how can I get acces to a field of an object in an ArrayList, when the object extends the type I wrote in the ArrayList generics ?
Heres the code:
ArrayList<ObjectA> objA = new ArrayList()<>;
objA.add( new ObjectA() );
objA.add( new ObjectAB() ); // ObjectAB extends ObjectA
now I would like to change the variablein ObjectAB, stored in the list
( only ObjectAB has this field, not ObjectA )
If I understand you correctly, you want to do something like this:
ArrayList<ObjectA> objA = new ArrayList()<>;
objA.add( new ObjectA() );
objA.add( new ObjectAB() ); // ObjectAB extends ObjectA
ObjectAB ab = (ObjectAB) objA.get(1);
ab.setFieldFromAB("foo");
So you have to cast item from list objA to type ObjectAB. If you are not sure the cast will succeed you might want to do some checks first like
if(objA.get(1) instanceof ObjectAB) {
ObjectAB ab = (ObjectAB) objA.get(1);
ab.setFieldFromAB("foo");
} else {
//doSomethingElse
}
When you retrieve back objects from the ArrayList, you will get Objects of type ObjectA only.
Because ArrayList<T> is the list of T type objects.
ObjectA obj0 = objA.get(0);
ObjectA obj1 = objA.get(1); // This you cannot get as the reference of ObjectAB
You need to explicitly downcast obj1 to ObjectAB
ObjectAB obj1B = (ObjectAB) obj1;
However if you really require explicit downcasting, then you are defeating the very purpose of the Generics - type safety. For sure there would be some better ways.
only ObjectAB has this field, not ObjectA?