SnakeYaml problem

mapping.yaml

mapping:
    nature:
        wetland:
            buffer: [5.0]
            mappings:
                natural: [wetland, mud]
        meadow:
            buffer: [5.0]
            mappings:
                landuse: [grass, meadow]
                natural: [fell, grassland]
    landmark:
        castle:
            buffer: [-5.0, 30.0, 50.0]
            mappings:
                bridge: [drawbridge]
                historic: [castle]

Snakeyaml.java

import java.util.List;
import java.util.Map;

import org.yaml.snakeyaml.Yaml;
import org.yaml.snakeyaml.constructor.Constructor;


public class SnakeTest {
	public static class TagMapping {

		public static class Mapping {
			public List<Double> buffer;
			public Map<String, List<String>> mappings;
		}

		public Map<String, Map<String, Mapping>> mapping;

	}		

	public static TagMapping parse(String str) {

		Yaml yaml = new Yaml(new Constructor(TagMapping.class));
		TagMapping mapping = (TagMapping) yaml.load(str);
		return mapping;
	}
	
public  static void main(String... args)
{
	String testString = 
			        "mapping:\n"+
					"    nature:\n"+
					"        wetland:\n"+
					"            buffer: [5.0]\n"+
					"            mappings:\n"+
					"                natural: [wetland, mud]\n"+
					"        meadow:\n"+
					"            buffer: [5.0]\n"+
					"            mappings:\n"+
					"                landuse: [grass, meadow]\n"+
					"                natural: [fell, grassland]\n"+
					"    landmark:\n"+
					"        castle:\n"+
					"            buffer: [-5.0, 30.0, 50.0]\n"+
					"            mappings:\n"+
					"                bridge: [drawbridge]\n"+
					"                historic: [castle]";
	
TagMapping m = parse(testString);
System.out.println(m.mapping.get("nature").get("wetland").mappings.get("natural").get(0));
}
	}

TagMapping.java

	public static class TagMapping {

		public static class Mapping {
			public List<Double> buffer;
			public Map<String, List<String>> mappings;
		}

		public Map<String, Map<String, Mapping>> mapping;

	}