Question

我不太清楚为什么我会得到一个NullPointerException，这是Java的新手，所以任何帮助都会非常感谢！

public class MainActivity extends AppCompatActivity {


    private TextView httpstuff;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView httpstuff = (TextView)findViewById(R.id.tvHttp);
        Button button = (Button) findViewById(R.id.button);
        button.setOnClickListener(
            new Button.OnClickListener(){
                public void onClick(View v){
                    new JSONTask().onPostExecute("http://samples.openweathermap.org/data/2.5/weather?q=Chicago&appid=93eea9a9277a0520d2f444c3ff8c62da");
                }
            }
    );

}

   class JSONTask extends AsyncTask<String,String,String> {
        @Override
        protected String doInBackground(String... urls) {
        HttpsURLConnection connection = null;
        BufferedReader reader = null;
        try {
            URL url = new URL("https://samples.openweathermap.org/data/2.5/weather?q=Chicago&appid=93eea9a9277a0520d2f444c3ff8c62da");
            connection = (HttpsURLConnection) url.openConnection();
            connection.connect();

            InputStream stream = connection.getInputStream();
            reader = new BufferedReader(new InputStreamReader(stream));
            StringBuffer buffer = new StringBuffer();
            String line;
            while ((line = reader.readLine()) != null) {
                buffer.append(line);
            }
            return buffer.toString();

        } catch(MalformedURLException e) {
            e.printStackTrace();
        } catch(IOException e) {
            e.printStackTrace();
        }             
        finally {
            if(connection != null){
                connection.disconnect();
            }
            try{
                if(reader != null){
                    reader.close();
                }
            }catch(IOException e) {
                e.printStackTrace();
            }

        }
            return null;
    }
   @Override
   protected void onPostExecute (String result){
       super.onPostExecute(result);
       httpstuff.setText(result);
        }
    }
}

Android Studio Logs表示NullPointerException错误发生在

httpstuff.setText(result);

在

new JSONTask().onPostExecute("http://samples.openweathermap.org/data/2.5/weather?q=Chicago&appid=93eea9a9277a0520d2f444c3ff8c62da");


private TextView httpstuff is a designated as a field but says that it is not initialized, but I use it in class JSONTask? Any ideas on why the exception is occurring? Thanks alot in advance!

Answer 1

 TextView httpstuff = (TextView)

删除第一个TextView。

在学习过程中始终尝试使用this.httpstuff（或MainActivity.this.httpstuff），直到您明白何时不使用this

httpstuff被指定为字段，但表示它未初始化

你正在使用Asynctask中的字段，是的，但你从未对其进行初始化，正如警告所说。

OnCreate有一个同名的局部变量

NullPointerException，简单的错误？

1 个答案: