Question

I am confused with the snippet from this code:

void stopTestThread() {

    // thread should cooperatively shutdown on the next iteration, because field is now null
    Thread testThread = m_logTestThread;
    m_logTestThread = null;
    if (testThread != null) {
      testThread.interrupt();
      try {testThread.join();} catch (InterruptedException e) {}
    }
  }

Does that mean testThread and m_logTestThread are different instances but point to the same object in the memory, so they are the same thread?

If so, what the purpose is of if (testThread != null) ?

Answer 1

Does that mean testThread and m_logTestThread are different instances
but point to the same object in the memory, so they are the same
thread?

This is partially true. Actually testThread and m_logTestThread are two different references not instances. And both the reference are pointing to the same Thread object. So, just making a reference m_logTestThread point to null does not make the testThread reference also point to null.

You can also see it in practice by a simple example: –

String str = "abc";
String strCopy = str;  // strCopy now points to "abc"
str = null;  // Nullify the `str` reference

System.out.println(strCopy.length()); // Will print 3, as strCopy still points to "abc"

So, even if you set one of the reference to null, the other reference still points to the same Thread object. An object is not eligible for Garbage Collection until it has 0 reference pointing to it, or there is a circular reference.

See this link: – Circular Reference – wiki page to know what exactly is Circular Refeference.

what the purpose is of “if (testThread != null)” ?

Its simple. You can infer from the condition that, it is checking whether the testThread reference is pointing to a null object.
The null check is done so that, you don’t get a NPE inside the if-construct, where you are trying to interrupt the Thread pointed to by that reference. So if that reference is pointing to null, then you don’t have any thread associated with that reference to interrupt.