The collections framework provides a core set of interfaces to define different collection behaviors. One of the core interfaces is the List interface. A List is a collection where elements are ordered in a sequence. Lists allow us to have duplicate elements and fine-grain control over where elements are inserted in the sequence. Like arrays, the position of a List is known as the index and is 0 based. Unlike arrays, which have a static size, Lists are dynamically sized.

The collections framework provides many List implementations, but we’ll focus on the ArrayList and LinkedList. The ArrayList is the overall preferred implementation for most use cases but the LinkedList performs better than an ArrayList if your program mostly inserts and deletes elements at the beginning or end of a list.

Let’s create a List using an ArrayList as its implementation and see some operations:

List<Integer> intList = new ArrayList<>(); // Empty `List`
intList.add(4); // 4
intList.add(6); // 4, 6
intList.add(3); // 4, 6, 3
intList.set(1, 3); // 4, 3, 3

int a = intList.get(2); // a = 3
int b = intList.indexOf(3); // b = 1

List<Integer> subIntList = intList.subList(1,3); // subIntList -> 3, 3

In the example above, we:

• Created a List reference named intList with an ArrayList implementation.
• Called add(), which appends elements to the end of the List. We can see the state of intList after each call.
• Called intList.set(1, 3), which replaces the element at index 1 (the second element in 0-based indexing) with 3.
• Called get(), which gets the value at index 2 (the third element in 0-based indexing).
• Called indexOf(), which returns the index of the first occurrence of 3 (the first 3 is at index 1).
• Called subList(), which returns a sublist in a new List with the elements specified by the starting index 1 (inclusive) and ending index 3 (exclusive).

We can iterate through a List using the enhanced for-loop. For example:

// Assuming `intList` has elements -> 1, 5, 2, 6, 1
for (Integer number: intList) {
  System.out.println(number);  
}
// OUTPUT TERMINAL: 1 5 2 6 1

In the example above, we used the enhanced for-loop, which iterates through the elements in intList from index 0 to the end of the list. Note that we use the int wrapper class Integer to iterate through the elements in intList. However, we could have also used an ‘int’.

Let’s practice creating a List and iterating through it.