Question

Let’s say I have a class called MemberNio (containing a SocketChannel and other nio specific objects) which extends Member class.

The method getMember(id) and getMembers return MemberNio objects.
The layers of my application that don’t need to know anything about the Nio stuff could just call the getMember method to get a member and use the supertype Member:

Member member = membersMgr.getMember(id);

But the problem occurs when I try to call getMembers:

List<Member> members =  membersMgr.getMembers(); // <- error, can't cast List<MemberNio> to List<Member>

That would force me to have MemberNio objects where I should only know about Member objects.

This is a recurring problem when I work with Lists and Interfaces/suptypes.

Answer 1

You can use: –

List<? extends Member> members =  membersMgr.getMembers();
for (Member member: members) {
     if (member instanceof MemberNio) {
          MemberNio memNio = (MemberNio)member;
          /** Do your stuff **/
     }
} 

To fetch the members using enhanced-for loop, we used the concept of :- Super type Reference pointing the Subclass object. So, no matter which subclass objects are stored in the list, we always use a super-class reference to point to them.. And then TypeCast accordingly by checking the actual instance type..

And make the return type of getMembers() the same.. It would work.

That way you can return List of Member or any class that extends your Member class.. And you don’t need to give the name of those class explicitly..