alex:cp$ cat main.cpp
#include <bits/stdc++.h>
using namespace std;

int main() {
    int n = 500000;
    string s = "";
    for (int i = 0; i < n; ++i) {
        s += "a";
    }
}
alex:cp$ cat Test.java
public class Test {
    public static void main(String[] args) {
        int n = 500000;
        String s = "";
        for (int i = 0; i < n; ++i) {
            s += "a";
        }
    }
}
alex:cp$ time ./main

CPU	74%
tot	0,157
usr	0,020
sys	0,096
alex:cp$ time java Test

CPU	105%
tot	6,010
usr	6,050
sys	0,277
alex:cp$