07 — Arrays

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▶️ java pkg1core/core8ArraysDemo.java

Declaring arrays

1int[] a = {5, 3, 8, 1, 9};       // inline init2int[] b = new int[3];              // default: all zeros3b[0] = 10;

• Fixed size once created
• Zero-indexed: first element is [0]
• arr.length is a field (not a method)

2D arrays (matrix)

1int[][] grid = {2    {1, 2, 3},3    {4, 5, 6}4};5System.out.println(grid[1][2]);   // 6

An array of arrays — rows can differ in length (jagged).

java.util.Arrays utilities

1Arrays.sort(sorted);2Arrays.binarySearch(sorted, 8);    // must be sorted first3Arrays.fill(arr, 7);4Arrays.copyOf(arr, 3);5Arrays.toString(arr);

Common patterns

1// Reverse in place2for (int i = 0, j = a.length - 1; i < j; i++, j--) {3    int t = a[i]; a[i] = a[j]; a[j] = t;4}5 6// Sum7int sum = 0;8for (int v : a) sum += v;

Arrays vs ArrayList

💡 Use arrays for fixed-size, performance-critical code; ArrayList for most application logic.

▶️ Dynamic array implementation: java pkg3datastructures/datastructures0DynamicArray.java

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