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Java · Basics

Arrays

An array stores a fixed number of values of the same type in numbered slots.

Beginner 5 min Lesson 8 of 31Examples target Java 21 LTS (they also run on Java 25).
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On this page · 5 min · Beginner

On this page

  1. In simple words
  2. Learning objectives
  3. Explanation
  4. Code example
  5. Step by step
  6. Real-world use
  7. Common mistakes
  8. Best practices
  9. Remember this
  10. Practice exercise
  11. Mini quiz
  12. Related topics
  13. My notes
Reading time
5 min
Difficulty
Beginner
Updated
Oct 30, 2026

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Interview questions

In simple words#

An array is an egg tray: a fixed number of slots, numbered from 0. You can change what’s in a slot, but you cannot add more slots.

Index
Slot number starting at 0.
length
Number of slots (a field, not a method).
Arrays utility
sort, toString, fill, copyOf…

Learning objectives#

  • Create and initialise arrays
  • Read and write by index
  • Use the Arrays utility class
  • Choose arrays vs ArrayList

Explanation#

int[] marks = new int[5]; creates five slots with default 0. int[] marks = {78, 92, 65}; creates and fills.

Indexes run from 0 to length - 1. Going outside throws ArrayIndexOutOfBoundsException.

java.util.Arrays has sort, toString, fill, copyOf, binarySearch and asList.

Arrays cannot grow. For a list that changes size, use ArrayList.

Code example: Class marks#

JavaClass marks Illustrative
import java.util.Arrays;


int[] marks = {78, 92, 65, 88, 71};
System.out.println("First: " + marks[0] + ", count: " + marks.length);


Arrays.sort(marks);
System.out.println("Sorted: " + Arrays.toString(marks));


int sum = 0;
for (int m : marks) sum += m;
System.out.printf("Average: %.1f%n", (double) sum / marks.length);


int[][] seats = new int[3][4];   // 3 rows × 4 seats
seats[1][2] = 1;                 // book row 2, seat 3
System.out.println(Arrays.deepToString(seats));
Expected output
First: 78, count: 5
Sorted: [65, 71, 78, 88, 92]
Average: 78.8
[[0, 0, 0, 0], [0, 0, 1, 0], [0, 0, 0, 0]]

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

  1. The array literal creates five values.
  2. Arrays.sort sorts in place.
  3. printf formats the average to one decimal.
  4. A 2D array models rows and seats.

Real-world use case#

A cinema booking screen stores seats as a 2D array of booked/free flags.

Common mistakes#

Printing an array directly

System.out.println(arr) prints something like [I@1b6d3586. Use Arrays.toString.

Index equal to length

The last index is length - 1.

Best practices#

  • Use ArrayList when size changes
  • Arrays.toString for printing
  • for-each when the index isn’t needed
  • Validate indexes

Remember this#

Index 0 to length - 1.

Fixed size — cannot grow.

Arrays.sort / Arrays.toString.

Practice exercise#

Your task: Find the highest mark in the array.

Show solution
Java Illustrative
int max = marks[0];
for (int m : marks) if (m > max) max = m;
// or: Arrays.stream(marks).max().getAsInt();

Mini quiz#

Arrays quizMultiple choice Question 1 of 1

What is the index of the last element in an array of length 10?

Score so far: 0/0

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My notes#

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