Java - 方法如何知道“this”指的是哪个对象？

Let's say we have a class

假设我们有一堂课

public class ThisTest {
    String username;

    ThisTest(String s) {
        this.username = s;
    }

    public void sayHello() {
        System.out.println("Hello " + this.username);
    }

    public static void main(String args[]) {
        ThisTest a = new ThisTest("Jack");

        ThisTest b = new ThisTest("George");

        a.sayHello();
        b.sayHello();
    }
}
public