java - 使用 AsyncTask 出现 NullPointerException

Question

我正在尝试向网站发送一个简单的帖子并收到响应，但我在实现 AsyncTask 来提供所需的辅助流时遇到问题。目前我收到一个java.lang.NullPointerException。你知道是什么原因造成的吗？

日志猫:

07-18 11:34:14.829: E/AndroidRuntime(8354): FATAL EXCEPTION: main
07-18 11:34:14.829: E/AndroidRuntime(8354): java.lang.NullPointerException
07-18 11:34:14.829: E/AndroidRuntime(8354):     at ca.waddlesoft.smpclient_android.MainActivity$SmpProcessor.onPostExecute(MainActivity.java:50)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at ca.waddlesoft.smpclient_android.MainActivity$SmpProcessor.onPostExecute(MainActivity.java:1)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at android.os.AsyncTask.finish(AsyncTask.java:602)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at android.os.AsyncTask.access$600(AsyncTask.java:156)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:615)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at android.os.Handler.dispatchMessage(Handler.java:99)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at android.os.Looper.loop(Looper.java:137)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at android.app.ActivityThread.main(ActivityThread.java:4514)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at java.lang.reflect.Method.invokeNative(Native Method)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at java.lang.reflect.Method.invoke(Method.java:511)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:980)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:747)
07-18 11:34:14.829: E/AndroidRuntime(8354):     at dalvik.system.NativeStart.main(Native Method)

我的代码:

package ca.waddlesoft.smpclient_android;

import java.net.MalformedURLException;
import java.net.URL;

import ca.waddlesoft.smpclient_android.R;

import android.os.AsyncTask;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.TextView;
import android.support.v4.app.NavUtils;

public class MainActivity extends Activity {
    private TextView tv;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        TextView tv;
        tv = (TextView)findViewById(R.id.TextView01);
        setContentView(R.layout.activity_main);
        SmpProcessor task = new SmpProcessor();
        task.execute();
    }

    private class SmpProcessor extends AsyncTask <Void,Void,String> {
        @Override
        protected String doInBackground(Void... voids){
        URL url = null;
        try {
            url = new URL("http://secure.g4apps.com");
            String app="A1";
            String data = "E1,E1,AT333AT333,9055627513,2012-02-02 02:02:02,2012-02-02 02:02:02,-454545.343434,434343.232323,A334A334A334A443X,1000000,3434";
            byte [] databyte=data.getBytes();
            String out = SmpClass.smpCall(url,app,data);
            //  String out = new String(smpresponse);
            return out;
        } catch (MalformedURLException e) {
            e.printStackTrace();
        }

        return null;
    }

    @Override
    protected void onPostExecute(String result) {
        tv.setText(result);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }
}

Answer 1

2个错误:

• 您在本地+在类中声明您的textView。只需在其中声明即可你的类(class)。
• 在调用 setContentView 之前初始化 textView

试试这个:

package ca.waddlesoft.smpclient_android;

import java.net.MalformedURLException;
import java.net.URL;

import ca.waddlesoft.smpclient_android.R;

import android.os.AsyncTask;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.TextView;
import android.support.v4.app.NavUtils;

public class MainActivity extends Activity {
    private TextView tv;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv = (TextView)findViewById(R.id.TextView01);

        SmpProcessor task = new SmpProcessor();
        task.execute();
    }

  private class SmpProcessor extends AsyncTask <Void,Void,String> {

            @Override
            protected String doInBackground(Void... voids){
                URL url = null;
                try {
                    url = new URL("http://secure.g4apps.com");
                    String app="A1";
                    String data = "E1,E1,AT333AT333,9055627513,2012-02-02 02:02:02,2012-02-02 02:02:02,-454545.343434,434343.232323,A334A334A334A443X,1000000,3434";
//                  byte [] databyte=data.getBytes();
                    String out = SmpClass.smpCall(url,app,data);
//                  String out = new String(smpresponse);
                    return out;
                } catch (MalformedURLException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                return null;
            }
            @Override
            protected void onPostExecute(String result) {
                    tv.setText(result);
            }
        }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }


}