☕ Java📅 Day 8

Arrays in Java

Declare, initialize and access single-dimensional and 2D arrays in Java.

🎯

Learning objectives

→Declare and initialize arrays two ways
→Access, modify and iterate array elements
→Work with 2D arrays (matrices)

💡 Key points

Array = fixed-size, ordered collection of same-type elements.
Zero-indexed: valid indices 0 to length - 1.
arr.length is a field (no parentheses), unlike String's length().
Once created, size cannot change. For dynamic sizing use ArrayList.

💻 Code examples(4)

#1Declare and initialize

java

int[] a = new int[5];              // {0, 0, 0, 0, 0}
int[] b = {10, 20, 30, 40, 50};    // literal
int[] c = new int[]{1, 2, 3};      // long form

String[] names = {"Amit", "Priya", "Ravi"};

new int[5] creates 5 slots initialized to 0. Literals infer size from list.

#2Access and modify

java

int[] marks = {85, 90, 78};
System.out.println(marks[0]);   // 85
marks[2] = 95;                  // update
System.out.println(marks.length); // 3

Reading marks[3] throws ArrayIndexOutOfBoundsException — always index < length.

#3Iterate — two ways

java

int[] arr = {5, 10, 15};

// index-based (need i for position)
for (int i = 0; i < arr.length; i++) {
    System.out.println(i + ": " + arr[i]);
}

// for-each (cleaner when index not needed)
for (int x : arr) {
    System.out.println(x);
}

Prefer for-each unless you need the index or want to modify elements by index.

#42D array (matrix)

java

int[][] grid = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9},
};
System.out.println(grid[1][2]);   // 6

for (int[] row : grid) {
    for (int val : row) {
        System.out.print(val + " ");
    }
    System.out.println();
}

int[][] = array of arrays. grid[row][col] to access. Rows can even have different lengths (jagged array).

🎯 Practice

Q1. What is the default value of int[] arr = new int[3]?+

All slots default to 0. Boolean arrays default to false, object arrays to null.

Q2. Find the largest element in int[] nums = {3, 8, 1, 12, 5}.+

int max = nums[0]; for (int n : nums) if (n > max) max = n; // max = 12

Q3. Why is int[] length a field but String's length() a method?+

Arrays are special language constructs with length exposed directly. String is a class — length() is a proper method.

📝 Notes

Common operations

// sum
int sum = 0;
for (int x : arr) sum += x;

// reverse in place
for (int i = 0, j = arr.length - 1; i < j; i++, j--) {
    int t = arr[i]; arr[i] = arr[j]; arr[j] = t;
}

// copy
int[] copy = arr.clone();

// sort
java.util.Arrays.sort(arr);

Array vs ArrayList

| | Array | ArrayList | |---|---|---| | Size | Fixed | Grows/shrinks | | Type | Any (primitive OK) | Objects only | | Methods | .length, indexing | rich API (add, remove, contains) | | Speed | Faster | Slightly slower |

Rule of thumb: fixed-size numeric data → array. Everything else → ArrayList.