Reading integer input in Java - Time & Space Complexity

Time Complexity: Reading integer input

O(n)

Understanding Time Complexity

We want to understand how the time to read an integer input grows as the input size changes.

Specifically, how does the program's work change when reading numbers?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int number = scanner.nextInt();
        System.out.println("You entered: " + number);
    }
}

This code reads one integer from the user and prints it back.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Reading characters from input until the integer is fully read.
• How many times: Depends on the number of digits in the integer input.

How Execution Grows With Input

Reading time grows with the number of digits typed by the user.

Input Size (digits)	Approx. Operations
1	Few character reads (about 1)
3	About 3 character reads
10	About 10 character reads

Pattern observation: The time grows roughly linearly with the number of digits entered.

Final Time Complexity

Time Complexity: O(n)

This means the time to read the integer grows linearly with the number of digits typed.

Common Mistake

[X] Wrong: "Reading an integer always takes the same time no matter how big it is."

[OK] Correct: Actually, the program reads each digit one by one, so more digits mean more work.

Interview Connect

Understanding how input reading scales helps you reason about program speed and user interaction delays.

Self-Check

"What if we read multiple integers in a loop? How would the time complexity change?"