JSON in salesforce apex

 JSONGenerator Class some frequently used Methods:

Method             Description

getAsString       Returns the generated JSON content, and also this method closes the JSON generator if it isn’t closed already.

writeBooleanField Writes a field name and value pair using the specified field name and Boolean value.

writeDateField    Writes a field name and value pair using the specified field name and date value. The date value is written in the ISO-8601 format.

writeEndArray     Writes the ending marker of a JSON array (‘]’).

writeEndObject    Writes the ending marker of a JSON object (‘}’).

writeNullField    Writes a field name and value pair using the specified field name and the JSON null literal value.

writeNumberField  Writes a field name and value pair using the specified field name and decimal, double, integer, long value.

writeStartArray   Writes the starting marker of a JSON array (‘[‘).

writeStartObject  Writes the starting marker of a JSON object (‘{‘).

writeStringField  Writes a field name and value pair using the specified field name and string value.

{

"ContactList" : [ {

    "ID" : "0032v00002j3qBCAAY",

    "FirstName" : "Bhole",

    "LastName" : "Shankar",

    }, {

    "ID" : "0032v00002j3qBDAAY",

    "FirstName" : "Brajesh",

    "LastName" : "Jha",

    } ]

}

public class JSONContacts {

    public static void JSONContactsMethod (){

        List<Contact> conList = [SELECT ID, firstName, lastName FROM Contact];

        if(conList.size()>0){

            JSONGenerator jsonGen = JSON.createGenerator(true);

            jsonGen.writeStartObject();

            jsonGen.writeFieldName('ContactList');

            jsonGen.writeStartArray();

            for(Contact con : conList){

                jsonGen.writeStartObject();

                jsonGen.writeStringField('ID',con.Id);

                jsonGen.writeStringField('FirstName',con.firstName);

                jsonGen.writeStringField('LastName',con.lastName);

                jsonGen.writeEndObject();

            }

            jsonGen.writeEndArray();

            jsonGen.writeEndObject();

            String jsonData = jsonGen.getAsString();

            System.debug('Json Data = ' + jsonData);

        }

        

    }

}