Metadata Card

• Prerequisites: Ch4 (Control Flow), Ch5 (Methods)
• Estimated time: 55 min
• Core difficulty:
• Reading mode: Steady progress
• Completion marker: Can declare, initialize, and iterate over arrays; understand index vs value; use multi-dimensional arrays

The Breakthrough · Tracing the Origins

A variable holds one value. What if you need a hundred? An array is a contiguous block of memory that holds multiple values of the same type.

Declaring and Initializing

// Declaration and allocation
int[] numbers = new int[5]; // [0, 0, 0, 0, 0]

// With initial values
int[] scores = {85, 92, 78, 90, 88};

// Using the new keyword with values
int[] prices = new int[]{10, 20, 30};

Indexing

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

System.out.println(scores[0]); // 85 (first element)
System.out.println(scores[1]); // 92
System.out.println(scores[2]); // 78

scores[1] = 95; // Modify
System.out.println(scores[1]); // 95

Critical: Array indices start at 0. The last index is length - 1. Accessing outside this range throws ArrayIndexOutOfBoundsException.

Length and Iteration

int[] arr = {10, 20, 30, 40, 50};
System.out.println("Length: " + arr.length); // 5

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

// Enhanced for loop (for-each)
for (int value : arr) {
 System.out.println(value);
}

Memory Layout

Stack:  scores (reference)
          |
Heap:   [85] [92] [78] [90] [88]
          0    1    2    3    4

The variable scores on the stack points to the array object on the heap.

Common Operations

int[] a = {1, 2, 3};
int[] b = a; // Both reference the SAME array
b[0] = 99;
System.out.println(a[0]); // 99! (changed)

// Copy: use Arrays.copyOf
int[] copy = Arrays.copyOf(a, a.length);

// Sort
Arrays.sort(arr);

// Fill
Arrays.fill(arr, 0); // Fill all with 0

// Search (must be sorted first)
int index = Arrays.binarySearch(arr, 42);

Multi-dimensional Arrays

int[][] matrix = {
 {1, 2, 3},
 {4, 5, 6},
 {7, 8, 9}
};

System.out.println(matrix[0][1]); // 2
System.out.println(matrix[1][2]); // 6

// Iteration
for (int row = 0; row < matrix.length; row++) {
 for (int col = 0; col < matrix[row].length; col++) {
 System.out.print(matrix[row][col] + " ");
 }
 System.out.println();
}

Final Challenge

1. Write a method that finds the maximum value in an int array
2. Write a method that reverses an array in-place
3. Implement linear search: find the index of a target value, return -1 if not found
4. Create a multiplication table as a 2D array (9x9)

Traveler's Notes

Arrays hold multiple values of the same type in contiguous memory.
Index starts at 0, ends at length-1.
Arrays in Java are objects on the heap; the variable holds a reference.
Array copying creates an independent copy; reference assignment shares the same array.
For-each loops (for (int x : arr)) are readable but don't give you the index.