为什么（我的）Java比C ++快25倍？

Posted 2021-05-07

当我想将它与C ++进行比较时，我正在玩Java中的一些FizzBu​​zz代码。

我在Java中运行下面的代码，在C ++中基本上使用相同的代码。

Java Eclipse IDE运行时：11秒

C ++代码阻止IDE运行时：202秒（比Java慢大约18倍）

C ++ Visual Studio IDE运行时：281秒（比Java慢大约25倍）

据我所知，我已经安装了所有这些IDE的最新版本。

为什么会这样？我所说的每一条信息都说C ++应该运行得更快。

Java代码：

public class Test {

public static void main(String []args) {

    double fizzes = 0;
    double buzzes = 0;
    double fizzbuzzes = 0;
    double normals = 0;
    double iterations = 1000000;

    for (int x = 0; x < iterations; x++) {


        int index = 0;
        String output = "";

        if (x % 3 == 0) {

            output = output + "Fizz";
            index = 1;
        }

        if (x % 5 == 0) {

            output = output + "Buzz";

            index = index + 2;
        }
        if (output == "") { //x%5!=0 && x%3!=0
            System.out.println(x);
            normals++;
        }
        else {
            System.out.println(output);
        }


        if (index == 3) {
            fizzbuzzes++;

        }
        else if (index == 2) {
            buzzes++;
        }
        else if (index == 1) {
            fizzes++;
        }


    }

    System.out.println("Normals " + (normals/iterations*100) + "%"); //percent
    System.out.println("Fizzes " + (fizzes/iterations*100)+ "%");
    System.out.println("Buzzes " + (buzzes/iterations*100)+ "%");
    System.out.println("FizzBuzzes " +(fizzbuzzes/iterations*100)+ "%");


}
}

C ++代码：

#include "stdafx.h"
#include "iostream"

using namespace std;


int main()
{

double fizzes = 0;
double buzzes = 0;
double fizzbuzzes = 0;
double normals = 0;
double iterations = 1000000;

for (int x = 0; x < iterations; x++) {


    int index = 0;
    string output = "";

    if (x % 3 == 0) {

        output = output + "Fizz";
        index = 1;
    }

    if (x % 5 == 0) {

        output = output + "Buzz";

        index = index + 2;
    }
    if (output == "") { //x%5!=0 && x%3!=0
        cout << x<< endl;
        normals++;
    }
    else {
        cout << (output)<< endl;
    }


    if (index == 3) {
        fizzbuzzes++;

    }
    else if (index == 2) {
        buzzes++;
    }
    else if (index == 1) {
        fizzes++;
    }


}

cout << "Normals " << (normals / iterations * 100) <<"%"<<  endl; //percentage
cout << "Fizzes " << (fizzes / iterations * 100) << "%" << endl;
cout << "Buzzes " << (buzzes / iterations * 100) << "%" << endl;
cout << "FizzBuzzes " << (fizzbuzzes / iterations * 100) << "%" << endl;

return 0;
}

这只是按百分比给出了Fizz / Buzzes的数量。

这只是我的IDE配置的结果吗？

编辑：将C ++代码更改为您的建议

#include "stdafx.h"
#include <iostream>
#include <string>


int main()
{

double fizzes = 0;
double buzzes = 0;
double fizzbuzzes = 0;
double normals = 0;
double iterations = 1000000;

for (int x = 0; x < iterations; x++) {


    int index = 0;
    std::string output = "";

    if (x % 3 == 0) {

        output = output + "Fizz";
        index = 1;
    }

    if (x % 5 == 0) {

        output = output + "Buzz";

        index = index + 2;
    }



    if (index == 3) {
        fizzbuzzes++;

    }
    else if (index == 2) {
        buzzes++;
    }
    else if (index == 1) {
        fizzes++;
    }

    if (output == "") { //x%5!=0 && x%3!=0
        //std::cout << x << "
";
        normals++;
    }
    //else {
    //  std::cout << output << "
";
    //}
}

std::cout << "Normals " << (normals / iterations * 100) <<"%"<<  "
"; //percentage
std::cout << "Fizzes " << (fizzes / iterations * 100) << "%" << "
";
std::cout << "Buzzes " << (buzzes / iterations * 100) << "%" << "
";
std::cout << "FizzBuzzes " << (fizzbuzzes / iterations * 100) << "%" << "
";

int z;
std::cin >> z; //so i can see the values
return 0;
}

还可以在发布模式下构建，在IDE之外，它可以立即运行。看起来像打印需要很多时间。

答案

这很有意思，我在我的PC上测试它，结果是一样的。

然后我尝试在java和c ++（使用-O3发布）中禁用打印，两者的时间使用时间<3秒。

所以问题是每次都是cout flush，但java在IDE中运行，IDE会缓冲所有输出，所以这应该是java比c ++更快的原因。

然后我尝试使用控制台运行java，并且比在IDE中运行需要更长的时间。

这应该与https://stackoverflow.com/users/3807729/galik将c ++ ouputput重定向到/ dev / null的测试一致

然后我将java代码更改为缓冲编写器，IDE内部的时间缩短为5秒。

import java.io.BufferedWriter;
import java.io.FileOutputStream;
import java.io.OutputStreamWriter;

public class Test {

    public static void main(String []args) throws Exception{

        double fizzes = 0;
        double buzzes = 0;
        double fizzbuzzes = 0;
        double normals = 0;
        double iterations = 1000000;


        BufferedWriter sout = new BufferedWriter(new OutputStreamWriter(new
                FileOutputStream(java.io.FileDescriptor.out),"ASCII"), 1024);

        for (int x = 0; x < iterations; x++) {


            int index = 0;
            StringBuilder output = new StringBuilder();

            if (x % 3 == 0) {

                output.append("Fizz");
                index = 1;
            }

            if (x % 5 == 0) {

                output.append("Buzz");

                index = index + 2;
            }
            if (output.length()==0) { //x%5!=0 && x%3!=0
                sout.write(String.valueOf(x));
                sout.write("
");
                normals++;
            }
            else {
                sout.write(output.toString());
                sout.write("
");
            }


            if (index == 3) {
                fizzbuzzes++;

            }
            else if (index == 2) {
                buzzes++;
            }
            else if (index == 1) {
                fizzes++;
            }


        }

        System.out.println("Normals " + (normals/iterations*100) + "%"); //percent
        System.out.println("Fizzes " + (fizzes/iterations*100)+ "%");
        System.out.println("Buzzes " + (buzzes/iterations*100)+ "%");
        System.out.println("FizzBuzzes " +(fizzbuzzes/iterations*100)+ "%");


    }
}