Question

I have a method which takes a Collection<Object> where the Object can be a String or CustomClass. It then takes each element of the collection and passes it to a method with an argument of Object like so:

public void foo(Collection<Object> c) {
    for(Object o : c)
        bar(o);
}

public void bar(Object o) {
    if(o instanceof String || o instanceof CustomClass) {
        ...
    }
}

bar works fine when I pass it a String or CustomClass, but when I try to pass a NavigableSet<String> to foo I get cannot find symbol; symbol : method foo(java.util.NavigableSet<java.lang.String>).

However if I change the the argument type in foo to Collection<String> it works fine, but this means I need to make a new foo(Collection<CustomClass>) method which will involve repeating code. Is there a way around this?

Answer 1

Collection<String> is not a subtype of Collection<Object>, therefore the compiler doesn’t find any compatible methods. Put

public <T> void foo(Collection<T> c) {
  for (T o : c) bar(o);
}