问为什么我的arraylist的Java Set方法不起作用？

EN

public class Student implements Comparable<Student>
{
   private String name;
   private double gradePoints = 0;
   private int units = 0;
   private int index=-1;
   public Student(String name)
   {
      this.name = name;
   }
   public int getIndex() {
       return index;
   }
   public void setIndex(int i) {
       index=i;
   }
   public Student(String name, double gpa, int units)
   {
      this.name = name;
      this.units = units;
      this.gradePoints = gpa * units;

   }
   
   public String getName()
   {
      return name;
   }
   
   public double gpa()
   {
      if(units > 0) 
          return gradePoints/units;
      return 0;
   }
   
   public void addGrade(double gradePointsPerUnit, int units)
   {
      this.units += units;
      this.gradePoints += gradePointsPerUnit * units;
   }
   
   
   public int compareTo(Student other)  //Do not change this method.  Ask me why if you like.
   {
      double difference = gpa() - other.gpa();
      if(difference == 0) return 0;
      if(difference > 0) return 14;     //Do not hardcode 14, or -12, into your code.
      return -12;
   }
}

import java.util.ArrayList;

public class heapgang {
    public static void main(String[] args) {
        ArrayList<Student> x= new ArrayList<Student>();
        Student a= new Student("bob",1,2);
        Student b= new Student("arav",3,4);
        x.add(a);
        x.add(b);
        x.set(0, b); // should change the first element in the arraylist to student named "arav"
        
    }
}