当main方法中声明了2个object时，代码是同时编译的吗？

Question

I am here investigating multi-thread operation. But I am currently feeling doubt about the main method execution sequence. Please explain to me these, the question is tagged below.

This is a simple program I am investigating

public class HelloWorld implements Runnable  {
    private Thread t;
    private String threadname;

    HelloWorld(String name){
        threadname= name;
        System.out.println("Create " +threadname);
    }

    public void run() {
        System.out.println("Running " +threadname);
        try {
            for(int i=4; i>0;i--) {
                System.out.println("Thread" +threadname +", "+i);
                Thread.sleep(50);
            }
        }catch(InterruptedException e) {
            System.out.println("Thread" +threadname +"interrupted ");
        }
        System.out.println("Thread" +threadname +"exiting ");
    }

    public void start() {
        System.out.println("Starting " +threadname);
        if(t==null)
        {
            t=new Thread(this, threadname);
            t.start();
        }
    }

    public static void main(String[] args) {
        HelloWorld obj1= new HelloWorld ("Thread-1");
        obj1.start();

        HelloWorld obj2= new HelloWorld ("Thread-2");
        obj2.start();
    }
}

actual result

Create Thread-1
Starting Thread-1
Create Thread-2
Starting Thread-2
Running Thread-1
ThreadThread-1, 4
Running Thread-2
ThreadThread-2, 4
ThreadThread-2, 3
ThreadThread-1, 3
ThreadThread-1, 2
ThreadThread-2, 2
ThreadThread-1, 1
ThreadThread-2, 1
ThreadThread-2exiting 
ThreadThread-1exiting 

My question:

Create Thread-1
Starting Thread-1
Create Thread-2(why here will switch from 1 to 2)
Starting Thread-2
Running Thread-1
ThreadThread-1, 4
Running Thread-2(At here, I understand the thread is switch when Thread.sleep(50) is being executed;)
ThreadThread-2, 4
ThreadThread-2, 3
ThreadThread-1, 3
ThreadThread-1, 2
ThreadThread-2, 2
ThreadThread-1, 1
ThreadThread-2, 1
ThreadThread-2exiting 
ThreadThread-1exiting 

Question:why here will switch from 1 to 2? Is the 2 objects at the main method run simultaneously?

Answer 1

This is because you are using multi threading and multi threading provides parallel execution.

In main method you have started two child threads obj1.start() and obj2.start(). Till obj1.start() there is only one child thread (main thread being the parent). But when you start second child thread obj2.start(), now there are two child threads which are being executed parallel that is why switching happened.Both of the thread will have separate path of execution and will run in parallel.

Also, switching is happening because of the thread scheduling algorithm (Round Robin, etc.) being used by CPU.