Which line indicate AUTOUNBOXING in the code snippet below?

ArrayList<Double> grades = new ArrayList<Double>();
grades.add(3.2);  // Line 1
grades.add(2.7);  // Line 2
double sum = 0.0; // Line 3
for (int i=0; i < grades.size(); i++) {
    sum += grades.get(i);  // Line  4
    System.out.println("Sum = " + sum);
}