Question

Is ArrayList an array or a list in java? what is the time complexity for the get operation, is it O(n) or O(1)?

Answer 1

An ArrayList in Java is a List that is backed by an array.

The get(index) method is a constant time, O(1), operation.

The code straight out of the Java library for ArrayList.get(index):

public E get(int index) {
    RangeCheck(index);
    return (E) elementData[index];
}

Basically, it just returns a value straight out of the backing array. (RangeCheck(index)) is also constant time)