多线程场景下，为什么主程序中的变量值没有更新？

package threadTest;

public class Main {

    public static void main(String[] args) throws InterruptedException {
        System.out.println("=======1========");
        Values va = new Values();
        va.setA(2);
        System.out.println("mainThread1: " + va.getA());

        System.out.println("=======2========");
        ThreadDemo td = new ThreadDemo(va);
        td.start();
        td.join();
        
        System.out.println("=======3=========");
        System.out.println("mainThread2: " + va.getA());
    }

}

class ThreadDemo extends Thread {

    private Values va; // 接收主程序的va对象

    public ThreadDemo(Values va) {
        this.va = va;
    }

    @Override
    public void run() {
        va.setA(10); // 修改传入的va对象
    }

}

class Values {

    private int a;

    public void setA(int a) {
        this.a = a;
    }

    public int getA() {
        return a;
    }

}