public class GradeSystem {
    private Map<String, List<Integer>> studentGrades = new HashMap<>();
    
    public void addGrade(String student, int grade) {
        studentGrades.computeIfAbsent(student, k -> new ArrayList<>()).add(grade);
    }
    
    public double getAverage(String student) {
        return studentGrades.getOrDefault(student, List.of())
            .stream()
            .mapToInt(Integer::intValue)
            .average()
            .orElse(0.0);
    }
    
    public Set<String> getTopStudents(double minAverage) {
        return studentGrades.entrySet().stream()
            .filter(entry -> getAverage(entry.getKey()) >= minAverage)
            .map(Map.Entry::getKey)
            .collect(Collectors.toSet());
    }
}

// Usage:
GradeSystem system = new GradeSystem();
system.addGrade("Alice", 90);
system.addGrade("Alice", 95);
system.addGrade("Bob", 80);

System.out.println(system.getAverage("Alice")); // 92.5
System.out.println(system.getTopStudents(85)); // [Alice]