Convert a JSON to a Map in Java

Need to convert a JSON to a Map in Java? First, you’ll need a library for JSON processing.

We will use the Jackson library for this example since it is one of the most used ones in Java. To see where to find Jackson jars, read Introduction to Java JSON. 

Let’s say we have a JSON like the following:

{
   "key1":"value1",
   "key2":"value2",
   "key3":"value3",
   "key4":"value4",
   "key5":"value5"
}

As you can see, the above JSON is pretty simple. It contains key-value pairs, just like the Map. To convert the above JSON to a Map, we need the ObjectMapper class and the TypeReference, since the Map is a generic type.

Converting a JSON to a Map in Java

Here is the program that parses JSON to a Map:

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;

class Test {

  public static void main(String[] args) throws IOException {
    ObjectMapper objectMapper = new ObjectMapper();

    // load the JSON file
    File file = new File("src/main/resources/json_file.json");

    JsonNode json = objectMapper.convertValue(file, JsonNode.class);

    String jsonString = objectMapper.writeValueAsString(json);

    Map<String, String> map = objectMapper.readValue(jsonString, new TypeReference<Map<String, String>>() {});

    System.out.println(map.get("key1"));
  }
}

{
   "user1":{
      "name":"Tom",
      "city":"London",
      "state":"UK"
   },
   "user2":{
      "name":"Megan",
      "city":"Tokyo",
      "state":"Japan"
   }
}

To convert it to a Map, we need the corresponding Java class (POJO).

Here is the complete program:

//POJO class
class User {

  private String name;
  private String city;
  private String state;

  //constructors, getters, setters and toString() method
}

class Test {

  public static void main(String[] args) throws IOException {
    ObjectMapper objectMapper = new ObjectMapper();

    // load the JSON file
    File file = new File("src/main/resources/json_file.json");

    JsonNode json = objectMapper.readValue(file, JsonNode.class);

    String jsonString = objectMapper.writeValueAsString(json);

    Map<String, User> map = objectMapper.readValue(jsonString, new TypeReference<Map<String, User>>() {});

    System.out.println(map.get("user1"));
  }
}