Can someone help me with my homework – Implementation of findMedian(…)

Problem

I have a task that reads as follows:

Implement an IntegerList class with a Vector list object attribute that contains a collection of integers. Implement the findMedian () method which, for a given object from o of the IntegerList class, returns a number m from the vector o.list such that there are at least half of the numbers less than or equal to m, and the numbers greater than or equal to m are also at least half:

For example, for [1, 4, 1, 3, 5, 7] it will be the number 4, and for [1, 1, 1, 2, 2] it will be the number 1 and only 1. If the vector is empty then the method is supposed to throw an IllegalArgumentException exception.

What I tried?

import java.util.Vector;    
public class IntegerList {  
    Vector<Integer> list =new Vector<Integer>();    
        
    public int findMedian(Vector<Integer>list){ 
       if(list.size() ==0){ 
           throw new IllegalArgumentException();    
       }    
        int result =0;  
    
    return result;  
    }   
}   
    
    
public class Main { 
    public static void main(String[] args){ 
        IntegerList v = new IntegerList();  
        v.list.add(2);  
        v.list.add(3);  
        v.list.add(4);  
        v.list.add(9);  
        System.out.println(v.findMedian(v.list));   
    
    
    }   
}   

My question:

Why this not working? What would you change to even better solve this problem?

Answer

It is working, but only on empty list. To make it work: we should apply what we learned ..and little try.

Voila:

public int findMedian(Vector<Integer> list) { 
   if (list == null || list.isEmpty()) { 
       throw new IllegalArgumentException("'list' is null or empty");    
   } // from here: list not null and list.size() > 0 ... :
   // sort the "list":    
   java.util.Collections.sort(list); // if you may not modify the list: create a copy first! (the list remains sorted, also when leaving the method...
   // return "middle" element:
   return list.get(list.size()/2); 
}   

javadoc