JAVA将“未知”类型的对象转换为另一个对象的类型

Question

Im new here and i want to start with my first not so easy to describe Questions. I think a piece of code can explain best.

public void erlaube(Geraet pGeraet){
    for(Object g : mGeraetetypen){                                          
        if(pGeraet.getClass().equals(g.getClass())){                        
            System.out.println("TRUE");
            mErlaubt.add((g.getClass())pGeraet); // MY PROBLEM                    
        }
        //System.out.println(pGeraet.getClass());
        //System.out.println(g.getClass());
    }
}

A "Geraet" (Device) is an abstract Class and it can be a Sensor or an Actor f.ex. the class Temperaturesensor.

Geraetetypen (Devicetypes) is an ArrayList and contains all available Sensors and Actors.

The for- and if-block checks if the parameter Object is a type of an Object contained in the Devicetypes list f.ex. Temperaturesensor. if it is, i want to add it as this datatype (Temperaturesensor) to the "mErlaubt" (mAllowed) ArrayList.

So it should do the same as This (cast):

mErlaubt.add( (Temperatursensor) pGeraet);

But i dont want to cast explicit with (Temperatursensor). I want to cast dynamic with the datatype i found from the Datatypelist that compares.

Maybe something like the line with the comment //MY PROBLEM, but this doesnt work.

I know this is hard and my description is not verry well, but i hope you understand what i try to do. Please help. If you dont understand something or have a Question please ask.. Thanks!

Answer 1

If your mErlaubt is an ArrayList of Geraet superclass, then you just have to use mErlaubt.add(pGeraet);

Your mErlaubt must then be defined like : List<Geraet> mErlaubt = new ArrayList<Geraet>();

Answer 2

You want to retrieve the class from a specific item of a list, and cast another object to this class? Apart from someClass.cast(someObject) , there won't be any solutions. But as far as i can see, this whole code doesn't make any sense. You'll add a single object multiple times. And apart from that i'd say that's rather a job for Generics than casting .

Answer 3

I think that this change should work as pGeraet is of Class Geraet so casting a variable to the super class should work.

   for(Object g : mGeraetetypen){                                          
        if(pGeraet.getClass().equals(g.getClass())){                        
            System.out.println("TRUE");
            mErlaubt.add((Geraet)pGeraet); 
        }

Answer 4

Thank you all! I want to share my verry simple solution to all. The class is a Deviceplace for Sensors/Actors on a houseautomation GUI. Each Deviceplace allows only a hand full of Sensor/Actors (f.ex. no Heating on a outside place)

public class Deviceplace {

ArrayList<Geraet> mErlaubt;    //allowed devices

public Deviceplace(){
    mErlaubt = new ArrayList<>();
}

public void erlaube(Geraet pGeraet){  //add allowed device
            mErlaubt.add(pGeraet);
}

public boolean isErlaubt(Geraet pGeraet){   // check if device is allowed on this place
        for(Geraet g : mErlaubt){
            if(g.getClass().isInstance(pGeraet)){
                return true;
            }
        }
        return false;
}

public static void main(String[] args) {
    Deviceplace place = new Deviceplace();
    place.erlaube(new Temperatursensor());          // ad Temperaturesensor as allowed device
    // now check if a device is allowed on this deviceplace
    System.out.println(place.isErlaubt(new Windsensor()));          // RETURNS false (Windsensor not allowed)
    System.out.println(place.isErlaubt(new Temperatursensor()));    // RETURNS true  (Temperaturesensor allowed)
}

}

Answer 5

Referring to your code sample , I'd suggest to store the allowed classes instead of instances of the allowed classes. You could use this modified code:

public class Deviceplace {

  Set<Class<? extends Geraet>> mErlaubt;

  public Deviceplace(){
    mErlaubt = new HashSet<>();
  }

  public void erlaube(Class<? extends Geraet> clazz) {
    mErlaubt.add(clazz);
  }

  // Option 1: no subtypes of the allowed types
  public boolean isErlaubt(Geraet pGeraet) {
    return mErlaubt.contains(pGeraet.getClass());
  }

  public static void main(String[] args) {
    Deviceplace place = new Deviceplace();
    place.erlaube(Temperatursensor.class);
    System.out.println(place.isErlaubt(new Windsensor()));
    System.out.println(place.isErlaubt(new Temperatursensor()));
  }

}

An alternative implementation of isErlaubt that also includes subtypes would be:

  // Option 2: including subtypes of the allowed types
  public boolean isErlaubt(Geraet pGeraet) {
    for(Class<? extends Geraet> clazz : mErlaubt){
      if (clazz.isAssignableFrom(pGeraet.getClass())) {
        return true;
      }
    }
    return false;
  }