/** Creates a new GradeManager. */
  public GradeManager() {
    // Create a new HashMap of the grades
    this.allGrades = new HashMap<LetterGrade, Integer>();

    // Add in all grades and set the occurance to 0
    for (LetterGrade gl : LetterGrade.values()) {
      allGrades.put(gl, 0);
    }
  }
 /**
  * Returns a string representation of the histogram of the grades.
  *
  * @return a string representation of the histogram of the grades.
  */
 public String getHistString() {
   StringBuffer sb = new StringBuffer();
   for (LetterGrade gl : LetterGrade.values()) {
     sb.append(gl + ":");
     for (int i = 0; i < this.allGrades.get(gl); i++) {
       sb.append("*");
     }
     sb.append("\n");
   }
   return sb.toString();
 }