Containers

Arrays

An array in Java is a container that holds multiple values of the same data type in a fixed-size, contiguous memory location.

int[] numbers = new int[5]; // Creates an array of size 5 with default values (0)
int[] nums = {10, 20, 30, 40, 50}; // Declares and initializes an array
 
numbers[0] = 100;
numbers[1] = 200;
numbers[2] = 300;
 
System.out.println(numbers[0]); // Output: 100
System.out.println(numbers[2]); // Output: 300
 
System.out.println(numbers.length); // Output: 3
 
for (int i = 0; i < numbers.length; i++) {
    System.out.println(numbers[i]);
}
 
for (int num : numbers) {
    System.out.println(num);
}
 
// multi dimensional array
int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};
 
System.out.println(matrix[0][1]); // Output: 2
 
for (int i = 0; i < matrix.length; i++) {
    for (int j = 0; j < matrix[i].length; j++) {
        System.out.print(matrix[i][j] + " ");
    }
    System.out.println();
}
 
Arrays.sort(numbers);
System.out.println(Arrays.toString(numbers));

Default Values in Arrays

When an array is created with new, elements get default values:

Data Type	Default Value
int[]	0
double[]	0.0
char[]	'\u0000' (null character)
boolean[]	false
String[]	null

---

Arraylists

ArrayList is a resizable array implementation in Java, part of the java.util package. Unlike arrays, ArrayList can grow or shrink dynamically.

ArrayList<Type> listName = new ArrayList<>();

import java.util.ArrayList;
 
public class Main {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>(); // Creates an empty ArrayList
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");
        System.out.println(names); // Output: [Alice, Bob, Charlie]
 
        System.out.println(names.get(1)); // Output: Bob
 
        names.set(1, "David");
        System.out.println(names); // Output: [Alice, David, Charlie]
        names.remove(0); // Removes "Alice"
        System.out.println(names.size()); // Output: 2
 
        for (int i = 0; i < names.size(); i++) {
            System.out.println(names.get(i));
        }
 
        // range base loop
        for (String name : names) {
            System.out.println(name);
        }
 
        //using forEach
        names.forEach(System.out::println);
 
        System.out.println(names.contains("Bob")); // Output: false
        Collections.sort(names) // sorting
 
        names.clear();
        System.out.println(names); // Output: []
    }
}

ArrayList vs. Arrays

Feature	Array	ArrayList
Size	Fixed	Dynamic (Resizable)
Performance	Faster	Slightly slower (due to resizing)
Type	Can store primitives	Stores only objects (Wrapper classes for primitives)
Flexibility	Low	High (Built-in methods)