Arrays and ArrayList

A Java array has a fixed size set at creation and no built-in way to grow — array.length gives you that size directly, since unlike C arrays, a Java array always knows its own length.

public class Main {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3, 4, 5};
        System.out.println("numbers[2] = " + numbers[2]);
        System.out.println("length = " + numbers.length);
    }
}

numbers[2] = 3
length = 5

ArrayList

ArrayList is Java's primary growable collection — it manages its own resizing as you add elements, and like all of Java's generic collections, it only holds reference types, so an ArrayList<Integer> stores boxed Integer objects rather than primitive int values directly.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        System.out.println(numbers);
        System.out.println("size = " + numbers.size());
    }
}

[1, 2, 3, 4]
size = 4

Next: strings — String immutability, concatenation, and the difference between == and .equals().