Task: Linux CPU utilization
C++: #include <fstream>
#include <iostream>
#include <numeric>
#include <unistd.h>
#include <vector>

std::vector<size_t> get_cpu_times() {
    std::ifstream proc_stat("/proc/stat");
    proc_stat.ignore(5, ' '); 
    std::vector<size_t> times;
    for (size_t time; proc_stat >> time; times.push_back(time));
    return times;
}

bool get_cpu_times(size_t &idle_time, size_t &total_time) {
    const std::vector<size_t> cpu_times = get_cpu_times();
    if (cpu_times.size() < 4)
        return false;
    idle_time = cpu_times[3];
    total_time = std::accumulate(cpu_times.begin(), cpu_times.end(), 0);
    return true;
}

int main(int, char *[]) {
    size_t previous_idle_time=0, previous_total_time=0;
    for (size_t idle_time, total_time; get_cpu_times(idle_time, total_time); sleep(1)) {
        const float idle_time_delta = idle_time - previous_idle_time;
        const float total_time_delta = total_time - previous_total_time;
        const float utilization = 100.0 * (1.0 - idle_time_delta / total_time_delta);
        std::cout << utilization << '%' << std::endl;
        previous_idle_time = idle_time;
        previous_total_time = total_time;
    }
}

Java: import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.text.NumberFormat;
import java.time.Duration;
import java.util.Timer;
import java.util.TimerTask;


public class Main {

    public static void main(String[] args) throws FileNotFoundException {
        
        final var period = Duration.ofSeconds(1);
        new Timer().schedule(new CpuUtilizationTask(), 0, period.toMillis());
    }

    
    static class CpuUtilizationTask extends TimerTask {

        private final String STAT_FILE_HEADER = "cpu  ";
        private final NumberFormat percentFormatter;
        private final RandomAccessFile statPointer;
        long previousIdleTime = 0, previousTotalTime = 0;

        public CpuUtilizationTask() throws FileNotFoundException {
            this.percentFormatter = NumberFormat.getPercentInstance();
            percentFormatter.setMaximumFractionDigits(2);
            var statFile = new File("/proc/stat");
            
            this.statPointer = new RandomAccessFile(statFile, "r");
        }

        @Override
        public void run() {

            try {
                var values = statPointer.readLine()
                        .substring(STAT_FILE_HEADER.length())
                        .split(" ");

                
                var idleTime = Long.parseUnsignedLong(values[3]);
                var totalTime = 0L;
                for (String value : values) {
                    totalTime += Long.parseUnsignedLong(value);
                }

                var idleTimeDelta = idleTime - previousIdleTime;
                var totalTimeDelta = totalTime - previousTotalTime;
                var utilization = 1 - ((double) idleTimeDelta) / totalTimeDelta;

                
                System.out.println(percentFormatter.format(utilization));

                previousIdleTime = idleTime;
                previousTotalTime = totalTime;

                
                statPointer.seek(0);
            } catch (IOException ioException) {
                ioException.printStackTrace();
            }
        }

    }
}