          | else
     *          |   result == piece.canSharePositionWith(this)
     */
    @Override
    public boolean canSharePositionWith(Piece piece) {
        if (piece == null)
            return false;

        // Different robots cannot share positions
        if (piece instanceof Robot)
            return (piece == this);

        // Robots can share their position with items
        if (piece instanceof Item)
            return true;

        return piece.canSharePositionWith(this);
    }


Wall class
----

    /**
     * @return  True if and only if the other piece equals this wall.
     *          | result == (piece == this)
     */
    @Override
    public boolean canSharePositionWith(Piece piece) {
        // Walls cannot share their positions with other pieces
        return (piece == this);
    }


Item class
---------/**
     * @return  False if the other piece is not effective.
     *          | if (piece == null)
     *          |   result == false
     * @return  True if the other piece is an item.
     *          | else if (piece instanceof Item)
     *          |   result == true
     * @return  Otherwise, the call is dispatched to the other piece.
     *          | else
     *          |   result == piece.canSharePositionWith(this)
     */
    @Override
    public boolean canSharePositionWith(Piece piece) {
        if (piece == null)
            return false;

        // Items can share their position with other items
        if (piece instanceof Item)
            return true;

        return piece.canSharePositionWith(this);
    }