Lecture 8: Arrays in Java

Introduction to Arrays

• Definition: Collection of data types (primitives or objects).
• Need: Simplifies storing multiple elements without repetitive code.
• Syntax: data_type[] array_name = new data_type[size]; // Example int[] rollNumbers = new int[5]; int[] rollNumbers2 = {23, 12, 45, 32, 15};
• Rules:
  • All elements must be of the same type.
  • Declared with square brackets.

Memory Management

• Stack and Heap memory.
• Declaration happens in Stack; Initialization in Heap.
• Dynamic memory allocation at runtime.
• Continuous memory allocation not guaranteed in Java.
• Indices start from 0.

Null and Default Values

• Integers initialized to 0.
• Strings and other objects to null.
• null: Special literal, default for reference types, not primitives.

Operations on Arrays

Input/Output

• Using loops to take input and display arrays.
• Enhanced for loop (for-each) for simpler iteration. for (dataType item : array) { System.out.println(item); }
• Arrays.toString() for printing arrays in a formatted manner.

Functions

• Passing arrays to functions: Object reference is passed, changes in functions affect the original array. static void change(int[] arr) { arr[0] = 99; }

2D Arrays

• Syntax: int[][] matrix = new int[rows][columns];
• Internally: Array of arrays, not necessarily continuous memory.
• Input and Output in 2D arrays involves nested loops. for (int row = 0; row < arr.length; row++) { for (int col = 0; col < arr[row].length; col++) { arr[row][col] = sc.nextInt(); } }

ArrayList

• Part of Collection Framework (java.util package).
• Syntax: ArrayList<dataType> list = new ArrayList<>();
• Dynamic size, non-primitive data types.
• Methods: add, set, remove, contains, get.
• How it works: Drops old array, copies to new larger array when needed.
• Example: ArrayList<Integer> list = new ArrayList<>(); list.add(67); list.add(45); list.add(32); System.out.println(list.contains(67)); // true
• Multi-dimensional ArrayLists possible (ArrayList of ArrayLists). ArrayList<ArrayList<Integer>> list = new ArrayList<>();

Practice Problems

• Reverse an array using two-pointer technique. static void reverse(int[] arr) { int start = 0, end = arr.length - 1; while (start < end) { int temp = arr[start]; arr[start] = arr[end]; arr[end] = temp; start++; end--; } }
• Find maximum element in an array and in a range. static int max(int[] arr) { int max = arr[0]; for (int i : arr) { if (i > max) max = i; } return max; }

Upcoming Topics

• Sorting techniques.
• More questions on arrays, searching, sorting.
• Advanced topics like sliding window, two-pointer method.