StudentBook.java

public class StudentBook implements Action{
 private Student[] students = new Student[10];
 private int index = 0;

 @Override
 public boolean add(int id, String firstName, String lastName, int yearOfBirth, float gpa) {
 if (id <= 0 || firstName == null || lastName == null || yearOfBirth <= 1920 || gpa < 0) {
 return false;
 }

 for (Student student:students) {
 if (student != null && student.getId() == id) {
 return false;
 }
 }

 if (index < students.length) {
 students[index++] = new Student(id, gpa, firstName, lastName, yearOfBirth);
 return true;
 }
 return false;
 }

 @Override
 public boolean remove(int id) {
 for (int i = 0; i < students.length; i++) {
 if (students[i] != null && students[i].getId() == id) {
 students[i] = null;
 return true;
 }
 }
 return false;
 } 

 @Override
 public boolean modify(int id, String firstName, String lastName, int yearOfBirth, float gpa) {
 for (int i = 0; i < students.length; i++) {
 if (students[i] != null && students[i].getId() == id) {
 students[i] = new Student(id, gpa, firstName, lastName, yearOfBirth);
 return true;
 } 
 }
 return false;
 }

 @Override
 public void displayAll() {
 System.out.println("Current student list: ");

 for (Student student : students) {
 if (student != null) {
 System.out.println(student);
 }
 }
 }
}