“类的无参数构造函数不存在”，但它确实存在

我收到错误No-args constructor for class MwVolleyApi$Page does not exist. Register an InstanceCreator with Gson for this type to fix this problem。我已经阅读了此内容并找到了两种类型的解决方案，这两种解决方案似乎都不适合我：

1. https://github.com/google/gson/issues/135 https://github.com/google/gson/issues/135- 使用静态嵌套类（我做了）
2. 类 XXX 的无参数构造函数不存在 https://stackoverflow.com/questions/29548983/no-args-constructor-for-class-xxx-does-not-exist- 为类创建一个无参数构造函数（我做了）

我缺少什么？

完整错误消息：

10-08 17:07:53.790: E/Volley(31902): [4551] NetworkDispatcher.run: Unhandled exception java.lang.RuntimeException: No-args constructor for class fr.free.nrw.commons.upload.MwVolleyApi$Page does not exist. Register an InstanceCreator with Gson for this type to fix this problem.
10-08 17:07:53.790: E/Volley(31902): java.lang.RuntimeException: No-args constructor for class fr.free.nrw.commons.upload.MwVolleyApi$Page does not exist. Register an InstanceCreator with Gson for this type to fix this problem.
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.MappedObjectConstructor.constructWithNoArgConstructor(MappedObjectConstructor.java:64)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.MappedObjectConstructor.construct(MappedObjectConstructor.java:53)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonObjectDeserializationVisitor.constructTarget(JsonObjectDeserializationVisitor.java:41)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonDeserializationVisitor.getTarget(JsonDeserializationVisitor.java:56)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:101)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonDeserializationVisitor.visitChild(JsonDeserializationVisitor.java:107)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonDeserializationVisitor.visitChildAsObject(JsonDeserializationVisitor.java:95)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonArrayDeserializationVisitor.visitArray(JsonArrayDeserializationVisitor.java:73)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:109)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonDeserializationVisitor.visitChild(JsonDeserializationVisitor.java:107)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonDeserializationVisitor.visitChildAsArray(JsonDeserializationVisitor.java:102)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonObjectDeserializationVisitor.visitArrayField(JsonObjectDeserializationVisitor.java:81)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.ObjectNavigator.navigateClassFields(ObjectNavigator.java:154)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:123)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonDeserializationVisitor.visitChild(JsonDeserializationVisitor.java:107)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonDeserializationVisitor.visitChildAsObject(JsonDeserializationVisitor.java:95)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonObjectDeserializationVisitor.visitObjectField(JsonObjectDeserializationVisitor.java:62)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.ObjectNavigator.navigateClassFields(ObjectNavigator.java:156)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:123)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonDeserializationContextDefault.fromJsonObject(JsonDeserializationContextDefault.java:73)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.JsonDeserializationContextDefault.deserialize(JsonDeserializationContextDefault.java:51)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.Gson.fromJson(Gson.java:495)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.Gson.fromJson(Gson.java:444)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.Gson.fromJson(Gson.java:396)
10-08 17:07:53.790: E/Volley(31902):    at com.google.gson.Gson.fromJson(Gson.java:372)
10-08 17:07:53.790: E/Volley(31902):    at fr.free.nrw.commons.upload.MwVolleyApi$QueryRequest.parseNetworkResponse(MwVolleyApi.java:140)
10-08 17:07:53.790: E/Volley(31902):    at com.android.volley.NetworkDispatcher.run(NetworkDispatcher.java:123)
10-08 17:07:53.790: E/fr.free.nrw.commons.upload.MwVolleyApi$LogResponseErrorListener(31902): com.android.volley.VolleyError: java.lang.RuntimeException: No-args constructor for class fr.free.nrw.commons.upload.MwVolleyApi$Page does not exist. Register an InstanceCreator with Gson for this type to fix this problem.

相关代码：

private static class QueryRequest extends JsonRequest<QueryResponse> {
    private static final String TAG = QueryRequest.class.getName();

    public QueryRequest(String url,
                        Response.Listener<QueryResponse> listener,
                        Response.ErrorListener errorListener) {
        super(Request.Method.GET, url, null, listener, errorListener);
    }

    @Override
    protected Response<QueryResponse> parseNetworkResponse(NetworkResponse response) {
        String json = parseString(response);
        QueryResponse queryResponse = GSON.fromJson(json, QueryResponse.class);
        return Response.success(queryResponse, cacheEntry(response));
    }

    private Cache.Entry cacheEntry(NetworkResponse response) {
        return HttpHeaderParser.parseCacheHeaders(response);
    }

    private String parseString(NetworkResponse response) {
        try {
            return new String(response.data, HttpHeaderParser.parseCharset(response.headers));
        } catch (UnsupportedEncodingException e) {
            return new String(response.data);
        }
    }
}


private static class QueryResponse {
    private Query query = new Query();

    private String printSet() {
        if (categorySet == null || categorySet.isEmpty()) {
            GpsCatExists.setGpsCatExists(false);
            Log.d(TAG, "gpsCatExists=" + GpsCatExists.getGpsCatExists());
            return "No collection of categories";
        } else {
            GpsCatExists.setGpsCatExists(true);
            Log.d(TAG, "gpsCatExists=" + GpsCatExists.getGpsCatExists());
            return "CATEGORIES FOUND" + categorySet.toString();
        }

    }

    @Override
    public String toString() {
        if (query != null) {
            return "query=" + query.toString() + "\n" + printSet();
        } else {
            return "No pages found";
        }
    }
}

private static class Query {
    private Page [] pages;

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder("pages=" + "\n");
        if (pages != null) {
            for (Page page : pages) {
                builder.append(page.toString());
                builder.append("\n");
            }
            builder.replace(builder.length() - 1, builder.length(), "");
            return builder.toString();
        } else {
            return "No pages found";
        }
    }
}

private static class Page {
    private int pageid;
    private int ns;
    private String title;
    private Category[] categories;
    private Category category;

    public Page() {
    }

    @Override
    public String toString() {

        StringBuilder builder = new StringBuilder("PAGEID=" + pageid + " ns=" + ns + " title=" + title + "\n" + " CATEGORIES= ");

        if (categories == null || categories.length == 0) {
            builder.append("no categories exist\n");
        } else {
            for (Category category : categories) {
                builder.append(category.toString());
                builder.append("\n");
                if (category != null) {
                    String categoryString = category.toString().replace("Category:", "");
                    categorySet.add(categoryString);
                }
            }
        }

        categoryList = new ArrayList<String>(categorySet);
        builder.replace(builder.length() - 1, builder.length(), "");
        return builder.toString();
    }
}

private static class Category {
    private String title;

    @Override
    public String toString() {
        return title;
    }
}

Gson 无法实例化私有嵌套类。做你的Page公共类：

public static class Page {...}