关于Java当中继承的疑问 报错

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这是我运行的Test.java文件的源码，Sub类继承的是Main类，MyThreadA和MyThreadB都是线程类

package service;

public class Test {

public static void main(String[] args) {
    // TODO Auto-generated method stub
    Sub subRef = new Sub();
    MyThreadA a = new MyThreadA(subRef);
    a.setName("A");
    a.start();
    MyThreadB b = new MyThreadB(subRef);
    b.setName("B");
    b.start();
}

}

如下是控制台的运行结果，我的疑问是为什么Main类里面的serviceMethod()方法也被调用了？线程MyThreadA和线程MyThreadB的run()方法不是只是调用了Sub类里面的serviceMethod()方法吗？

int sub 下一步 sleep begin threadName=A time=1521894980829
int sub 下一步 sleep begin threadName=B time=1521894980829
int sub 下一步 sleep end threadName=A time=1521894985834
int sub 下一步 sleep end threadName=B time=1521894985834
int main 下一步 sleep begin threadName=A time=1521894985835
int main 下一步 sleep end threadName=A time=1521894990838
int main 下一步 sleep begin threadName=B time=1521894990838
int main 下一步 sleep end threadName=B time=1521894995840

如下是MyThreadA类

package service;

public class MyThreadA extends Thread {
    
    private Sub sub;

    public MyThreadA(Sub sub) {
        super();
        this.sub = sub;
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        sub.serviceMethod();
    }
    
    
}

如下是MyThreadB类

package service;

public class MyThreadB extends Thread {
    
    private Sub sub;

    public MyThreadB(Sub sub) {
        super();
        this.sub = sub;
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        sub.serviceMethod();
    }
    
    
    
}

如下是Main类

package service;

public class Main {
    synchronized public void serviceMethod() {
        try {
            System.out.println("int main 下一步 sleep begin threadName="
                    + Thread.currentThread().getName() + " time="
                    + System.currentTimeMillis());
            Thread.sleep(5000);
            System.out.println("int main 下一步 sleep end threadName="
                    + Thread.currentThread().getName() + " time="
                    + System.currentTimeMillis());
        } catch (Exception e) {
            // TODO: handle exception
            e.printStackTrace();
        }
    }
}

如下是Sub类，继承Main类

package service;

public class Sub extends Main {
    @Override
    public void serviceMethod() {
        try {
            System.out.println("int sub 下一步 sleep begin threadName="
                    + Thread.currentThread().getName() + " time="
                    + System.currentTimeMillis());
            Thread.sleep(5000);
            System.out.println("int sub 下一步 sleep end threadName="
                    + Thread.currentThread().getName() + " time="
                    + System.currentTimeMillis());
            super.serviceMethod();
        } catch (Exception e) {
            // TODO: handle exception
            e.printStackTrace();
        }
    }
}