addGradesOneByOne.java

String[][] peopleToGrades = {
  {"Tremaine", "A"},
  {"Jeremy", "A-"},
  {"Yuta", "B"},
  {"Steph", "A+"},
  {"Klay", "B+"},
  {"Damian", "A"},
  {"Klay", "A"},
  {"Jeremy", "B-"},
};

HashMap<String, ArrayList<String>> peopleToGradesMap = new HashMap<>();

for (int i = 0; i < peopleToGrades.length; i++) {
  if (!peopleToGradesMap.containsKey(peopleToGrades[i][0])) {    // [i][0] is the name of the person
    peopleToGradesMap.put(peopleToGrades[i][0], new ArrayList<String>());   // ArrayList is empty at this point
    peopleToGradesMap.get(peopleToGrades[i][0]).add(peopleToGrades[i][1]);
  } else {
    peopleToGradesMap.get(peopleToGrades[i][0]).add(peopleToGrades[i][1]);
  }
}