obj = obj.getClass().getSuperclass().cast(obj);

This line does not do what you expect it to do. Casting an Object does not actually change it, it just tells the compiler to treat it as something else.

E.g. you can cast a List to a Collection, but it will still remain a List.

However, looping up through the super classes to access fields works fine without casting:

Class<?> current = yourClass;
while(current.getSuperclass()!=null){ // we don't want to process Object.class
    // do something with current's fields
    current = current.getSuperclass();
}

BTW, if you have access to the Spring Framework, there is a handy method for looping through the fields of a class and all super classes:
ReflectionUtils.doWithFields(baseClass, FieldCallback)
(also see this previous answer of mine: Access to private inherited fields via reflection in Java)

Try the following:

Field[] fields = c.getClass().getFields();

If you want all superclass fields, see the following:

Retrieving the inherited attribute names/values using Java Reflection

You can do it like this. You need to add more condition as per your requirement in filter:

public static List<Field> getAllFields(List<Field> fields, Class<?> type) {
    fields.addAll(
            Arrays.stream(type.getDeclaredFields())
                    .filter(field -> field.isAnnotationPresent(NotNull.class))
                    .collect(Collectors.toList())
    );
    if (type.getSuperclass() != null) {
        getAllFields(fields, type.getSuperclass());
    }
    return fields;
}

Call example:

List<Field> fieldList = new ArrayList<>();
getAllFields(fieldList,B.class);

Field[] fields = YourClassName.class.getFields();

returns an array of all public variables of the class.

getFields() return the fields in the whole class-heirarcy. If you want to have the fields defined only in the class in question, and not its superclasses, use getDeclaredFields(), and filter the public ones with the following Modifier approach:

Modifier.isPublic(field.getModifiers());

The YourClassName.class literal actually represents an object of type java.lang.Class. Check its docs for more interesting reflection methods.

The Field class above is java.lang.reflect.Field. You may take a look at the whole java.lang.reflect package.

Here's a simple way that inspects class declared fields:

static Set<Field> getFields(Class<?> cls) {
    Set<Field> set = new HashSet<>();

    for (Field f : cls.getDeclaredFields()) {
        set.add(f);

        //a filter to avoid standard classes. Update accordingly
        if (f.getType().getName().startsWith("com.foo.bar")) {
            set.addAll(getFields(f.getType()));
        }
    }

    return set;
}

And a simple call:

public static void main(String[] args) {
    Set<Field> all = getFields(Employee.class);
    for (Field f : all) {
        System.out.println(String.format("%s.%s", 
             f.getDeclaringClass().getSimpleName(), f.getName()));
    }
}

With your example classes, the above prints:

Employee.firstName
Employee.address
Address.street
Address.streetNum
Employee.id

And here's an equivalent Java8+ version (only inspecting 2 levels of the tree):

Stream.of(Employee.class.getDeclaredFields())
        .flatMap(f -> Stream.concat(Stream.of(f), 
                Stream.of(f.getType().getDeclaredFields())))
        .filter(f -> f.getDeclaringClass()
                      .getPackage()
                      .getName()
                      .startsWith("com.foo.bar"))
        .map(f -> String.format("%s.%s", 
                    f.getDeclaringClass().getSimpleName(), f.getName()))
        .forEach(System.out::println);

The filter above is used to limit inspected classes to those in the current package (change accordingly). Without it, one gets String's fields, etc.

You may also want to call distinct() to remove duplicates if they may show up.