My handwritten test methods get overwritten. How do I avoid this?

Put all your handwritten code between markers. Always! If you want to write a test method by yourself, write it between the marker at the class level. The marker is already there. (The instance under test is defined within this marker as well.)

// JUnitDoclet begin class
  org.junitdoclet.demo.SampleClass sampleclass = null;
  
  // and now a handwritten test method
  public void testHandwritten() {
    // ...
  } 
// JUnitDoclet end class