in Java, why does the code b = = (b = a) not always return true? in a multithreaded environment

a can be modified by multiple threads.
b initializes to b = a
when printing b = = (b = a), the result is not necessarily true.

however, isn"t the priority of the operator b = an and then b = = b?

the code is as follows:

public class Demo implements Runnable{

    public static double a = 0;

    @Override
    public void run() {
        a = Math.random()*100;
        double b = a;
        System.out.println(b == (b = a));
    }

     public static void main(String args[]) {

         for(int i=0;i<10;iPP) {
             new Thread(new Demo()).start();
         }
     }
    
}

print result:

false
true
true
true
false
true
false
true
true
true