Fix and add the documentation for Built-in Serializers and Deserializers

UserGuide.md

@@ -378,12 +378,67 @@ This approach is practical only if the array appears as a top-level element or i
 
 ### Built-in Serializers and Deserializers
 
-Gson has built-in serializers and deserializers for commonly used classes whose default representation may be inappropriate, for instance
+Gson has built-in serializers and deserializers for commonly used classes whose default representation may be inappropriate.
+Gson provides built-in serializers and deserializers for basic Java types (e.g., primitive types, strings, arrays) as well as some commonly used classes like `Date`, `BigInteger`, `BigDecimal`, and more. When you use Gson to convert Java objects to JSON or JSON to Java objects, these built-in serializers and deserializers handle the conversion automatically for these basic types and classes.
 
-* `java.net.URL` to match it with strings like `"https://github.com/google/gson/"`
-* `java.net.URI` to match it with strings like `"/google/gson/"`
+For example, suppose you have a Java class `Person`:
 
-For many more, see the internal class [`TypeAdapters`](gson/src/main/java/com/google/gson/internal/bind/TypeAdapters.java).
+```java
+public class Person {
+private String name;
+private int age;
+// constructors, getters, setters, etc.
+}
+```
+You can serialize an instance of `Person` to JSON using Gson like this:
+
+```java
+Person person = new Person("John Doe", 30);
+Gson gson = new Gson();
+String json = gson.toJson(person);
+```
+The output JSON would be: `{"name":"John Doe","age":30}`.
+
+And you can deserialize the JSON back to a `Person` object like this:
+
+```java
+String json = "{\"name\":\"Jane Smith\",\"age\":25}";
+Person person = gson.fromJson(json, Person.class);
+```
+The `gson.fromJson()` method uses the built-in deserializers to convert the JSON back into a `Person` object.
+
+Gson also allows you to customize serialization and deserialization by providing your own custom serializers and deserializers for specific types if needed.
+
+Below is a list of some of the classes supported by Gson's built-in serializers and deserializers:
+
+* Primitive Types:
+    * `int`, `long`, `float`, `double`, `boolean`, `char`, `byte`, `short` and their wrapper types.
+* Arrays:
+    * Arrays of primitive types and their wrappers.
+    * Arrays of objects.
+* Collections:
+    * `java.util.Collection`: Collection of objects, and collection subtypes such as:
+      * `java.util.List`: List of objects.
+      * `java.util.Set`: Set of objects.
+      * `java.util.Map`: Map of key-value pairs.
+* Date and Time:
+    * java.util.Date: Serialized as a Unix timestamp (milliseconds since January 1, 1970, 00:00:00 GMT).
+    * java.sql.Date: Serialized as a string in the format "yyyy-MM-dd".
+    * java.sql.Time: Serialized as a string in the format "HH:mm:ss".
+    * java.sql.Timestamp: Serialized as a string in the format "yyyy-MM-dd HH:mm:ss".
+* BigInteger and BigDecimal:
+    * java.math.BigInteger: Serialized as a string.
+    * java.math.BigDecimal: Serialized as a string.
+* Enum Types:
+    * Enum constants are serialized using their name by default. You can also customize the serialization using the `@SerializedName` annotation on an enum constant.
+* Optional:
+    * java.util.Optional: Serializes the value if present; otherwise, it serializes as null.
+* Custom Objects:
+    * Gson can automatically serialize and deserialize custom Java objects using reflection. By default, it serializes the non-static, non-transient fields of an object. 
+
+It's important to note that Gson's built-in serializers and deserializers can handle nested objects and collections of objects as well. If a class is not supported by Gson's default behavior, you can provide custom serialization and deserialization logic using Gson's `TypeAdapter` class.
+
+For more of the built-in serializers and deserializers, see the internal class [`TypeAdapters`](gson/src/main/java/com/google/gson/internal/bind/TypeAdapters.java).
 
 You can also find source code for some commonly used classes such as JodaTime at [this page](https://sites.google.com/site/gson/gson-type-adapters-for-common-classes-1).