TraceableHttpClientRequest implements HttpClientRequest { @Override public void addHeader(String header, String value) { Collection<String> existingValues = request.headers().get(header); if (existingValues == null) { existingValues = new ArrayList<>(); } existingValues.add(value); request.headers().put(header, existingValues); } TraceableHttpClientRequest(Request request); @Override void addHeader(String header, String value); @Override URI getUri(); @Override String getHttpMethod();  }
@Test public void testAddHeader() { httpRequest.addHeader("testK", "testV"); assertTrue(request.headers().get("testK").contains("testV")); }