Question

I have something like

public void setCustomHandler(final List<ClassA> customHandler) {
    this.customHandler = customHandler;
}

Now I create a classB and call setCustomHandler(List<B>). Eclipse is telling me to create

setCustomHandler(final List<ClassB> customHandler){
}

How can I just assign

List<ClassB objects> to List<ClassA objects>?

Answer 1

Short answer: You can’t.

List<ClassB> is not a subtype of List<ClassA> (even if ClassB is a subtype of ClassA):

List<ClassA> handlerList;
List<ClassB> bHandlers = new ArrayList<ClassB>();

handlerList = bHandlers; // Type mismatch: cannot convert from
                         // List<ClassB> to List<ClassA>

---

If you have the control of the receiving class, you could perhaps let it accept List<? extends ClassA> instead:

List<? extends ClassA> handlerList;
List<ClassB> bHandlers = new ArrayList<ClassB>();

handlerList = bHandlers;  // Compiles fine.