I'm trying create a hashmap and I can't seem to find a way to retrieve an attribute's value in the map.

I create new key-value pairs by using instance variables in a subclass `Student`.

How can I retrieve all entries that share the same attribute value 'A' when they are created from a subclass?

```
Map<String, Student> students = new HashMap<>();
students.put("0001", new Student("Mike Myers","60 Hey", 'A'));
students.put("0002", new Student("Victor Hughes","21 ddd", 'F'));
students.put("0003", new Student("Elisabeth Carter","56 fff", 'A'));

```

If I do 

```
for (Map.Entry<String, Student> entry : students.entrySet())
{
   System.out.println(entry.getValue());
}

```

then I get the values of all attributes (name, address, category). So I wonder how I can get the keys that have the category 'A' only?