Heapsort Java using object array

Question

I am working on a personal project to test heapsort on a group of objects. I am using HS to organize a set of students. using the example of a school project I already did using Selection sort I put this code together:

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

      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();


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

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

      }

   }

I already checked to see whether I have the right helper code in other parts and I do. I keep getting an

ArrayOutofBounds Error

at the code where I call this method.

How do I successfully implement the algorithm using the Student Object calls?

PS: Helper code as follows

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;

   }

Answer 1

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);
  }
}

Answer 2

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.