尝试在空对象引用上调用虚拟方法“int java.lang.字符串长度（）” - Android

Question

我是Android的新手，我一直在尝试使用PHP和MYSQL进行JSON解析，当我运行应用程序时收到此错误。

错误:

致命异常：主进程：com.example.admin。androidjsonparsing，PID:13399 java.lang.NullPointerException：尝试调用虚拟方法“int java.lang.String”。org.json.JSONTokener.nextCleanInternal（JSONToker.java:116）org.json.SONTOkener.netxtValue（JSONTokener.java:94）org.json.JSONArray.（JSONArray.java:92）org.jsson.JSONArray（MainActivity.java:22）在android.os.AsyncTask.finish（Async任务.java:695在android.app.ActivityThread.main（ActivityThread.java:6669）的android.os.Looper.loop（Looper.java:193）上的androids.os.Handler.dispatchMessage（Handler.java:106），java.lang.reflect.Method。在com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run（RuntimeInit.java:493）在com.anderoid.internall.os.ZygoteInit.main（Zygote Init.java:858）调用（本机方法）

这里是代码：

MainActivity.java

package com.example.admin.androidjsonparsing;

import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

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

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class MainActivity extends AppCompatActivity {

ListView listView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    listView = (ListView) findViewById(R.id.listView);
    getJSON("http://192.168.1.4/AndroidJSONParsing/getdata.php");
}


private void getJSON(final String urlWebService) {

    class GetJSON extends AsyncTask<Void, Void, String> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }


        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            Toast.makeText(getApplicationContext(), s, Toast.LENGTH_SHORT).show();
            try {
                loadIntoListView(s);
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }

        @Override
        protected String doInBackground(Void... voids) {
            try {
                URL url = new URL(urlWebService);
                HttpURLConnection con = (HttpURLConnection) url.openConnection();
                StringBuilder sb = new StringBuilder();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(con.getInputStream()));
                String json;
                while ((json = bufferedReader.readLine()) != null) {
                    sb.append(json + "\n");
                }
                return sb.toString().trim();
            } catch (Exception e) {
                return null;
            }
        }
    }
    GetJSON getJSON = new GetJSON();
    getJSON.execute();
}

private void loadIntoListView(String json) throws JSONException {
    JSONArray jsonArray = new JSONArray(json);
    String[] heroes = new String[jsonArray.length()];
    for (int i = 0; i < jsonArray.length(); i++) {
        JSONObject obj = jsonArray.getJSONObject(i);
        heroes[i] = obj.getString("name");
    }
    ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, heroes);
    listView.setAdapter(arrayAdapter);
}

}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.admin.androidjsonparsing.MainActivity">


<!-- this is our listview where we will display the fetched data -->
<ListView
    android:id="@+id/listView"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_alignParentTop="true" />

</RelativeLayout>

Answer 1

出现问题是因为您< code>String json不是有效的json，可能是因为它是< code>null或< code >格式错误的json，然后尝试以下操作:

if (s!=null && !TextUtils.isEmpty(s)) loadIntoListView(s);

private void loadIntoListView(String json) throws JSONException {
    JSONArray jsonArray = new JSONArray(json);
    try {
        for (int i = 0; i < jsonArray.length(); i++) {
            JSONObject obj = jsonArray.getJSONObject(i);
            heroes[i] = obj.getString("name");
        }
    } catch(JSONException e) {
        //handler json exception
    }
    ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, heroes);
    listView.setAdapter(arrayAdapter);
}

Answer 2

试试这个代码...

    @Override
protected void onPostExecute(String s) {
    super.onPostExecute(s);
    if (!TextUtils.isEmpty(s) && s!=null) {
        Toast.makeText(getApplicationContext(), s, Toast.LENGTH_SHORT).show();
        try {
            loadIntoListView(s);
        } catch (JSONException e) {
            e.printStackTrace();
        }
    }
}

Answer 3

检查空字符串。

       @Override
       protected void onPostExecute(String s) {
       super.onPostExecute(s);
       if(s!=null){

        Toast.makeText(getApplicationContext(),s,Toast.LENGTH_SHORT).show();
                try {
                      loadIntoListView(s);
                    } catch (JSONException e) {
                    e.printStackTrace();
                }
            }
    }