I would like to create a java stream that will collect only elements that are instanceof class A and at the same time are distinct by x. I am using java 8. What I have in the beggining is an List of objects of class C.

Class A extends B{
   int x;
}

Class B extends C{

}

Class C(){

}

but other classes also extend class C.

What I did so far:

List<C> input = ...; 
List<C> result = input.stream().filter(r -> r instanceof A).collect(Collectors.toList());

And this works fine however for the first part, however now I would like to also compare all elements and get List of distinct objects(compare by value of int x).

>Solution :

You can define a method that would expect an intance of Class in order to filter out any subtype of A from the source list.

That would work only if equals/hashCode contract was properly implemented by each subclass.

public static <T extends A> List<T> getItemsOfType(List<A> source, Class<T> itemClass) {
    return source.stream()
        .filter(item -> item instanceof T)
        .map(itemClass::cast)
        .distinct()
        .collect(Collectors.toList());
}

public static void main(String[] args) {
    List<A> input = // initializing the input list
    List<C> result = getItemsOfType(input, C.class);
}