Iterate through nested hashmap

for (Map.Entry<String, HashMap<String, Student>> letterEntry : students.entrySet()) {
    String letter = letterEntry.getKey();
    // ...
    for (Map.Entry<String, Student> nameEntry : letterEntry.getValue().entrySet()) {
        String name = nameEntry.getKey();
        Student student = nameEntry.getValue();
        // ...
    }
}

...and the var keyword in Java 10 can remove the generics verbosity:

for (var letterEntry : students.entrySet()) {
    String letter = letterEntry.getKey();
    // ...
    for (var nameEntry : letterEntry.getValue().entrySet()) {
        String name = nameEntry.getKey();
        Student student = nameEntry.getValue();
        // ...
    }
}

Java 8 lambdas and Map.forEach make bkail's answer more concise:

outerMap.forEach((letter, nestedMap) -> {
    //...
    nestedMap.forEach((name, student) -> {
        //...
    });
    //...
});