Array

An array is a fixed-size, indexed collection of elements of the same data type.

int[] arr = new int[5];
int[] nums = {1, 2, 3, 4};

Stored in contiguous memory
Index-based access (0-based)
Length is fixed once created

🧠 Key Characteristics

Fast access: O(1)
Fixed size (cannot grow/shrink)
Can store primitives or objects
Supports single-dimensional & multi-dimensional arrays

📏 Array Length

arr.length

⚠️ length is a property, not a method.

🔁 Traversing an Array

Classic For Loop

for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}

Enhanced For Loop

for (int num : arr) {
    System.out.println(num);
}

⚙️ Common Arrays Utility Methods (java.util.Arrays)

Arrays.sort(arr)
Arrays.binarySearch(arr, key) // Works only on sorted arrays :: Time: O(log n)
Arrays.equals(arr1, arr2)
Arrays.copyOf(arr, newLength)
Arrays.copyOfRange(arr, from, to)
Arrays.fill(arr, value)
Arrays.toString(arr)

📌 These are frequently asked in interviews.

🧩 Java 8+ Array Enhancements

Streams from Arrays

Arrays.stream(arr)

Examples:

int sum = Arrays.stream(arr).sum();

int max = Arrays.stream(arr).max().orElse(0);

long count = Arrays.stream(arr)
                   .filter(n -> n == 0)
                   .count();

⚠️ Streams are not in-place.

🔄 Converting Array ↔ List

Array to List

List list = Arrays.asList(1, 2, 3);

⚠️ Fixed-size list (cannot add/remove).

List to Array

Integer[] arr = list.toArray(new Integer[0]);