检查 JSON 上是否存在字段

有时检查 JSON 上是否存在字段是有用的，以避免代码中存在某些字段。

要实现这一点，请使用 JSONObject#has(String) 或方法，如下例所示：

示例 JSON

{
   "name":"James"
}

Java 代码

String jsonStr = " { \"name\":\"James\" }";
JSONObject json = new JSONObject(jsonStr);
// Check if the field "name" is present
String name, surname;

// This will be true, since the field "name" is present on our JSON.
if (json.has("name")) {
    name = json.getString("name");
}
else {
    name = "John";
}
// This will be false, since our JSON doesn't have the field "surname".
if (json.has("surname")) {
    surname = json.getString("surname");
}
else {
    surname = "Doe";
}

// Here name == "James" and surname == "Doe".