Chapter 9: Arrays

Introduction to Arrays

• An array is a data structure that stores a collection of elements of the same type, indexed starting at 0.

Declaring Arrays

• Arrays can be declared as:

  • int[] numbers; (preferred)

  • int numbers[]; (valid but less common)

• Must be initialized with a specific size or values:

  • int[] numbers = new int[5]; (array of 5 integers)

  • int[] numbers = {1, 2, 3, 4, 5}; (initialized with values)

Accessing Array Elements

• Access elements via their index:

  • numbers[0] accesses the first element.

Array Length

• Use numbers.length to get the number of elements.

Multidimensional Arrays

• Arrays can be multidimensional:

  • Example: int[][] matrix = new int[3][4];

  • Accessed via multiple indices: matrix[0][1]

Enhanced For Loop

• Simplifies iteration:

  • for (int num : numbers) { System.out.println(num); }

Passing Arrays to Methods

• Arrays are passed by reference:

  • Example: public static void printArray(int[] arr) { for (int num : arr) System.out.println(num); }

Common Array Operations

• Copying: System.arraycopy() copies elements.

• Sorting: Use Arrays.sort().

• Searching: Use linear search or binary search (for sorted arrays).

Array of Objects

• Can store objects, e.g., String[] names = new String[3];.

Array Exceptions

• ArrayIndexOutOfBoundsException occurs for invalid indices (0 to length-1). Always ensure indices are within bounds.

Declaration Example

• int i1,i2,i3,i4;

• Array Syntax:

  • First, the data type: int, double, String.

  • Use brackets: int[] alhs or int alhs[].

  • Example: int[] alhs = new int[6];

  • The 6 indicates a fixed number of components (must be a positive integer).

Initialization Behavior

• Java initializes array components to 0. For a newly created array of size 6, all cells start at 0.

One-dimensional Array Declaration

• Example of correct vs. incorrect:

  • Incorrect: int alpha[], beta;

  • Correct: int[] alpha, beta; (applies prison to both)