Convert JSON to Map in Java

Java provides two very strong libraries to work with JSON data, i.e., JACKSON and Gson libraries. We often need to convert JSON responses into a map to work with the returned JSON data easily.

We can easily convert JSON data into a map because the JSON format is essentially a key-value pair grouping and the map also stores data in key-value pairs.

Let's understand how we can use both JACKSON and Gson libraries to convert JSON data into a Map. We also understand that how we can use both libraries to convert Map data into JSON.

Suppose, we have a Sample.json file in the system that contains the following data:

{  
  "Name" : "Donal",  
  "Mobile" : "89346724",  
  "Designation" : "Sr. Salesforce Developer",  
  "Pet" : "Dog",  
  "Address" : "AMERICA"  
}  

JACKSON Library

In order to convert JSON data into Java Map, we take help of JACKSON library. We add the following dependency in the POM.xml file to work with JACKSON library.

<dependencies>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.5.3</version>
    </dependency>
</dependencies>

Let's implement the logic of converting JSON data into a map using ObjectMapper, File and TypeReference classes.

JacksonConvertJSONToMap.java

 // import required classes and packages
package javaTpoint.JavaExample;
import java.io.File;	// for reading file data
import java.util.Map;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;
// create JacksonConvertJSONToMap class to convert JSON data into Java Map
public class JacksonConvertJSONToMap {
	// main() method start
    public static void main(String args[]) {
    	// create instance of the ObjectMapper class to map JSON data
    	ObjectMapper mapper = new ObjectMapper();
    	// create instance of the File class 
        File fileObj = new File("C:\\Users\\rastogi ji\\OneDrive\\Desktop\\Sample.json");
        // use try-catch block to convert JSON data into Map
        try {
        	// read JSON data from file using fileObj and map it using ObjectMapper and TypeReference classes
            Map<String, Object> userData = mapper.readValue(
            		fileObj, new TypeReference<Map<String, Object>>() {
            }); 
            // print all key-value pairs 
            System.out.println("Name : " + userData.get("Name"));
            System.out.println("Mobile : " + userData.get("Mobile"));
            System.out.println("Designation : " + userData.get("Designation"));
            System.out.println("Pet : " + userData.get("Pet"));
            System.out.println("Address : " + userData.get("Address")); 
        } catch (Exception e) {
        	// show error message
            e.printStackTrace();
        } 
    }
}

Output:

Let's take another example of the Jackson library to understand how we can convert a Java map into JSON because we often need to pass map data to API as JSON. So, in this example, we convert Map data into JSON and store it in a file.

JacksonConvertMapToJson.java

// import required classes and packages
package javaTpoint.JavaExample;

import java.io.File;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

import com.fasterxml.jackson.databind.ObjectMapper;

//create JacksonConvertMapToJSON class to convert Map data into JSON
public class JacksonConvertMapToJSON {
	
	// main() method start
    public static void main(String args[]) {
        
    	// create instance of the ObjectMapper class
    	ObjectMapper mapper = new ObjectMapper();
    	
    	// declare and initialize map (key is of type String and value is of type Object)
        Map<String, Object> userData = new HashMap<String, Object>();
        
        // declare variables and array to store user entered data
    	String name, price, model;
    	String colors[];
    	
    	// create an instance of the Scanner class
    	Scanner sc = new Scanner(System.in);
    	
    	// take inputs from the user and store them to the variables 
    	System.out.println("Enter the name of the car: ");
    	name = sc.nextLine();
    	
    	System.out.println("Enter the modal number of the car: ");
    	model = sc.nextLine();
    	
    	System.out.println("Enter the price of the car: ");
    	price = sc.nextLine();
    	
    	colors = new String[3];
    	colors[0] = "Red";
    	colors[1] = "Black";
    	colors[2] = "White";
    	
    	// close Scanner class object
		sc.close();
    	
    	// fill userData map
    	userData.put("Car", name);
    	userData.put("Price", price);
    	userData.put("Model", model);
    	userData.put("Colors", colors);
 
       // use try-catch block to convert Java map into JSON
        try {
        	
        	// use ObjectMapper class to convert Map data into JSON and write it into Sample.json file
            mapper.writeValue(new File("C:\\Users\\rastogi ji\\OneDrive\\Desktop\\Sample.json"), userData);
            System.out.println("Map data successfully written to the Sample.json file.");
        } catch (Exception e) {
        	// handle exception
            e.printStackTrace();
        } 
    }
}

Output:

Gson Library

Gson library is another library which we can use to convert JSON data into map or map data into JSON. In order to use Gson library, we need to add the following dependency in our POM.xml file.

<dependencies>
 	<dependency>
 		<groupId>com.google.code.gson</groupId>
 		<artifactId>gson</artifactId>
 		<version>2.8.3</version>
 	</dependency>
</dependencies>

GsonConvertJSONToMap.java

//import required classes and packages
package javaTpoint.JavaExample;
import java.util.HashMap;
import java.util.Map;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.io.IOException;
import java.nio.file.Files;  
import java.nio.file.Paths;  

//create GsonConvertJSONToMap class to convert JSON data into Java Map
public class GsonConvertJSONToMap {
	// main() method start
	public static void main(String args[]) {
		// create variable loc that store location of the Sample.json file 
		String loc = "C:\\Users\\rastogi ji\\OneDrive\\Desktop\\Sample.json";
		String result;
		try {
			// read byte data from the Sample.json file and convert it into String
			result = new String(Files.readAllBytes(Paths.get(loc)));
			// store string data into Map by using TypeToken class
	    	 Map<String, Object> userData = new Gson().fromJson(result, new TypeToken<HashMap<String, Object>>() {
	    		        }.getType()); 
	         // print all key-value pairs 
	         System.out.println("Name : " + userData.get("Name"));
	         System.out.println("Mobile : " + userData.get("Mobile"));
	         System.out.println("Designation : " + userData.get("Designation"));
	         System.out.println("Pet : " + userData.get("Pet"));
	         System.out.println("Address : " + userData.get("Address"));
		} catch (IOException e1) {
			// TODO Auto-generated catch block
			e1.printStackTrace();
		}
	}
}

Output:

Let's take another example of the Gson library to understand how to convert a Java map into JSON. Using the Gson library is a little different from the Jackson library.

GsonConvertMapToJson.java

//import required classes and packages
package javaTpoint.JavaExample;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import com.google.gson.Gson;
//create GsonConvertMapToJson class to convert Map data into JSON
public class GsonConvertMapToJson {
	// main() method start
	public static void main(String args[]) {
	 	// declare and initialize map(key is of type String and value is of type Object)
	     	Map<String, Object> userData = new HashMap<String, Object>();
	     	// declare variables and array to store user entered data
	 	String name, price, model;
	 	String colors[];
	 	// create an instance of the Scanner class
	 	Scanner sc = new Scanner(System.in);
	 	
	 	// take inputs from the user and store them to the variables 
	 	System.out.println("Enter the name of the car: ");
	 	name = sc.nextLine();
	 	
	 	System.out.println("Enter the modal number of the car: ");
	 	model = sc.nextLine();
	 	
	 	System.out.println("Enter the price of the car: ");
	 	price = sc.nextLine();
	 	
	 	colors = new String[3];
	 	colors[0] = "Red";
	 	colors[1] = "Black";
	 	colors[2] = "White";
	 	
	 	// close Scanner class object
			sc.close();
	 	
	 	// fill userData map
	 	userData.put("Car", name);
	 	userData.put("Price", price);
	 	userData.put("Model", model);
	 	userData.put("Colors", colors);
	 	
  	// use try-catch block to convert Java map into JSON
        		try (FileWriter file = new FileWriter("C:\\Users\\rastogi ji\\OneDrive\\Desktop\\Sample.json")) {
        			// create instance of the Gson 
        			Gson gsonObj = new Gson();
            			// convert userData map to json string
        			String jsonStr = gsonObj.toJson(userData);
            			// use write() of File to write json string into file
        			file.write(jsonStr); 
   			// use flush() method to flushes stream
            			file.flush();
           			System.out.println("Map data successfully written to the Sample.json file.");
 	} catch (IOException e) {
            			// error handling and exceptions 
   			e.printStackTrace();
      		}
  	}
}

Output: