First try at Interfaces

Question

I am taking my first try at Interfaces and having trouble. I created the following 2 interfaces (which may be incorrect). I need to implement it using the class that follows, using these functions, but I'm stuck - can you help?
public interface Openings {
* Add an opening that reduces the Wall's surface area calculation by
* the area of the opening.
* @param fs a <code>FlatSurface</code> value
* @return a <code>boolean</code> value that is true when the flat
* surface was added, else false.
public boolean addOpening(FlatSurface s);
     //Wall length
     int side1Length = 17;
     int side2Length = 8;     
public interface FlatSurface {
// ceiling, window, and door instances
     float ceilingHeight = 8.5F;
     float doorWidth = 3.5F;
     float doorHeight = 6.75F;
     int windowWidth = 3;
     int windowHeight = 4;
/** The length of the surface as measured in feet. */
double getLength();
/** Set the length of the surface as measured in feet. */
void setLength(double feet);
/** The width of the surface as measured in feet. */
double getWidth();
/** Set the width of the surface as measured in feet. */
void setWidth(double feet);
/** Calculate and return the area of the surface in square feet. */
double area();
public class Wall extends Surface implements Openings {
//make north, east, south, and west walls in the Room superclass
//The implementation of the Openings interface should manage windows and the door appropriately for each of the four walls
     double northWall = (side1Length * ceilingHeight) - windowArea;
     double eastWall = (side2Length * ceilingHeight) - windowArea;
     double southWall = (side1Length * ceilingHeight) - (2 * windowArea);
     double westWall = (side2Length * ceilingHeight) - doorArea;
     double wallArea = northWall + eastWall + southWall + westWall;
     double ceilingArea = side1Length * side2Length;
}

Answer 1

Thanks for the reply. That helps with the hierarchy, the following explains how it will work. Now I'm just stumbling with the coding of Wall & Surface. I included the new code after the explained hierarchy - can you help?
Public Surface class (Surface.java) implements the FlatSurface interface. This class makes ceiling, window, and door instances within the Room class.
Public Wall class (Wall.java) that extends Surface and implements the Openings interface. This class makes north, east, south, and west walls in the Room class. The implementation of the Openings interface should manage windows and the door appropriately for each of the four walls.
Room (base, superclass)
| * has 4 Walls, one Ceiling
+- Surface
| * makes ceiling, window, and door instances within the Room class
+- FlatSurface
| * has dimensions
+- Wall
| * makes north, east, south, and west walls in the Room class;
+- Openings
| * public boolean addOpening(FlatSurface s);
// FlatSurface interface is a two dimentional area with sides that are measured in feet.
public interface FlatSurface {
/** The length of the surface as measured in feet. */
double getLength();
/** Set the length of the surface as measured in feet. */
void setLength(double feet);
/** The width of the surface as measured in feet. */
double getWidth();
/** Set the width of the surface as measured in feet. */
void setWidth(double feet);
/** Calculate and return the area of the surface in square feet. */
double area();
//Openings interface - add an opening that reduces the Wall's surface area calculation by the area of the opening.
public interface Openings {
* @param fs a <code>FlatSurface</code> value
* @return a <code>boolean</code> value that is true when the flat
* surface was added, else false.
public boolean addOpening(FlatSurface s);
All those "xLength" and "xArea" values are fixed and now I think I should declare and use them in Wall & Surface to make the calculations? -
public class Surface implements FlatSurface {
// ceiling, window, and door instances
     float ceilingHeight = 8.5F;
     float doorWidth = 3.5F;
     float doorHeight = 6.75F;
     int windowWidth = 3;
     int windowHeight = 4;
     // Declare and initialize doorArea
     float doorArea = doorWidth * doorHeight;
     // Declare and initialize windowArea
     int windowArea = windowWidth * windowHeight;
I created the original program as a single Room.java that functions perfectly - do you want to see that? I am now attempting to extend it into interfaces.