Question

我想解析JSON对普通String的回复，但当我尝试返回jsonString时，我收到以下消息：

Incompatible types:
Required:
android.content.Loader
<java.lang.String>
Found:
java.lang.String

你能告诉我，我做错了什么吗？

package com.example.entropy.booklist;

import android.app.LoaderManager;
import android.content.Loader;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.widget.ListView;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;

import static android.R.attr.data;

/**
 * Created by entropy on 18/07/17.
 */

public class AllInOne extends AppCompatActivity implements LoaderManager.LoaderCallbacks<String> {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.all_in_one);
        getLoaderManager().initLoader(0, null, this);

    }

    String jsonStr ;

    @Override
    public Loader<String> onCreateLoader(int id, Bundle args) {
        HttpURLConnection urlConnection = null;
        BufferedReader reader = null;

        String line;
        try {
            URL url = new URL("https://earthquake.usgs.gov/fdsnws/event/1/query?format=geojson&starttime=2012-01-01&endtime=2012-12-01&minmagnitude=6");
            urlConnection = (HttpURLConnection) url.openConnection();
            urlConnection.setRequestMethod("GET");
            urlConnection.connect();
            InputStream inputStream = urlConnection.getInputStream();
            StringBuffer buffer = new StringBuffer();
            if (inputStream == null)
            {return null;}

            reader = new BufferedReader(new InputStreamReader(inputStream));
            while ((line = reader.readLine()) != null) buffer.append(line);
           if (buffer.length() == 0)
            {return null;}

            jsonStr = buffer.toString();



        } catch (IOException e) {
            return null;

        } finally {
            if (urlConnection != null) urlConnection.disconnect();
            if (reader != null) {
                try {
                    reader.close();

                } catch (final IOException e) {
                    Log.e("MainActivity", "Error closing stream", e);
                }
            }
        }
        return jsonStr;
    }

Answer 1

首先，您的错误消息绝对正确＆＃34;不兼容的类型：必需：android.content.Loader找到：java.lang.String＆＃34;因为您要返回String，但方法需要Loader<String>。

其次，你以错误的方式使用加载器。数据加载不应在onCreateLoader()。请查看此链接以获取正确的信息，请在继续执行任何实现之前阅读本文档。 https://developer.android.com/guide/components/loaders.html

另外，请查看同一文档中的AsyncTaskLoader。这将满足您从服务器加载数据的目的。

Answer 2

您应该按照签名建议的方法onCreateLoader返回一个Loader。因此，创建一个AsyncTaskLoader<String>，在其中完成您的工作（它应该在后台完成，因为您通过网络获取数据）。

你应该这样做。

@Override
    public Loader<String> onCreateLoader(int id, Bundle args) {
        return new AsyncTaskLoader<String>(this) {

            @Override
            protected String loadInBackground() {
                HttpURLConnection urlConnection = null;
                BufferedReader reader = null;

                String line;
                try {
                    URL url = new URL("https://earthquake.usgs.gov/fdsnws/event/1/query?format=geojson&starttime=2012-01-01&endtime=2012-12-01&minmagnitude=6");
                    urlConnection = (HttpURLConnection) url.openConnection();
                    urlConnection.setRequestMethod("GET");
                    urlConnection.connect();
                    InputStream inputStream = urlConnection.getInputStream();
                    StringBuffer buffer = new StringBuffer();
                    if (inputStream == null)
                    {return null;}

                    reader = new BufferedReader(new InputStreamReader(inputStream));
                    while ((line = reader.readLine()) != null) buffer.append(line);
                    if (buffer.length() == 0)
                    {return null;}

                    jsonStr = buffer.toString();



                } catch (IOException e) {
                    return null;

                } finally {
                    if (urlConnection != null) urlConnection.disconnect();
                    if (reader != null) {
                        try {
                            reader.close();

                        } catch (final IOException e) {
                            Log.e("MainActivity", "Error closing stream", e);
                        }
                    }
                }
                return jsonStr;
            }
        }
    }

Answer 3

谢谢大家。我看到了我做错了什么。感谢litelite的回答，我已经阅读了更多内容并设法修复了这个问题：

public class AllInOne extends AppCompatActivity implements LoaderManager.LoaderCallbacks<String> {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.all_in_one);
    getLoaderManager().initLoader(0, null, this);
    BookAdapter adapter = new BookAdapter(this, books);
    ListView listView = (ListView) findViewById(R.id.list);
    listView.setAdapter(adapter);

}

public static String jsonStr;

@Override
public Loader<String> onCreateLoader(int id, Bundle args) {
    return new GetResponse(this);
}


private static class GetResponse extends AsyncTaskLoader<String> {

    public GetResponse(Context context) {
        super(context);
    }

    @Override
    public String loadInBackground() {
        HttpURLConnection urlConnection = null;
        BufferedReader reader = null;

        String line;
        try {
            URL url = new URL("https://earthquake.usgs.gov/fdsnws/event/1/query?format=geojson&starttime=2012-01-01&endtime=2012-12-01&minmagnitude=6");
            urlConnection = (HttpURLConnection) url.openConnection();
            urlConnection.setRequestMethod("GET");
            urlConnection.connect();


            InputStream inputStream = urlConnection.getInputStream();
            StringBuffer buffer = new StringBuffer();
            if (inputStream == null) {
                return null;
            }

            reader = new BufferedReader(new InputStreamReader(inputStream));
            while ((line = reader.readLine()) != null) buffer.append(line);
            if (buffer.length() == 0) {
                return null;
            }
            jsonStr = buffer.toString();


        } catch (IOException e) {
            return null;

        } finally {
            if (urlConnection != null) urlConnection.disconnect();
            if (reader != null) {
                try {
                    reader.close();

                } catch (final IOException e) {
                    Log.e("MainActivity", "Error closing stream", e);
                }
            }
        }

        return jsonStr;

    }
}

Loader <string> onCreateLoader不返回String

3 个答案: