Memory leak using autoboxing

I was following tutorial and below was the example for auto boxing memory leak.

package com.example.memoryleak;
public class Adder {
  public long addIncremental(long l) {
    Long sum=0L;
    sum =sum+l;
    return sum;
  }
  public static void main(String[] args) {
    Adder adder = new Adder();
    for(long ;i<1000;i++)  {
      adder.addIncremental(i);
      }
    }
 }

Now, I could understand that unnecessary objects would be created because of autoboxing but how it caused memory leak, the way I understand is that memory leak is caused when you are holding a strong reference to a dead object. Now, in this case once I have came out of the FOR loop there would be no strong references to those Long objects then how it caused memory leak?

Please note I want to understand how it caused memory leak, I know those objects were unnecessary.