New object as parameter?

Question

In the Treehouse Defense game there is the following method in class Map: public bool OnMap(Point point).
This method is called by the constructor of class MapLocation:
 public MapLocation(int x, int y, Map map) : base(x, y)
        {
            if (!map.OnMap(this))
            {
                throw new OutOfBoundsException(this + " is outside the boundaries of the map.");
            }
        }
I'm confused because it seems to me that the method map.OnMap would only accept a variable called point (of type Point) as its parameter. Why does it accept something else, i.e. in this case the X and Y values of a variable of type MapLocation that is being constructed?
Also, the method public bool OnMap(Point point) seems to indicate to me that a variable called point is declared. Is this not an object? if it is an object, how can it be replaced by the X and Y values of an object which does not exist yet?

Allan Clark · Accepted Answer

When you see the method;
```c#
public bool OnMap(Point point)
```
The parameter part is saying "this method accepts any object that is a point and while inside this method we will call it 'point' ." So the method does not care what the Object is called when it is passed into the method, just they type and what it will be called inside the method. 
As for why the method accepts a MapLocation object, just looking at the constructor you provided 
```c#
public MapLocation(int x, int y, Map map) : base(x, y)
```
where it says 'base(x,y)' is calling the constructor of the parent class of this object. While I haven't looked at the code I would venture to guess that the base class here is of type Point. This will be noted at the top next to the name of the class should look like this:
```c#
public class MapLocation : Point
```
Since MapLocation extends Point it is a point so it can be passed in anywhere a Point is expected.

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Kimmo Ojala

Kimmo Ojala

New object as parameter?

1 Answer

Allan Clark

Allan Clark

Kimmo Ojala

Kimmo Ojala