Builtins example: Grades

Section 3.4 Builtins example: Grades

As an example of these two concepts, let’s look at a simple example, that builds on the LetterGrade class we saw in Subsection 2.2.1. Here it is again as a reference:

public class LetterGrade {
  public String letter;
  public double points;

  public LetterGrade(String letter, double points) {
    this.letter = letter;
    this.points = points;
  }
  ...
}

We will now create a class that is responsible for managing the grades in our application. We will call this class a GradeManager for lack of a better term. This class will do two things:

It holds a list of all the grades, in increasing order, and can return a copy of that list on request
It holds a map of the grades, using their letters as the keys. Using this map, it can quickly return to its user the grade object corresponding to a particular letter.

Let’s take a look. For simplicity we will only add a few letter grades, and omit plus/minus:

public class GradeManager {
  private List<LetterGrade> grades = new ArrayList<>();
  private Map<String, LetterGrade> gradesByLetter = new HashMap<>();

  GradeManager() {
    addGrade("W", 0.0);
    addGrade("F", 0.0);
    addGrade("D", 1.0);
    addGrade("C", 2.0);
    addGrade("B", 3.0);
    addGrade("A", 4.0);
  }

  private void addGrade(String letter, double points) {
    LetterGrade grade = new LetterGrade(letter, points);
    grades.add(grade);
    gradesByLetter.put(letter, grade);
  }

  List<LetterGrade> getGrades() {
    return new ArrayList<>(grades);
  }

  LetterGrade gradeForLetter(String letter) {
    return gradesByLetter.getOrDefault(letter, null);
  }
}

You should by now have a basic understanding of what this code does; if you don’t, refer to the corresponding documentation and the previous sections. Notice how we use a helper function addGrade to create a new grade and add it to the two entities used to represent the grades. It is marked as private since no-one else needs to know about it.

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