Schema model: update type and nullable

Nullable has been removed, type is now permitted to be an array of
permitted types and may include null.

Attempt to keep the old methods as far as they don't clash with the new
ones and can still be defined in a way that makes sense.

api/src/main/java/org/eclipse/microprofile/openapi/models/media/Schema.java

@@ -47,7 +47,8 @@ public interface Schema extends Extensible<Schema>, Constructible, Reference<Sch
      * The values allowed for the in field.
      */
     enum SchemaType {
-        INTEGER("integer"), NUMBER("number"), BOOLEAN("boolean"), STRING("string"), OBJECT("object"), ARRAY("array");
+        INTEGER("integer"), NUMBER("number"), BOOLEAN("boolean"), STRING("string"), OBJECT("object"), ARRAY(
+                "array"), NULL("null");
 
         private final String value;
 
@@ -596,27 +597,72 @@ default Schema required(List<String> required) {
     void removeRequired(String required);
 
     /**
-     * Returns the type property from this Schema.
+     * Returns the type property of this Schema instance. Defines the types which are valid.
      *
-     * @return the type used in this Schema. Default value <b>must</b> be <code>null</code>
-     **/
-    SchemaType getType();
+     * @return a copy List (potentially immutable) of the allowed types
+     */
+    List<SchemaType> getType();
+
+    /**
+     * Sets the type property of this Schema instance. Defines the types which are valid.
+     *
+     * @param types
+     *            a list of the allowed types
+     */
+    void setType(List<SchemaType> types);
+
+    /**
+     * Sets the type property of this Schema instance. Defines the types which are valid.
+     *
+     * @param types
+     *            a list of the allowed types
+     * @return current Schema instance
+     * @since "4.0"
+     */
+    default Schema type(List<SchemaType> types) {
+        setType(types);
+        return this;
+    }
+
+    /**
+     * Adds a type to the type list.
+     *
+     * @param type
+     *            the type to add to the type list
+     * @return current Schema instance
+     * @since "4.0"
+     */
+    Schema addType(SchemaType type);
+
+    /**
+     * Removes a type from the type list.
+     *
+     * @param type
+     *            the type to remove from the type list
+     * @since "4.0"
+     */
+    void removeType(SchemaType type);
 
     /**
-     * Sets the type used by this Schema to the string given.
+     * Sets the type property of this Schema instance to a single type.
      *
      * @param type
-     *            the type used by this Schema or <code>null</code> for reference schemas
+     *            the required type
+     * @since "4.0"
+     * @deprecated use {@link #setType(List)}
      */
+    @Deprecated(since = "4.0")
     void setType(SchemaType type);
 
     /**
-     * Sets the type used by this Schema to the string given.
+     * Sets the type property of this Schema instance to a single type.
      *
      * @param type
-     *            the type used by this Schema or <code>null</code> for reference schemas
+     *            the required type
      * @return the current Schema instance
+     * @deprecated use {@link #setType(List)}
      */
+    @Deprecated(since = "4.0")
     default Schema type(SchemaType type) {
         setType(type);
         return this;
@@ -847,18 +893,23 @@ default Schema format(String format) {
     }
 
     /**
-     * Returns the nullable property from this Schema instance which indicates whether null is a valid value.
+     * Returns whether the type property allows the object to be {@code null}
      *
-     * @return the nullable property
-     **/
+     * @return whether null is allowed
+     * @deprecated use {@link #getType()} and check if the result contains {@link SchemaType#NULL}
+     */
+    @Deprecated(since = "4.0")
     Boolean getNullable();
 
     /**
-     * Sets the nullable property of this Schema instance. Specify true if this Schema will allow null values.
+     * Updates the type property to either permit or disallow {@code null}
      *
      * @param nullable
-     *            a boolean value indicating this Schema allows a null value.
+     *            a boolean value indicating whether this Schema allows a null value.
+     * @deprecated use {@link #setType(List)}, {@link #addType(SchemaType)}, or {@link #removeType(SchemaType)} to add
+     *             or remove {@link SchemaType#NULL}
      */
+    @Deprecated(since = "4.0")
     void setNullable(Boolean nullable);
 
     /**