Calculating average of an array list?

I'm trying to use the below code to calculate the average of a set of values that a user enters and display it in a jTextArea but it does not work properly. Say, a user enters 7, 4, and 5, the program displays 1 as the average when it should display 5.3

  ArrayList <Integer> marks = new ArrayList();
  Collections.addAll(marks, (Integer.parseInt(markInput.getText())));

  private void analyzeButtonActionPerformed(java.awt.event.ActionEvent evt) {
      analyzeTextArea.setText("Class average:" + calculateAverage(marks));
  }

  private int calculateAverage(List <Integer> marks) {
      int sum = 0;
      for (int i=0; i< marks.size(); i++) {
            sum += i;
      }
      return sum / marks.size();
  }

What is wrong with the code?

With Java 8 it is a bit easier:

OptionalDouble average = marks
            .stream()
            .mapToDouble(a -> a)
            .average();

Thus your average value is average.getAsDouble()

return average.isPresent() ? average.getAsDouble() : 0; 

Why use a clumsy for loop with an index when you have the enhanced for loop?

private double calculateAverage(List <Integer> marks) {
  Integer sum = 0;
  if(!marks.isEmpty()) {
    for (Integer mark : marks) {
        sum += mark;
    }
    return sum.doubleValue() / marks.size();
  }
  return sum;
}

Update: As several others have already pointed out, this becomes much simpler using Streams with Java 8 and up:

private double calculateAverage(List <Integer> marks) {
    return marks.stream()
                .mapToDouble(d -> d)
                .average()
                .orElse(0.0)
}