Java · Lesson 4 of 9

Loops & arrays

for, enhanced for, while, and working with arrays.

An array holds a fixed number of values of one type: int[] marks = {72, 88, 65};. Index from 0; marks.length gives the size. Reading past the end throws ArrayIndexOutOfBoundsException.

for (int i = 0; i < n; i++) — counted loop.
for (int m : marks) — enhanced for: every element, read-only access.
while (cond) and do { } while (cond) (runs at least once).
break and continue work as in other languages.

java.util.Arrays has helpers: Arrays.sort(arr), Arrays.toString(arr).

Example program

Run with JDK 17+: java Main.java

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] marks = {72, 88, 65, 91, 58};

        int total = 0;
        for (int m : marks) {
            total += m;
        }
        System.out.println("Average: " + (double) total / marks.length);

        int highest = marks[0];
        for (int i = 1; i < marks.length; i++) {
            if (marks[i] > highest) highest = marks[i];
        }
        System.out.println("Highest: " + highest);

        Arrays.sort(marks);
        System.out.println(Arrays.toString(marks));
    }
}

Output

Average: 74.8
Highest: 91
[58, 65, 72, 88, 91]

Save the code as Main.java and run java Main.java (JDK 11+ runs single files directly). Names and figures are fictional.

Key points

Arrays have fixed length.
Enhanced for is simplest for reading every item.
Arrays.sort / Arrays.toString save time.

Quick check

What is the last valid index of an array of length 5?

5 4 6 0

Practice task

Count how many marks in the array are 70 or above and print the count.

Interview corner

Array vs ArrayList?

An array has a fixed size and can hold primitives. ArrayList grows automatically, holds objects only and offers methods like add and remove.

How do you reverse an array in place?

Swap elements from both ends moving inward: for i from 0 to n/2, swap arr[i] and arr[n-1-i].