### Question:
PropagatorStack implements Propagator { public Format format() { return format; } PropagatorStack(Format format); Format format(); PropagatorStack pushPropagator(Propagator propagator); SpanContext extract(C carrier); void inject(SpanContext context, C carrier);  }### Answer:
@Test public void testCtor_empty() { PropagatorStack propagatorStack = new PropagatorStack(Builtin.TEXT_MAP); assertTrue(propagatorStack.propagators.isEmpty()); assertEquals(propagatorStack.format(), Builtin.TEXT_MAP); }
@Test public void testCtor_customFormat() { Format format = new Format() {}; PropagatorStack propagatorStack= new PropagatorStack(format); assertTrue(propagatorStack.propagators.isEmpty()); assertEquals(propagatorStack.format(), format); }