从Map Java 8中的Map获取字符串

Question

What do you think is the best way to find a value in a map inside another map.

    Map <String, String> map1 = new HashMap<>();

    map1.put("map1|1", "1.1");
    map1.put("map1|2", "1.2");
    map1.put("map1|3", "1.3");
    map1.put("map1|4", "1.4");

    Map <String, String> map2 = new HashMap<>();

    map2.put("map2|1", "2.1");
    map2.put("map2|2", "2.2");
    map2.put("map2|3", "2.3");
    map2.put("map2|4", "2.4");


    Map<String, Map> mapOfMaps = new HashMap<>();

    mapOfMaps.put("MAP|map1", map1);
    mapOfMaps.put("MAP|map2", map2);

Now if I need the value of "MAP|map2" (inside mapOfMaps) and "map2|3" (inside map2) will be "2.3"

I tried to do something like:

System.out.println("x="+getValue(mapOfMaps,"MAP|map2", "map2|4"));

public static String getValue (Map<String, Map> map,String mapfind, String val) {

     Map<Object, Object> mp = map.entrySet().stream()
                .filter(x -> x.getKey().equals(mapfind))
                .collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));

     System.out.println("--------"+mp);

     return (String) mp.get(val);
 }

but the result is:

--------{MAP|map2={map2|1=2.1, map2|4=2.4, map2|2=2.2, map2|3=2.3}}
x=null

Can you help me with some ideas?

Answer 1

Instead of declaring mapOfMaps by its raw type it should be defined as

Map<String, Map<String, String>> mapOfMaps = new HashMap<>();

The corresponding getValue method would look like this:

  public static String getValue(Map<String, Map<String, String>> mapOfMaps, String mapfind, String val) {
    Map<String, String> innerMap = mapOfMaps.get(mapfind);
    return innerMap != null ?
      innerMap.get(val) :
      null;
  }

Using Optional we can write it as follows:

  public static String getValue(Map<String, Map<String, String>> mapOfMaps, String mapfind, String val) {
    return Optional.ofNullable(mapOfMaps.get(mapfind))
      .map(m -> m.get(val))
      .orElse(null);
  }

If we kept mapOfMaps declared by its raw type we would get in the first version of getValue a type safety warning about an unchecked conversion and in the second version we would need explicitly cast the result to String . Since we use mapOfMaps only to map String keys to String values we should declare it accordingly.

---

Further reading: What is a raw type and why shouldn't we use it?

Answer 2

I think the easiest way to get your desired output would be to use map.get(mapfind).get(val) . But if you want to achieve it using your existing code, you could call values() on the collected map and call filter to get a second level filter. Below is the code snippet of your modified method

public static String getValue(Map<String, Map> map, String mapfind, String val) {
    Map mp = map.entrySet().stream().filter(x -> x.getKey().equals(mapfind))
            .collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()))

            .values().stream().filter(y -> y.containsKey(val)).findAny().orElse(null);

    System.out.println("--------" + mp);
    if (mp == null)
        return "";
    return (String) mp.get(val);
}

Answer 3

public static String getValue (Map<String, Map> map,String mapfind, String val) {
    Map childMap = map.get(mapfind);
    if (childMap == null) {
        return null;
    }
    return childMap.containsKey(val) ? childMap.get(val).toString() : null;
}