Question

以下是我的pojo课程，

public class LocalTrack {
    private long id;
    private String title;
    private String artist;
    private String album;
    private String path;
    private long duration;
    private String index;

  //constructor

    //getter setters for above fields
}

我使用以下代码检索先前保存的json响应。

  Map<String, ?> all = tinydb.getAll();
            Iterator       it  = all.entrySet().iterator();
            while (it.hasNext()) {
                Map.Entry pairs = (Map.Entry) it.next();
                LocalTrack localTrack = gson.fromJson(pairs.getValue().toString(), LocalTrack.class);
            }

pairs.getValue（）有以下json。

{"album":"http://MyMp3Song.Com","artist":"04 - Thai Mannai Vanakka","index":"#","path":"/storage/sdcard1/Download/04 - Thai Mannai Vanakkam(MyMp3Song.Com).mp3","title":"04 - Thai Mannai Vanakkam(MyMp3Song.Com)","duration":369424,"id":32004}

当我说gson.fromJson(pairs.getValue().toString(), LocalTrack.class)时，它会返回以下错误，

java.lang.NullPointerException：尝试调用虚方法 'java.lang.Object com.google.gson.Gson.fromJson（java.lang.String， java.lang.Class）'对空对象引用在 android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2364）

为什么会这样？

Answer 1

＆＃39; gson＆＃39;对象需要初始化。这是一个链接.. mkyong on how to use gson

Gson gson = new Gson();
Map<String, ?> all = tinydb.getAll();
            Iterator       it  = all.entrySet().iterator();
            while (it.hasNext()) {
                Map.Entry pairs = (Map.Entry) it.next();
                LocalTrack localTrack = gson.fromJson(pairs.getValue().toString(), LocalTrack.class);
            }

通过gson反序列化对象会导致空指针异常

1 个答案: