Google Gson Interview Questions and Answers
Freshers / Beginner level questions & answers
Ques 1. What is Google-Gson?
Google Gson is a Java library that can be used to convert Java Objects into respective JSON format. In another way, it can used to convert the JSON into equivalent java objects. There are some other java libraries also capable of doing this conversion, but Gson stands among very few which do not require any pre-annotated java classes OR sourcecode of java classes in any way.
Gson also support the old java classes which had not support of generics in them for type information. It just work with these legacy classes smoothly.
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Ques 2. What are the two ways to create Gson objects?
Gson object can be created in two ways. First way gives you a quick Gson object ready for faster coding, while second way uses GsonBuilder to build a more sophisticated Gson object.
//First way to create a Gson object for faster coding
Gson gson = new Gson();
//Second way to create a Gson object using GsonBuilder
Gson gson = new GsonBuilder()
.disableHtmlEscaping()
.setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE)
.setPrettyPrinting()
.serializeNulls()
.create();
When using GsonBuilder, there are plenty of other useful options you can provide to Gson object.
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Ques 3. How to convert Java objects to JSON format?
To convert the java objects to JSON format, use toJson() method.
Employee employee = new Employee();
employee.setId(1);
employee.setFirstName("Arindam");
employee.setLastName("Ghosh");
employee.setRoles(Arrays.asList("FINANCE", "MANAGER"));
Gson gson = new Gson();
System.out.println(gson.toJson(employee));
Output:
{
"id":1,
"firstName":"Rob",
"lastName":"Bosch",
"roles":["FIINANCE","MANAGER"]
}
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Ques 4. How to convert JSON to Java Objects?
To convert the JSON to java object, use fromJson() method.
Gson gson = new Gson();
System.out.println(
gson.fromJson("{'id':1,'firstName':'Arindam','lastName':'Ghosh','roles':['FINANCE','MANAGER']}",
Employee.class));
Output:
Employee [id=1, firstName=Arindam, lastName=Ghosh, roles=[FINANCE, MANAGER]]
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Experienced / Expert level questions & answers
Ques 5. What is Instance Creator? Why and when do we require this?
In most of the cases, Gson library is smart enough to create instances even if any class does not provide default no-args constructor. But, if you found any problem using a class having no no-args constructor, you can use InstanceCreator support. You need to register the InstanceCreator of a java class type with Gson first before using it.
For example, Department.java does not have any default constructor.
public class Department
{
public Department(String deptName)
{
this.deptName = deptName;
}
private String deptName;
public String getDeptName()
{
return deptName;
}
public void setDeptName(String deptName)
{
this.deptName = deptName;
}
@Override
public String toString()
{
return "Department [deptName="+deptName+"]";
}
}
And our Employee class has reference of Department as:
public class Employee
{
private Integer id;
private String firstName;
private String lastName;
private List<String> roles;
private Department department; //Department reference
//Other setters and getters
}
To use Department class correctly, you need to register an InstanceCreator for Department.java as below:
class DepartmentInstanceCreator implements InstanceCreator<Department> {
public Department createInstance(Type type)
{
return new Department("None");
}
}
//Now use the above InstanceCreator as below
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(Department.class, new DepartmentInstanceCreator());
Gson gson = gsonBuilder.create();
System.out.println(
gson.fromJson("{'id':1,'firstName':'Arindam','lastName':'Ghosh','roles':['FINANCE','MANAGER'],'department':{'deptName':'Finance'}}",
Employee.class));
Output:
Employee [id=1, firstName=Arindam, lastName=Ghosh, roles=[FINANCE, MANAGER], department=Department [deptName=Finance]]
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Ques 6. How to custom Serialization and De-serialization in Gson?
Many times, we need to write/read the JSON values which are not default representation of java object. In that case, we need to write custom serializer and deserializer of that java type.
In our example, I am writing serializer and deserializer for java.util.Date class, which will help writing the Date format in "dd/MM/yyyy" format.
DateSerializer.java
import java.lang.reflect.Type;
import java.text.SimpleDateFormat;
import java.util.Date;
import com.google.gson.JsonElement;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
public class DateSerializer implements JsonSerializer<Date>
{
private static final SimpleDateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");
public JsonElement serialize(Date date, Type typeOfSrc, JsonSerializationContext context)
{
return new JsonPrimitive(dateFormat.format(date));
}
}
DateDeserializer.java
import java.lang.reflect.Type;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
public class DateDeserializer implements JsonDeserializer<Date>
{
private static final SimpleDateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");
public Date deserialize(JsonElement dateStr, Type typeOfSrc, JsonDeserializationContext context)
{
try
{
return dateFormat.parse(dateStr.getAsString());
}
catch (ParseException e)
{
e.printStackTrace();
}
return null;
}
}
Now you can register these serializer and deserializer with GsonBuilder as below:
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(Date.class, new DateSerializer());
gsonBuilder.registerTypeAdapter(Date.class, new DateDeserializer());
Complete example of serializer and deserializer is as below:
Employee employee = new Employee();
employee.setId(1);
employee.setFirstName("Arindam");
employee.setLastName("Ghosh");
employee.setRoles(Arrays.asList("FINANCE", "MANAGER"));
employee.setBirthDate(new Date());
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(Date.class, new DateSerializer());
gsonBuilder.registerTypeAdapter(Date.class, new DateDeserializer());
Gson gson = gsonBuilder.create();
//Convert to JSON
System.out.println(gson.toJson(employee));
//Convert to java objects
System.out.println(gson.fromJson("{'id':1,'firstName':'Arindam','lastName':'Ghosh','roles':['FINANCE','MANAGER'],'birthDate':'17/06/2014'}", Employee.class));
Output:
{
"id":1,
"firstName":"Bob",
"lastName":"Richardson",
"roles":["FINANCE","MANAGER"],
"birthDate":"17/06/2014"
}
Employee [id=1, firstName=Arindam, lastName=Ghosh, roles=[FINANCE, MANAGER], birthDate=Tue Jun 17 00:00:00 IST 2014]
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Ques 7. How to provide Pretty Printing for JSON Output Format?
The default JSON output that is provide by Gson is a compact JSON format. This means that there will not be any white-space in the output JSON structure. To generate a more readable and pretty looking JSON use setPrettyPrinting() in GsonBuilder.
Gson gson = new GsonBuilder().setPrettyPrinting().create();
String jsonOutput = gson.toJson(employee);
Output:
{
"id": 1,
"firstName": "Lokesh",
"lastName": "Gupta",
"roles": [
"ADMIN",
"MANAGER"
],
"birthDate": "17/06/2014"
}
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Ques 8. What is Versioning Support in Gson?
This is excellent feature you can use, if the class file you are working has been modified in different versions and fields has been annotated with @Since. All you need to do is to use setVersion() method of GsonBuilder.
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(Date.class, new DateSerializer());
gsonBuilder.registerTypeAdapter(Date.class, new DateDeserializer());
//Specify the version like this
gsonBuilder.setVersion(1.0);
Gson gson = gsonBuilder.create();
Fields added in various versions in Employee.java
public class Employee
{
@Since(1.0)
private Integer id;
private String firstName;
private String lastName;
@Since(1.1)
private List<String> roles;
@Since(1.2)
private Date birthDate;
//Setters and Getters
}
Now test the version feature:
//Using version 1.0 fields
gsonBuilder.setVersion(1.0);
Output:
{
"id":1,
"firstName":"Bob",
"lastName":"Richardson"
}
//Using version 1.1 fields
gsonBuilder.setVersion(1.1);
Output:
{
"id":1,
"firstName":"Bob",
"lastName":"Richardson",
"roles":["FINANCE","MANAGER"]
}
//Using version 1.2 fields
gsonBuilder.setVersion(1.2);
Output:
{
"id":1,
"firstName":"Bob",
"lastName":"Richardson",
"roles":["FINANCE","MANAGER"],
"birthDate":"17/06/2014"
}
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Most helpful rated by users:
- What is Google-Gson?
- How to convert Java objects to JSON format?
- What are the two ways to create Gson objects?
- What is Instance Creator? Why and when do we require this?
- How to custom Serialization and De-serialization in Gson?