Update java-extension.md

Author: taowen

java-extension.md

@@ -44,3 +44,23 @@ when a `Date` class to read, the registered decoder will be called.
 JsonIterator iter = JsonIterator.parse("1481365190000");
 iter.read(Date.class); // Sat Dec 10 18:19:50 CST 2016
 ```
+
+Generic types can also be customized by `TypeLiteral`. Here is an example:
+
+```
+TypeLiteral<List<Integer>> listOfIntType = new TypeLiteral<List<Integer>>() {
+};
+JsonIterator.registerTypeDecoder(listOfIntType, new Decoder() {
+    @Override
+    public Object decode(JsonIterator iter) throws IOException {
+        ArrayList<Integer> list = new ArrayList<Integer>();
+        for (char c : iter.readString().toCharArray()) {
+            list.add(c - '0');
+        }
+        return list;
+    }
+});
+JsonIterator iter = JsonIterator.parse("\"1481365190000\"");
+List<Integer> listOfInt = iter.read(listOfIntType);
+System.out.println(listOfInt); // [1, 4, 8, 1, 3, 6, 5, 1, 9, 0, 0, 0, 0]
+```