Java ArrayList

ArrayList is a resizable array that implements List. A normal array has a fixed size. ArrayList can grow when you add. Duplicates are allowed. Order is kept — first add stays first. This is the list you use in almost every fresher program.

Let’s take this on the board. ArrayList<String> names = new ArrayList<>(); names.add("Asha"); names.add("Ravi"); print names.size(); print names.get(0);. Output: 2 then Asha. Index from 0, same as array. get(1) is Ravi. get(2) throws IndexOutOfBoundsException — last valid index is size() - 1. Don’t write names.get(names.size()).

Fast get(i) is why we pick ArrayList day to day. Adding in the middle is slower because later items shift, like inserting a bench in the middle of a row. If you only add at the end and read by index, ArrayList is perfect. If you need unique names, use HashSet. If you insert/delete a lot at both ends, then think LinkedList — not on day one.

Cannot store raw int. ArrayList<Integer> marks = new ArrayList<>(); marks.add(90); works because of autoboxing (int → Integer). ArrayList<int> is illegal. Write the type in < >. Raw ArrayList without generics is old style and unsafe.