## Chapter 9: Arrays (A Row of Boxes) Imagine you want to store marks of 50 students. Creating `marks1`, `marks2`... is a nightmare. An **Array** is one variable that holds many values of the same type. Think of it as a row of lockers. ### 1. The "0" Confusion (Indexing) In Java, we count from **0**, not 1. **Example 1: Creating and Filling (Basic Example)** ```java int[] marks = new int[5]; // 5 boxes marks[0] = 90; // 1st box marks[4] = 85; // 5th (last) box ``` **Example 2: The Manager's Scream (Common Mistake)** ```java int[] arr = new int[5]; System.out.println(arr[5]); // CRASH! ArrayIndexOutOfBoundsException ``` *Teacher's Note:* If a hotel has 5 rooms numbered 0 to 4, and you ask for room 5, the manager (Java) screams! This is the most common error in Chapter 9. ### 2. The `length` Trap **Example 3: No Brackets! (Explanation Focused)** ```java int size = arr.length; // Correct int size = arr.length(); // ERROR! ``` *Teacher's Note:* Arrays use `.length`, but Strings use `.length()`. Students mix this up for months. Remember: Arrays are simple "boxes," so they have a simple property. Strings are "tools," so they use a method. ### 3. Using Loops with Arrays **Example 4: The Classroom Roll Call (Real-Life Example)** ```java String[] students = {"Ravi", "Neha", "Aman"}; for (int i = 0; i < students.length; i++) {     System.out.println("Present: " + students[i]); } ``` **Example 5: 2D Arrays (Future Hint)** ```java int[][] matrix = new int[3][3]; ``` *Teacher's Note:* Later, we'll learn about "Arrays of Arrays" for things like maps, grids, or Tic-Tac-Toe boards.