Google GSON is JSON parser and also generator for java, which is widely use to convert object to/from Json. To learn more about Gson one can refer to Gson tutorial. GSON library is used to convert any object to Json and vice-versa. Here in this tutorial we’ll be discussing below popular examples to convert different object to/from Json.

1. Convert Object to/from Json.
2. Convert List of objects to/from Json.
3. Convert Map containing objects.

Simple POJO which will be used as an object in all examples.

public class Emp {
	private int id;
	private String firstName;
	private String lastName;
 
	public Emp() {}
 
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getFirstName() {
		return firstName;
	}
	public void setFirstName(String firstName) {
		this.firstName = firstName;
	}
	public String getLastName() {
		return lastName;
	}
	public void setLastName(String lastName) {
		this.lastName = lastName;
	}
 
	@Override
	public String toString() {
		StringBuilder sb = new StringBuilder();
		sb.append("Id : " + id);
		sb.append(", FirstName : " + firstName);
		sb.append(", LastName : " + lastName);
 
		return sb.toString();
	}
}

1) Convert Object to/from Json

• Example to convert Emp object to Json.

  Emp emp = new Emp();
  emp.setId(1);
  emp.setFirstName("Rahul");
  emp.setLastName("Anand");
  		
  Gson gson = new Gson();
  String jsonString = gson.toJson(emp);
  System.out.println(jsonString);

  Output

  {"id":1,"firstName":"Rahul","lastName":"Anand"}

• Example to convert Json to Emp object.

  String jsonString = "{\"id\":1,\"firstName\":\"Rahul\",\"lastName\":\"Anand\"}";
  Gson gson = new Gson();
  Emp emp = gson.fromJson(jsonString, Emp.class);
  System.out.println(emp);

  Output

  Id : 1, FirstName : Rahul, LastName : Anand

2) Convert List of objects to/from Json

• Example to convert List of Emp objects to Json.

  List<Emp> employees = new ArrayList<Emp>(); 
  Emp emp1 = new Emp();
  emp1.setId(1);
  emp1.setFirstName("Rahul");
  emp1.setLastName("Anand");
  		
  Emp emp2 = new Emp();
  emp2.setId(2);
  emp2.setFirstName("Daniel");
  emp2.setLastName("Anderson");
  		
  employees.add(emp1);
  employees.add(emp2);
  		
  Gson gson = new Gson();
  String jsonString = gson.toJson(employees);
  System.out.println(jsonString);

  Output

  [{"id":1,"firstName":"Rahul","lastName":"Anand"},{"id":2,"firstName":"Daniel","lastName":"Anderson"}]

• Example to convert Json to List of Emp objects.

  String jsonString = "[{\"id\":1,\"firstName\":\"Rahul\",\"lastName\":\"Anand\"},{\"id\":2,\"firstName\":\"Daniel\",\"lastName\":\"Anderson\"}]";
  Gson gson = new Gson();
  List<Emp> employees = gson.fromJson(jsonString, new TypeToken<List<Emp>>(){}.getType());
  System.out.println(employees);

  Json object in user readable format.

  [
    {
      "id": 1,
      "firstName": "Rahul",
      "lastName": "Anand"
    },
    {
      "id": 2,
      "firstName": "Daniel",
      "lastName": "Anderson"
    }
  ]
  

  Output

  [Id : 1, FirstName : Rahul, LastName : Anand, Id : 2, FirstName : Daniel, LastName : Anderson]

3) Convert Map containing objects.

• Example to convert Map containing Emp objects to Json.

  Map<Integer, Emp> employees = new HashMap<Integer, Emp>(); 
  Emp emp1 = new Emp();
  emp1.setId(1);
  emp1.setFirstName("Rahul");
  emp1.setLastName("Anand");
  		
  Emp emp2 = new Emp();
  emp2.setId(2);
  emp2.setFirstName("Daniel");
  emp2.setLastName("Anderson");
  		
  employees.put(1, emp1);
  employees.put(2, emp2);
  		
  Gson gson = new Gson();
  String jsonString = gson.toJson(employees);
  System.out.println(jsonString);

  Output

  {"1":{"id":1,"firstName":"Rahul","lastName":"Anand"},"2":{"id":2,"firstName":"Daniel","lastName":"Anderson"}}

• Example to convert Json to Map.

  String jsonString = "{\"1\":{\"id\":1,\"firstName\":\"Rahul\",\"lastName\":\"Anand\"},\"2\":{\"id\":2,\"firstName\":\"Daniel\",\"lastName\":\"Anderson\"}}";
  Gson gson = new Gson();
  Type type = new TypeToken<Map<Integer, Emp>>(){}.getType();
  Map<Integer, Emp> employees = gson.fromJson(jsonString, type);
  System.out.println(employees);

  Json String in user readable format

  {
    "1": {
      "id": 1,
      "firstName": "Rahul",
      "lastName": "Anand"
    },
    "2": {
      "id": 2,
      "firstName": "Daniel",
      "lastName": "Anderson"
    }
  }
  

  Output

  {1=Id : 1, FirstName : Rahul, Last Name : Anand, 2=Id : 2, FirstName : Daniel, Last Name : Anderson}

  Stay tuned for more tutorials and examples.