How do I implement this method with a Java interface? What I have is totally wrong and I am not sure how to implement it.

// Analyzable.java

public interface Analyzable {
double getAverage();
GradedActivity getHighest();
GradedActivity getLowest();
}

// CourseGrades.java

public class CourseGrades implements Analyzable {
  
public GradedActivity getHighest() {
GradedActivity highest = grades[0].getScore();
  
for (int i = 0; i < grades.length; i++) {
if (grades[i].getScore() > highest) {
highest = grades[i].getScore();
}
}
  
return highest;
}

}

// GradeActivity.java

public class GradedActivity {
private double score;
  
public void setScore(double score) {
this.score = score;
}
  
public double getScore() { return score; }
  
// Get letter grades
public char getLetterGrade(double grade) {
char letterGrade;
  
if (grade >= 90) {
letterGrade = 'A';
}
else if (grade >= 80) {
letterGrade = 'B';
}
else if (grade >= 70) {
letterGrade = 'C';
}
else if (grade >= 60) {
letterGrade = 'D';
}
else {
letterGrade = 'F';
}
  
return letterGrade;
  
}
}