我还是Android编程的新手,我一直在尝试制作一个简单的气象应用程序进行练习。当我输入有效的城市名称时,此代码可以正常工作,但是当名称无效时,我想使用toast
向用户显示错误的。但这导致崩溃,我尝试删除了toast
,但我的应用仍然崩溃。
这是代码:
public class MainActivity extends AppCompatActivity {
EditText cityName;
TextView resultTextView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
cityName = (EditText) findViewById(R.id.cityName);
resultTextView = (TextView) findViewById(R.id.resultTextView);
}
public void findWeather(View view)
{
Log.i("Button", "pressed");
InputMethodManager mgr = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);
mgr.hideSoftInputFromWindow(cityName.getWindowToken(), 0);
try {
String encodedCityName = URLEncoder.encode(cityName.getText().toString(), "UTF-8");
DownloadTask task = new DownloadTask();
task.execute("http://api.openweathermap.org/data/2.5/weather?q=" + encodedCityName +"&appid=***");
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
Toast.makeText(getApplicationContext(), "Could not find city", Toast.LENGTH_LONG);
}
}
public class DownloadTask extends AsyncTask<String, Void, String>
{
@Override
protected String doInBackground(String... strings) {
String result = "";
URL url;
HttpURLConnection urlConnection;
try {
url = new URL(strings[0]);
urlConnection = (HttpURLConnection) url.openConnection();
InputStream in = urlConnection.getInputStream();
InputStreamReader reader = new InputStreamReader(in);
int data = reader.read();
while(data != -1)
{
char current = (char) data;
result += current;
data = reader.read();
}
return result;
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
try{
String message = "";
JSONObject jsonObject = new JSONObject(s);
String weatherInfo = jsonObject.getString("weather");
Log.i("weather", weatherInfo);
JSONArray arr = new JSONArray(weatherInfo);
for(int i = 0; i < arr.length(); i++)
{
JSONObject jsonPart = arr.getJSONObject(i);
String main = "";
String description = "";
main = jsonPart.getString("main");
description = jsonPart.getString("description");
if (!main.isEmpty() && !description.isEmpty())
{
message += main + ": " + description + "\r\n";
}
}
if(!message.isEmpty())
{
resultTextView.setText(message);
}
else
{
Toast.makeText(getApplicationContext(), "Could not find city", Toast.LENGTH_LONG);
}
} catch (JSONException e) {
e.printStackTrace();
}
}
}
和错误消息:
12-22 22:11:51.204 4858-4858 / com.example.aiden.whatstheweather E / Android运行时:致命异常:主进程:com.example.aiden.whatstheweather,PID:4858
java.lang.NullPointerException:尝试在org.json.JSONTokener上的org.json.JSONTokener.nextCleanInternal(JSONTokener.java:116)上的null对象引用上调用虚拟方法'int java.lang.String.length()'。 org.json.JSONObject处的nextValue(JSONTokener.java:94).com.example.aiden.whatstheweather.MainActivity $ DownloadTask.onPostExecute处的org.json.JSONObject。(JSONObject.java:176)处的nextValue(JSONTokener.java:94) (MainActivity.java:102),位于com.example.aiden.whatstheweather.MainActivity $ DownloadTask.onPostExecute(MainActivity.java:65),位于android.os.AsyncTask.finish(AsyncTask.java:695),位于android.os.AsyncTask。 android.os.Looper.loop(Looper)处的android.os.AsyncTask $ InternalHandler.handleMessage(AsyncTask.java:712)处的-wrap1(未知源:0)android.os.Handler.dispatchMessage(Handler.java:105)处的.java:164),请访问android.app.ActivityThread。com.android.internal.os.Zygote $ MethodAndArgsCaller.run(Zygote.java:240)处java.lang.reflect.Method.invoke(本地方法)处的main(ActivityThread.java:6541) os.ZygoteInit.main(ZygoteInit.java:767)
如果发生任何异常,doInBackground()
将返回null
。如下处理。
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
if(s!=null){
// Do you work here on success
}else{
// null response or Exception occur
}
}
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