<分区>
我对 Android 编程还是有点陌生，我一直在尝试制作一个简单的天气应用程序来练习。当我输入一个有效的城市名称时，此代码可以正常工作，但是我想使用一个 toast 在名称无效时向用户显示错误。但这会导致崩溃，我尝试删除 toast，但我的应用程序仍然崩溃。
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();
}
}
}
和错误信息:
12-22 22:11:51.204 4858-4858/com.example.aiden.whatstheweather E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.aiden.whatstheweather, PID: 4858
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:116) at org.json.JSONTokener.nextValue(JSONTokener.java:94) at org.json.JSONObject.(JSONObject.java:159) at org.json.JSONObject.(JSONObject.java:176) at com.example.aiden.whatstheweather.MainActivity$DownloadTask.onPostExecute(MainActivity.java:102) at com.example.aiden.whatstheweather.MainActivity$DownloadTask.onPostExecute(MainActivity.java:65) at android.os.AsyncTask.finish(AsyncTask.java:695) at android.os.AsyncTask.-wrap1(Unknown Source:0) at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:712) at android.os.Handler.dispatchMessage(Handler.java:105) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6541) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)