Adding user to yml file as Block mapping nested in a block sequence, using SnakeYAML Java

I've modified your code and got the expected result. Below is how the code look like:

public class YAMLSample {  final String fileName = "/tmp/rest.yml";  public void writeToYML() throws IOException {    log( "Write to YML" );    Map<String, Object> user = new HashMap<>();    user.put( "name", "user5" );    user.put( "type", "allow" );    user.put( "auth_key", "user5:user5" );    user.put( "kibana_access", "ro" );    user.put( "indices", new String[] { ".kibana*", "abc", "def" } );    Map<String, Object> user2 = new HashMap<>();    user2.put("name", "user2");    user2.put("type", "allow");    user2.put("auth_key", "user2:user2");    user2.put("kibana_access", "ro");    user2.put("indices", new String[] { ".kibana*", "abc", "def" });    List<Map<String, Object>> list = new ArrayList<>();    list.add(user);    list.add(user2);    Map<String, List<Map<String, Object>>> config = new HashMap<>();    config.put( "access_control_rules", list );    DumperOptions options = new DumperOptions();    options.setIndent( 6 );    options.setIndicatorIndent( 4 );    options.setDefaultFlowStyle(DumperOptions.FlowStyle.AUTO);    Yaml yaml = new Yaml(options);    FileWriter writer = new FileWriter(fileName, true);    yaml.dump( config, writer );    log( "DONE!" );  }  public static void main(String[] args) throws IOException {    new YAMLSample().writeToYML();  }  public void log(String str) {    System.out.println(str);  }}

Basically I added this two options to your Dumper
options.setIndicatorIndent(4); options.setDefaultFlowStyle(DumperOptions.FlowStyle.AUTO);

and updated from 5 to 6 the options.setIndent(6);