StudentManagement

public class StudentManagement {

    // TODO: khai bao thuoc tinh students la array chua cac doi tuong thuoc lop Student (max. 100)
    public Student[] students = new Student[100];
    private static int size = 0;

    public static boolean sameGroup(Student s1, Student s2) {
        return (s1.getGroup().equals(s2.getGroup()));
    }

    public void addStudent(Student newStudent){
        students[size] = newStudent;
        size++;
    }

    public String studentsByGroup() {
        String ans = "";
        String[] gr = new String[100];
        int index = 0;
        int k = 0;
        int grsize = 1;
        gr[0] = students[0].getGroup();
        //tao mang chua group ko trung nhau
        for(int i=1; i<size; i++){
            boolean check = true;
            for(int j=0; j<grsize; j++){
                if(students[i].getGroup().equals(gr[j])) {
                    check = false;
                    break;
                }
            }
            if(check){
                gr[grsize] = students[i].getGroup();
                grsize++;
            }
        }
        for(int i=0; i<grsize; i++){
            ans += gr[i] + "\n";
            for(int j =0; j<size; j++){
                if(students[j].getGroup().equals(gr[i])) ans += students[j].getInfo() + "\n";
            }
        }

        return ans;
    }

    public void removeStudent(String id) {
        for(int i=0; i< size; i++){
            if(students[i].getId().equals(id)){
                for(int j=i; i<size; i++){
                    students[j] = students[j+1];
                }
                break;
            }
        }
        size --;
        /*
        Student[] newarray = new Student[99];
        int k = 0;
        for(int i=0; i<students.length; i++){
            if(students[i].getId().equals(id)) continue;
            newarray[k++] = students[i];
        }
        students = newarray;

         */
    }

    public static void main(String[] args) {
        Student a = new Student("a", "12", "a@gmail.com");
        Student b = new Student("b", "32", "b@gmail.com");
        Student c = new Student();
        c.setName("c");
        c.setId("1111");
        c.setGroup("k64ca");
        c.setEmail("c@gmail.com");
        StudentManagement list = new StudentManagement();
        list.addStudent(a);
        list.addStudent(b);
        list.addStudent(c);
        System.out.println(list.studentsByGroup());
    }
}