从父类类型方法返回子对象

Question

I am trying to write a method that creates random plants and returns the object of created plant as Plant . In example below the method createPlant() is of type Plant and returns an object of child class Tree . As it turns out my way of thinking is erroneous. The error provided by Eclipse is: "This method must return a result of type Plant". So how should I go about creating such method?

public abstract class Plant {
    ...
}

public class Tree extends Plant {
    ...
}

public class Bush extends Plant {
    ...
}

public class Map {
    private Plant plant;
    ...
    public static Plant createPlant(float x, float y) { // This method must return a result of type Plant
        Random generator = new Random();            
        switch (generator.nextInt(2)) {
            case 0:
                return new Tree(x, y);
            case 1:
                return new Bush(x, y);
        }
    }
}

Answer 1

No, from an object oriented perspective this is absolutely okay. A plant is a general entity, whereas the tree is specialized in some way.

The main point is that the tree is also a plant (is-a-relation) so a method returning a plant can return anything that is at least as general as a plant but also may be more specialized.

Answer 2

Add default case of null .

    switch (generator.nextInt(2)) {
        case 0:
            return new Tree(x, y);
        case 1:
            return new Bush(x, y);
        default:                         // Requires default case
            return null;
    }

Or create a dummy NoPlant class

  public class NoPlant extends Plant {
     ...
  }

Now use in this way

    switch (generator.nextInt(2)) {
        case 0:
            return new Tree(x, y);
        case 1:
            return new Bush(x, y);
        default:                         // Requires default case
            return new NoPlant();
    }

---

--EDIT--

Try in this way also

    int random=generator.nextInt(2); // return either 0 or 1

    if(random==0){
        return new Tree(x,y);
    }else{
        return new Bush(x, y);
    }