Heapsort Java using object array

You didn't give the caller portion. But I found something that is risky.

int right = 2*i+2;
int left = 2*i+1;
Student leftStudent = studentList[left];
Student rightStudent = studentList[right];
int max;

if(left <= size && leftStudent.getGrades() > studentList[i].getGrades())
    max = leftStudent.getGrades();          
else            
    max = studentList[i].getGrades();


if(right <= size && rightStudent.getGrades() > studentList[max].getGrades())
    max = rightStudent.getGrades();

You checked if left and right are smaller than size, very good. But you need to do this before using these variables. When you are writing,

Student leftStudent = studentList[left];
Student rightStudent = studentList[right];

how do you know if left and right aren't accessing the memory, you haven't created yet? So my suggestion is, edit these parts to these:

int right = 2*i+2;
int left = 2*i+1;
if(left > size || right > size)
    // will you return or anything else?
    // if size if equal to list size, then you need to add >=
    // instead of only >

I'm writing again, you haven't posted the full code. But in your code, this is definitely dangerous. Change this code according to your code's logic.

You should add this line in heapify method if(left>size || right>size) return;

Your heapify method should be something like below

public static void heapify(Student[] studentList, int i, int size)
 {  

      int right = 2*i+2;
      int left = 2*i+1;
      if(left>size || right>size) return;
      Student leftStudent = studentList[left];
      Student rightStudent = studentList[right];
      int max;

      if(left <= size && leftStudent.getGrades() > studentList[i].getGrades())
          max = leftStudent.getGrades();          
      else            
       max = studentList[i].getGrades();


      if(right <= size && rightStudent.getGrades() > studentList[max].getGrades())
          max = rightStudent.getGrades();


      if(max != studentList[i].getGrades())
      {

         switchNodes(studentList, i, max);
         heapify(studentList, max, size);

      }

   }

Complete working code

class Student
{
    int id;
    int grade;
    public int getGrades()
    {
        return grade;
    }
    public Student(int id,int grade)
    {
        this.id=id;
        this.grade=grade;
    }
}
public class HelloWorld{

     public static void heapify(Student[] studentList, int i, int size)
 {  

      int right = 2*i+2;
      int left = 2*i+1;
      if(left>size || right>size) return;
      Student leftStudent = studentList[left];
      Student rightStudent = studentList[right];
      int max;

      if(left <= size && leftStudent.getGrades() > studentList[i].getGrades())
          max = leftStudent.getGrades();          
      else            
       max = studentList[i].getGrades();


      if(right <= size && rightStudent.getGrades() > studentList[max].getGrades())
          max = rightStudent.getGrades();


      if(max != studentList[i].getGrades())
      {

         switchNodes(studentList, i, max);
         heapify(studentList, max, size);

      }

   }

public static void makeHeap(Student[] studentList)
{

    for(int i = studentList.length/2; i>=0; i--)
        heapify(studentList, i, studentList.length-1);




}

public static Student[] heapSort(Student[] studentList)
{

     makeHeap(studentList);
      int sizeOfHeap = studentList.length-1;
      for(int i = sizeOfHeap; i>0; i--)       
      {

         switchNodes(studentList, 0, i);
         sizeOfHeap--;
         heapify(studentList, 0, sizeOfHeap);

      }     

    return studentList;
}

   public static void switchNodes(Student[] studentList,int i, int j)
   {

       Student temp = studentList[i];
       studentList[i] = studentList[j];
       studentList[i] = temp;

   }
  public static void main(String[] args)
  {
      Student[] students= new Student[10];
      for(int i=0;i<10;i++)
      {
       students[i]=new Student(i,i);   
      }
      makeHeap(students);
  }
}