JSON Serialization and Deserialization

Map<String, Object> requestData = new Map<String, Object>{
    'accountName' => 'Riverbend Farms',
    'amount' => 5000
};

String jsonString = JSON.serialize(requestData);
System.debug(jsonString); // {"accountName":"Riverbend Farms","amount":5000}

1. Map<String, Object> requestData = new Map<String, Object>{ — Module 16's Map, this time holding mixed-type values as Object.
2. 'accountName' => 'Riverbend Farms', — a String value.
3. 'amount' => 5000 — an Integer value, in the same map.
4. }; — closes the map literal.
5. (blank line)
6. String jsonString = JSON.serialize(requestData); — converts the entire map into JSON text.
7. System.debug(jsonString); // {"accountName":"Riverbend Farms","amount":5000} — the resulting JSON string.

JSON.serialize converts an Apex Map (Module 16), List, or custom class into its JSON text representation — exactly what a real HTTP callout's request body typically needs to contain.

String responseBody = '{"status":"success","rate":45.50}';
Map<String, Object> parsed = (Map<String, Object>) JSON.deserializeUntyped(responseBody);

Decimal rate = (Decimal) parsed.get('rate');

1. String responseBody = '{"status":"success","rate":45.50}'; — raw JSON text, as if just returned from a callout.
2. Map<String, Object> parsed = (Map<String, Object>) JSON.deserializeUntyped(responseBody); — parses it into a generic map — every value is typed Object until cast.
3. (blank line)
4. Decimal rate = (Decimal) parsed.get('rate'); — Module 18's casting, needed to use the value as a specific type.

JSON.deserializeUntyped parses JSON text into nested Map/List structures without needing a matching Apex class defined in advance — flexible, but every value needs an explicit cast (Module 18's polymorphism-adjacent casting) to use it as a specific type.

public class ShippingRateResponse {
    public String status;
    public Decimal rate;
}

String responseBody = '{"status":"success","rate":45.50}';
ShippingRateResponse parsed = (ShippingRateResponse) JSON.deserialize(responseBody, ShippingRateResponse.class);

System.debug(parsed.rate); // 45.50, already a Decimal, no casting needed

1. public class ShippingRateResponse { — a plain class, shaped to match the JSON's fields.
2. public String status; — matches the JSON's "status" key by name.
3. public Decimal rate; — matches "rate".
4. } — closes the class.
5. (blank line)
6. String responseBody = '{"status":"success","rate":45.50}'; — the same raw JSON.
7. ShippingRateResponse parsed = (ShippingRateResponse) JSON.deserialize(responseBody, ShippingRateResponse.class); — JSON.deserialize, not deserializeUntyped, maps fields onto this specific class directly.
8. (blank line)
9. System.debug(parsed.rate); // 45.50, already a Decimal, no casting needed — the field is already the right type.

JSON.deserialize (not deserializeUntyped) maps JSON fields directly onto a matching Apex class's fields by name — this is Module 34's DTO/wrapper-class pattern in reverse: instead of Apex producing a shape for JSON to consume, JSON produces data mapped directly onto an Apex shape.