// Returned from other method, and the run time type is Map<String, B>
Map<String, ? extends A> map = ...

// Option 1: cannot pass.
Iterator<Map.Entry<String, ? extends A>> iterator = map.entrySet().iterator();

// Option 2: cannot pass either.
// Where B is a subclass extends from A
Iterator<Map.Entry<String, B>> iterator = map.entrySet().iterator();

Map<String, ? extends A> map = new HashMap<>();
for (Map.Entry<String, ? extends A> entry : map.entrySet()) {
    String key = entry.getKey();
    A value = entry.getValue();
}

for (Map.Entry<String, ? extends A> entry : map.entrySet())
{
   String key = entry.getKey();
    A value = entry.getValue();
}

// ERROR
Iterator<Map.Entry<String, ? extends A>> iterator = map.entrySet().iterator();

Iterator<? extends Map.Entry<String, ? extends A>> iterator = map.entrySet().iterator();

Iterator<Map.Entry<String, ? extends A>>

Iterator<? extends Map.Entry<String, ? extends A>>

Set<? extends Map.Entry<String, ? extends A>>

Set<Map.Entry<String, ? extends A>>