Can not find symbol - method Size() - arraylist

Question

I have been getting trouble with my program and can't figure out why I am getting this error which has been bugging me. Any suggestion will be of huge assistance! I don't know if i am missing something or something like that. I have correctly imported the Array Class.
private String title;
private ArrayList<Student> students;
private int courseId;
private static int courseIdList = 0;
/**
*
*/
public Course(String tile)
{
this.title = title;
courseId = courseIdList;
courseIdList++;
students = new ArrayList<Student>();
}
/**
* #param returns the title
*/
public String getTitle(String title)
{
return title;
}
/**
* #return returns the ID
*/
public int getId()
{
return courseId;
}
/**
*
*/
public void addStudent(Student students)
{
for(int i = 0; i < students.size(); i++)
{
if(students.getId() != students.get(i).getId())
{
students.add(i);
return;
}
}
}
/**
*
*/
public void removeStudent(Student students)
{
for(int i = 0; i < students.size(); i++)
{
if(students.getId() == students.get(i).getId())
{
students.remove(i);
return;
}
}
}
/**
* #return if class is above or equal to 10 then return true, else return false;
*/
public boolean isFull(Student students)
{
if(students.size() >= 30)
{
return true;
}
else
{
return false;
}
}
/**
* #returns if the class is below ten then return true, else return false
*/
public boolean cancel(Student students)
{
if( students.size() < 10)
{
return true;
}
else
{
return false;
}
}
/**
*
*/
public ArrayList<Student> getStudents()
{
return students;
}
/**
*
*/
public boolean inClass(Student students)
{
for(int i = 0; i < students.size(); i++)
{
if(student.getId() == students.get(i).getId())
{
return true;
}
}
return false;
}
/**
* boolean returns true if a student's ID mathes and false if their ID does not.
*/
public boolean equals(Student s)
{
if(this.getId() == s.getId())
{
return true;
}
return false;
}
/**
*
*/
public double getAverage()
{
double sum = 0;
for(Student s : students)
{
sum += s.getGrade();
}
double avg = sum / students.size();
return avg;
}
/**
*
*/
public void getHighestGrade()
{
int highestGrade = 0;
for(Student s : students)
{
if(students.getGrade() > s.get(i).getGrade())
{
students.highestGrade(i);
}
}
return new Student(" ", 0,0);
}
/**
*
*/
public ArrayList<Student> getWarnings()
{
for(i = 0; i < students.size; i++)
{
if (students.getGrade() <= 70)
{
return students;
}
}
}
/**
*
*/
public void removeSeniors(Student students)
{
for(i = 0; i < students.size; i++)
{
if(students.getId() == students.get(i).subString(0,2).equalsTo(17))
{
students.remove(i);
}
}
}
/**
*
*/
public void sortByGrade()
{
}
/**
*
*/
public void sortByAlpha()
{
}
/**
*
*/
public String toString()
{
String printOut = "";
printOut += "Course name: " + title + "\nStudent ID: " + courseId;
return printOut;
for(Student students : students)
{
printOut += students.toString() + "\n";
}
}
}

Answer 1

Now its the opposite :) students is a List and not a Student. You need to loop over the list and process each student.
public void doSomething(List<Student> students) {
// does not work because students is a List of students not a student.
students.getId(); // ERROR
// loop over the list and then get the id of a single student
for(Student student : students) {
int id = student.getId();
// do something with the id
}
}
size() is a method of the List interface
getId() is a method of your Student class.