code: public void setETag(String eTag) {
  		if (eTag != null) {
  			Assert.isTrue(eTag.startsWith("\"") || eTag.startsWith("W/"),
  					"Invalid eTag, does not start with W/ or \"");
  			Assert.isTrue(eTag.endsWith("\""), "Invalid eTag, does not end with \"");
  		}
  		set(ETAG, eTag);
  	}
docs: Set the (new) entity tag of the body, as specified by the {@code ETag} header.