Java Jackson JsonNode 的get(i)方法疑惑

Question

        String str = "{\"data\":{\"birth_day\":7,\"birth_month\":6},\"errcode\":0,\"msg\":\"ok\",\"ret\":0}";

        ObjectMapper mapper = new ObjectMapper();
        JsonNode root = mapper.readTree(str);

        for (int i = 0; i < root.size() ; i++) {
            System.out.println(root.get(i));
        }

        for (Iterator<?> iter = root.fieldNames();iter.hasNext();){
            System.out.println(iter.next());
        }

// null
// null
// null
// null
// data
// errcode
// msg
// ret

为什么前面输出的结果都是Null？

Answer 1

public JsonNode get(int index)

Method for accessing value of the specified element of an array node. For other nodes, null is always returned.

For array nodes, index specifies exact location within array and allows for efficient iteration over child elements (underlying storage is guaranteed to be efficiently indexable, i.e. has random-access to elements). If index is less than 0, or equal-or-greater than node.size(), null is returned; no exception is thrown for any index.

参考

http://fasterxml.github.io/jackson-databind/javadoc/2.0.0/

http://fasterxml.github.io/jackson-databind/javadoc/2.0.0/com/fasterxml/jackson/databind/JsonNode.html#get(int)