Arrays Class

You could write a loop to print, sort or search an array every time, but you shouldn't have to. Java's java.util.Arrays class is a toolbox of ready-made, well-tested methods for arrays. All of them are static, so you call them on the class: Arrays.sort(marks).

The Methods You'll Use Most

Sort, Search, Fill, Compare

import java.util.Arrays;
 
public class Toolbox {
    public static void main(String[] args) {
        int[] marks = {86, 64, 92, 78};
        Arrays.sort(marks);
        System.out.println("Sorted: " + Arrays.toString(marks));
        System.out.println("92 is at index " + Arrays.binarySearch(marks, 92));
        int[] seats = new int[5];
        Arrays.fill(seats, -1);
        System.out.println("Seats: " + Arrays.toString(seats));
        int[] same = {64, 78, 86, 92};
        System.out.println("equals: " + Arrays.equals(marks, same));
        System.out.println("==:     " + (marks == same));
    }
}

Sorted: [64, 78, 86, 92]
92 is at index 3
Seats: [-1, -1, -1, -1, -1]
equals: true
==:     false

How it works

1. Line 6 sorts marks in place: the array itself changes; nothing is returned.
2. Line 8 searches the sorted array; 92 is at index 3.
3. Line 10 sets every seat to -1, a common "not assigned" marker.
4. Lines 13 and 14: Arrays.equals compares the values; == checks whether they're the same array, and they aren't.

Try it yourself: search for 80 with binarySearch. What number comes back?

Two Traps

binarySearch needs a sorted array. On an unsorted array, the result is meaningless. When the key isn't found, it returns -(insertionPoint) - 1: a negative number that tells you where the key would go. For 80 in [64, 78, 86, 92], it would go at index 2, so the result is -3.

Arrays.asList has a fixed size. It returns a List backed by the array: you can change elements with set, but add or remove throws UnsupportedOperationException. For a list you can grow, write new ArrayList<>(Arrays.asList(...)) (collections are covered later in the course).

Sorting Text and Part of an Array

Arrays.sort also sorts String[] alphabetically, and Arrays.sort(a, from, to) sorts just a range. For descending order or custom orders, you'll pass a comparator once you've learned lambdas, later in the course.

Common Mistakes

Printing an array with println(a). Use Arrays.toString(a).

Comparing arrays with == or a.equals(b). Both check whether it's the same array. Use Arrays.equals(a, b).

Searching an unsorted array. Sort first, or loop through it yourself.

Interview Corner

Practice

1. Easy: Sort {"Mysuru", "Agra", "Kochi", "Delhi"} and print it with Arrays.toString.
2. Harder: Read 5 numbers, sort them, and print the smallest, the largest and the middle (median) value.

Key Takeaways