Question

i have a problem . I’m trying to add and them retrieve objects of the same class to ArrayList , but when i try to retrieve them i get incompatible types erroe

import java.util.*;

public class Test{

Test(){
    main();
}

List test = new ArrayList();

public void main(){
    test.add(new Square(10));

    Iterator i = test.iterator();
    while(i.hasNext()){
        Square temp = i.next();
    }
}
}

Answer 1

Yes – you’re not using generics anywhere, so i.next(); is just returning Object as far as the compiler is concerned.

Options:

• Use generics

• Cast to Square:

  Square temp = (Square) i.next();
  

Personally I’d favour using generics – it’s much nicer to have type safety where you can 🙂

// Not clear why this is an instance variable - and please make fields private :)
List<Square> test = new ArrayList<Square>();

public void main() {
    test.add(new Square(10));

    Iterator<Square> i = test.iterator();
    while(i.hasNext()) {
        Square temp = i.next();
        // Use square here
    }
}

Two asides:

• The enhanced for loop can replace your while loop:

  for (Square square : test)
  

• It’s odd to have an instance method called main with no parameters; it’s more common to have public static void main(String[] args). There’s nothing illegal about what you’ve done here, just odd.