Java – create JSON strings using gson

I'm finishing class,

public class Student {
    public int id;
    public String name;
    public int age;    
}

Now I want to create a new student,

//while create new student
Student stu = new Student();
stu.age = 25;
stu.name = "Guna";
System.out.println(new Gson().toJson(stu));

This gives me the following output,

{"id":0,"name":"Guna","age":25} //Here I want string without id,So this is wrong

So here I want string

{"name":"Guna","age":25}

If I want to edit old students

//While edit old student
Student stu2 = new Student();
stu2.id = 1002;
stu2.age = 25;
stu2.name = "Guna";
System.out.println(new Gson().toJson(stu2));

Now the output is

{"id":1002,"age":25} //Here I want the String with Id,So this is correct

How to use a field [at some time] to create a JSON string without a field [at a certain point in time]

Any help will be very considerable

thank you.

Solution

Better yet, use the @ expose annotation

public class Student {
    public int id;
    @Expose
    public String name;
    @Expose
    public int age;
}

And get the JSON string from the object using the following method

private String getJsonString(Student student) {
    // Before converting to GSON check value of id
    Gson gson = null;
    if (student.id == 0) {
        gson = new GsonBuilder()
        .excludeFieldsWithoutExposeAnnotation()
        .create();
    } else {
        gson = new Gson();
    }
    return gson.toJson(student);
}

If set to 0, it ignores the ID column, otherwise it returns a JSON string with fields