Understanding Java Arrays

What Is an Array?

An array is a fixed-size, ordered collection of elements of the same type. Once created, its length cannot change. Arrays are objects on the heap.

Declaration

The brackets can appear after the type or after the variable name. The first style is preferred.

java
int[] numbers;       // preferred
int numbers2[];      // valid but less common

String[] names;
double[] prices;

Creation and Initialization

Size-based (default values)

java
int[] nums = new int[5];       // {0, 0, 0, 0, 0}
boolean[] flags = new boolean[3]; // {false, false, false}
String[] words = new String[4];   // {null, null, null, null}

Default values: 0 for numeric types, false for boolean, null for object types.

Array Initializer (inline)

java
int[] primes = {2, 3, 5, 7, 11};
String[] days = {"Mon", "Tue", "Wed", "Thu", "Fri"};

Anonymous Array

java
int[] primes = new int[]{2, 3, 5, 7, 11};
printArray(new int[]{1, 2, 3}); // pass directly without a variable

Accessing Elements

Array indices are zero-based. Accessing an out-of-bounds index throws ArrayIndexOutOfBoundsException at runtime.

java
int[] arr = {10, 20, 30};
System.out.println(arr[0]);  // 10
System.out.println(arr[2]);  // 30
arr[1] = 99;
System.out.println(arr.length); // 3 (field, not method!)

Iterating Arrays

java
int[] scores = {85, 90, 78, 92};

// Traditional for loop
for (int i = 0; i < scores.length; i++) {
    System.out.println(scores[i]);
}

// Enhanced for loop (read-only iteration)
for (int score : scores) {
    System.out.println(score);
}

The enhanced for loop cannot modify the array elements or track the current index.

Multidimensional Arrays

Java supports arrays of arrays. Each "row" can have a different length (jagged arrays).

java
// Rectangular 2D array
int[][] matrix = new int[3][4]; // 3 rows, 4 columns

// Inline initialization
int[][] grid = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

System.out.println(grid[1][2]); // 6 (row 1, column 2)

// Jagged array
int[][] jagged = new int[3][];
jagged[0] = new int[]{1};
jagged[1] = new int[]{2, 3};
jagged[2] = new int[]{4, 5, 6};

The Arrays Utility Class

java.util.Arrays provides static helper methods for common array operations.

sort

Sorts the array in place in ascending order.

java
int[] nums = {5, 3, 1, 4, 2};
Arrays.sort(nums);
System.out.println(Arrays.toString(nums)); // [1, 2, 3, 4, 5]

String[] words = {"banana", "apple", "cherry"};
Arrays.sort(words);
System.out.println(Arrays.toString(words)); // [apple, banana, cherry]

binarySearch

Performs a binary search on a sorted array. Returns the index if found, or a negative value if not found. The array must be sorted first.

java
int[] sorted = {1, 2, 3, 4, 5};
System.out.println(Arrays.binarySearch(sorted, 3));  // 2
System.out.println(Arrays.binarySearch(sorted, 9));  // negative (not found)

equals

Returns true if two arrays have the same length and all corresponding elements are equal.

java
int[] a = {1, 2, 3};
int[] b = {1, 2, 3};
int[] c = {1, 2, 4};

System.out.println(Arrays.equals(a, b)); // true
System.out.println(Arrays.equals(a, c)); // false
System.out.println(a == b);              // false (different objects)

compare

Compares two arrays lexicographically. Returns negative, zero, or positive.

java
System.out.println(Arrays.compare(new int[]{1, 2}, new int[]{1, 3})); // negative
System.out.println(Arrays.compare(new int[]{1, 2}, new int[]{1, 2})); // 0

mismatch

Returns the index of the first element that differs, or -1 if the arrays are equal.

java
System.out.println(Arrays.mismatch(new int[]{1, 2, 3}, new int[]{1, 2, 4})); // 2
System.out.println(Arrays.mismatch(new int[]{1, 2}, new int[]{1, 2}));       // -1

fill

Sets every element to a given value.

java
int[] arr = new int[5];
Arrays.fill(arr, 7);
System.out.println(Arrays.toString(arr)); // [7, 7, 7, 7, 7]

toString

Returns a readable string representation (use this for printing, not arr.toString()).

java
int[] arr = {1, 2, 3};
System.out.println(arr.toString());          // [I@6d06d69c (not useful)
System.out.println(Arrays.toString(arr));    // [1, 2, 3]

Arrays Summary Table

Exam Tips

• length is a field on arrays, not a method (arr.length, not arr.length()).
• Using binarySearch on an unsorted array produces undefined results.
• == on arrays compares references; use Arrays.equals() for content.
• A multidimensional array declaration like int[][] arr = new int[3][] leaves the second dimension uninitialized — you must assign each row separately.