Experiment

Let suppose you have million of digits that you want to print on the screen one next to each other using Java. There are two approaches that I identified :

• You can either choose to print each digit at the time (one digit next to each other) like so :

import java.util.Random;
public class Generate_Million_Digits {
	public static void main(String[] args) {
		Random rand = new Random();
		long start = System.currentTimeMillis();
		for (int i = 1; i <= 1000000; i++) {
			System.out.print(rand.nextInt(10));
		}	
		System.out.println();
		long end = System.currentTimeMillis();
		System.out.println("Elapsed Time in milli seconds: " + (end - start));
	}
}

The elapsed Time in milli seconds is : 5248

• Or you can decide to concatenate all the digit you want to print in a special dat structure like a String or String Builder and then just print the string builder once like so :

import java.util.Random;
public class Generate_Million_Digits {
	public static void main(String[] args) {
		Random rand = new Random();
		long start = System.currentTimeMillis();
		StringBuilder sb = new StringBuilder();
		for (int i = 1; i <= 1000000; i++) {
			sb.append(rand.nextInt(10));
		}	
		System.out.println(sb.toString());
		long end = System.currentTimeMillis();
		System.out.println("Elapsed Time in milli seconds: "+ (end - start));
	}
}

The elapsed Time in milli seconds: 53

Story

The story is that I was recently solving a problem on Codeforces.com using Java 1.8.0_241. As part of my solution I needed to print a series of digits. My first intuition was to use the first approach I described above because it avoided me using additional data structures, but I failed into the Time limit Exceeded issue with one of the test case. So I ask myself how to fix the TLE. I then tried the second approach. After trying the second approach, the test that was failing with TLE passed with the second approach, therefore I was able to solve the problem. The good thing is that in order to solve the problem, I had to introduce a data structure (here a StringBuilder) to avoid making many prints.

Lesson

In Java, when you have to print many digits one next to each other, please prioritize concatenating all the digits into a data structure (StringBuilder, it is mutable) and print the bulk once, instead of printing one digit at the time one after each other.