code: public AppendObjectResponse appendObject(String bucketName, String key, String value, ObjectMetadata metadata) {
          try {
              return this.appendObject(bucketName, key, value.getBytes(DEFAULT_ENCODING), metadata);
          } catch (UnsupportedEncodingException e) {
              throw new BceClientException("Fail to get bytes.", e);
          }
      }
docs: Uploads the specified string and object metadata to Bos under the specified bucket and key name.
  
  @param bucketName The name of an existing bucket, to which you have Write permission.
  @param key The key under which to store the specified file.
  @param value The string containing the value to be uploaded to Bos.
  @param metadata Additional metadata instructing Bos how to handle the uploaded data
  (e.g. custom user metadata, hooks for specifying content type, etc.).
  @return An AppendObjectResponse object containing the information returned by Bos for the newly created object.