Update api.md

Author: taowen

api.md

@@ -289,46 +289,35 @@ List<Integer> val = iter.read(new TypeLiteral<ArrayList<Integer>>(){});
 
 The type literal instance might be reused globally, which is thread-safe.
 
-## Bind callback
-
-The downside of binding is there is always exception. We want the field to be string, but some input put the field as int. Being able to customize the binding is crucial. The answer is callback. Add callback hook in the binding process, then we can use iterator api to take back the control.
-
-Register type decoder to parse element of specific type to use your callback
-
-```java
-JsonIterator.registerTypeDecoder(Date.class, new Decoder() {
-    @Override
-    public Object decode(JsonIterator iter) throws IOException {
-	return new Date(iter.readLong());
-    }
-});
-JsonIterator iter = JsonIterator.parse("1481365190000");
-Date date = iter.read(Date.class);
-assertEquals(1481365190000L, date.getTime());
-JsonIterator.clearDecoders();
-```
+## Bind to existing object
 
-Register field decoder to special handle the fields chosen
+Java version also support bind to existing object
 
 ```java
-public class CustomizedObject {
-    public String field2;
+public static class TestObj1 {
     public String field1;
+    public String field2;
 }
 
-JsonIterator.registerFieldDecoder(CustomizedObject.class, "field1", new Decoder(){
+TestObj1 testObj = new TestObj1();
+testObj.field2 = "world";
+JsonIterator iter = JsonIterator.parse("{ 'field1' : 'hello' }".replace('\'', '"'));
+iter.read(testObj);
 
-    @Override
-    public Object decode(JsonIterator iter) throws IOException {
-	return Integer.toString(iter.readInt());
-    }
-});
-JsonIterator iter = JsonIterator.parse("{'field1': 100}".replace('\'', '"'));
-CustomizedObject myObject = iter.read(CustomizedObject.class);
-assertEquals("100", myObject.field1);
+System.out.println(testObj.field1); // "hello"
+System.out.println(testObj.field2); // "world"
 ```
 
-same api available in go version.
+Object and map will be reused as it is. Array will not be reused. Collection will be reused, but with elements cleared.
+
+## Bind callback
+
+The downside of binding is there is always exception. We want the field to be string, but some input put the field as int. Being able to customize the binding is crucial. The answer is callback. Add callback hook in the binding process, then we can use iterator api to take back the control.
+
+The callback itself is too complicated here, please checkout extension guides:
+
+* Java bind-api extension guide: http://jsoniter.com/java-extension.html
+* Go bind-api extension guide: http://jsoniter.com/go-extension.html
 
 # Any API