Why is my Java program running faster than my C++ program , both doing the same thing

Question

I wrote a program in both C++ and Java to print "Hello World" 100,000 times, but I noticed that the C++ code takes too long compared to the Java code; The Java code takes about 6 seconds averagely and the C++ code takes about 18 seconds averagely, both run from the command line; Can someone please explain why, thanks.

The name of the program is first.java and first.cpp for Java and C++ respectively I used: java first.java; and first.exe; both from the command line

g++ --version g++ (Rev6, Built by MSYS2 project) 11.2.0

java --version java 13.0.2, 2020-01-14

Java Code

class first {
    public static void main(String... args) {
        long start = System.currentTimeMillis();

        for (int i = 0; i < 100000; i++) {
            System.out.println("Hello World");
        }

        long end = System.currentTimeMillis();

        long dur = end - start;
        System.out.println(dur / 1000);
    }

}

C++ Code

#include <iostream>
#include <string>
#include <chrono>

using namespace std;

int main()
{
    auto start = std::chrono::system_clock::now();
    for (int i = 0; i < 100000; i++)
    {
        cout << "Hello World" << endl;
    }
    auto end = std::chrono::system_clock::now();

    std::chrono::duration<double> elapsed_seconds = end - start;
    cout << elapsed_seconds.count() << endl;
}

Answer 1

There are several relevant differences between your C++ and Java code:

1. By default C++ IO streams synchronise their state with the underlying C streams. This takes time. To avoid this (which you can do only if you know that your code does not mix C and C++ IO operations!), add the following to the beginning of your main code:

   std::ios_base::sync_with_stdio(false);
   

2. cout << endl; is equivalent to cout << "\n" << flush; (which, in turn, is equivalent to cout << "\n"; cout.flush();). The flush call is absent from your Java code. You could add it to your Java code or, better, remove it from your C++ code: you almost never need to use endl/flush. Instead, just use

   cout << "Hello World\n";
   

   As noted by Peter in the comments, most systems flush the stdout stream on newline anyway (at least when attached to a terminal) so one might expect this not to make a difference. However, it does make a (substantial!) difference e.g. when piping the output to a file.

3. Your Java benchmark code truncates fractional seconds. To show those fractions of seconds (relevant since the code runs in <1s!), change the relevant line to

   System.out.println(dur / 1000.0);
   

4. Be sure to compile your C++ code with optimisations enabled; with GCC/clang/ICC, you do this by passing -O2. MSVC has a similar flag, /O2 (there are higher optimisation levels but they have particular issues; -O2 is pretty much the default setting people use).

5. Conversely, java first.java will first compile the code every time you invoke it. To make the comparison fair, be sure to run javac first.java ahead of time, and then execute the code via java first.

Making these changes causes the C++ code to overtake the Java code on my system. This is most noticeable when increasing the loop size from 100,000 to 1,000,000: the C++ code now runs in milliseconds, while the Java code takes several seconds (be sure to pipe the output to a file! Otherwise you will be purely measuring the latency/rendering speed of your terminal, not the performance of the code).