Converting Object to Map: Cast vs ObjectMapper

6324 :

What is the difference between below two methods of converting Object to Map? And which method is better, assuming both methods work for converting an Object to a Map?

Cast:

Object o = some object;
Map<String, Object> map = (Map<String, Object>) o;

ObjectMapper:

Object o = some object;
Map<String, Object> map = new ObjectMapper().readValue((String) o, new TypeReference<Map<String, Object>>() {});

xingbin :

That's depends what the input is.

• (Map<String, Object>) o is used for casting conversion, so the runtime type of o must be Map, otherwise ClassCastException will be thrown:

  Object o = new HashMap<>();
  Map<String, Object> map = (Map<String, Object>) o; // ok
  
  Object o = new ArrayList<>();
  Map<String, Object> map = (Map<String, Object>) o; //ClassCastException
  
  Object o = new String();
  Map<String, Object> map = (Map<String, Object>) o; //ClassCastException
  

• ObjectMapper().readValue(String content, JavaType valueType):

  Method to deserialize JSON content from given JSON content String.

  which means the input should be a String in valid json format. For example:

  Object input = "{\"key1\":{},\"key2\":{}}";
  Map<String, Object> map = new ObjectMapper().readValue((String) input, new TypeReference<Map<String, Object>>() {});
  
  System.out.println(map.size()); // 2