Java 多线程同步：为什么我的代码运行结果不稳定？

import java.util.concurrent.executorservice;
import java.util.concurrent.executors;
import java.util.concurrent.timeunit;

public class multithreadtest {

    private static volatile int lt_num = 0;
    private static executorservice executor = executors.newfixedthreadpool(20);

    public static void main(string[] args) {
        for (int i = 0; i < 20; i++) {
            lt_num = i;
            executor.execute(() -> num(lt_num));
        }
        executor.shutdown();
        executor.awaittermination(1, timeunit.seconds);
    }

    public static void num(int lt_num) {
        try {
            thread.sleep(1);
        } catch (interruptedexception e) {
            e.printstacktrace();
        }
        system.out.println(num++);
    }
}

public static void main(String[] args) {
    for (int i = 0; i < 20; i++) {
        synchronized (MultiThreadTest.class) {
            lt_num = i;
            executor.execute(() -> num(lt_num));
        }
    }
}