Type mismatch Error : Cannont convert from ArrayList<SubClass1> to List<SuperClass>
Here is my class Structure, and where I am getting error
class Su开发者_JAVA百科perClass{
//variables
}
class SubClass1 extends SuperClass{
//variables
}
class SubClass2 extends SuperClass{
//variables
}
class AClass{
List<SuperClass> list;
public AClass(boolean b){
if(b)
list = new ArrayList<SubClass1>();//getting error here
else
list = new ArrayList<SubClass2>();//and here
}
void addObjects(SuperClass obj){
list.add(obj);
}
}
How can I solve this? Should I change my design ? How?
ADDITION:
When I changed
`List<SuperClass> list;`
to
List<? extends SuperClass> list;
I am not getting the previous error but I am getting another error while adding objects,
The method add(capture#2-of ? extends SuperClass) in the type List<capture#2-of ? extends SuperClass> is not applicable for the arguments (SuperClass)
It seems you're trying to create a list that only contains objects from the particular subclass. In this case you just need the generics to play nice at compile time. (Generics are erased at runtime :) )
class AClass<T extends SuperClass> {
List<T> list;
public AClass(){
list = new ArrayList<T>();
}
void addObjects(T obj){
list.add(obj);
}
}
You should use a bounded wildcard in your ArrayList declaration:
class AClass{
List<? extends SuperClass> list;
public AClass(boolean b){
if(b)
list = new ArrayList<SubClass1>();
else
list = new ArrayList<SubClass2>();
}
}
}
The ?
is a wildcard and defines an unknown type. But by using a bounded wildcard you can assure that it is an unknown subtype of SuperClass.
For further information about wildcards see here.
Concerning you're other problem:
The type of the parameter to list.add() is ? extends
SuperClass-- an unknown subtype of SuperClass. Since we don't know what type it is, we don't know if it is a supertype. it might or might not be such a supertype, so it isn't safe to pass a SubClass1 or SubClass2 there.
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