Why do I get NullpointerException at public void update() method?, what can i do to fix this?

public class CourseGrade implements Comparable { private Professor professor; private Course course; private Map grades; private int numStudents;

public CourseGrade (Professor professor, Course course) { this.professor=professor; this.course=course; this.numStudents =0; Map grades = new HashMap<>(); } public void updateGrade (String grade, int numStudents) { if (grades.containsKey(grade)) { grades.put(grade, numStudents); } }