無法實例化活動ComponentInfo {...}：java.lang.NullPointerException

Question

在學習android我做了一個ICMP ping程序。我添加了一個AsyncTask線程來阻止「主要網絡」，現在我的編程正在拋出這個運行時異常（標題中的那個）。我是新來的android;如果你們能幫助我，那真是太棒了。這裏有問題的logcat錯誤：

05-30 13:52:49.644: E/AndroidRuntime(620): FATAL EXCEPTION: main 
05-30 13:52:49.644: E/AndroidRuntime(620): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.icmpping.Joe/com.icmpping.Joe.PingAppActivity}: java.lang.NullPointerException 
05-30 13:52:49.644: E/AndroidRuntime(620): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1880) 
05-30 13:52:49.644: E/AndroidRuntime(620): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981) 
05-30 13:52:49.644: E/AndroidRuntime(620): at android.app.ActivityThread.access$600(ActivityThread.java:123) 
05-30 13:52:49.644: E/AndroidRuntime(620): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147) 
05-30 13:52:49.644: E/AndroidRuntime(620): at android.os.Handler.dispatchMessage(Handler.java:99) 
05-30 13:52:49.644: E/AndroidRuntime(620): at android.os.Looper.loop(Looper.java:137) 
05-30 13:52:49.644: E/AndroidRuntime(620): at android.app.ActivityThread.main(ActivityThread.java:4424) 
05-30 13:52:49.644: E/AndroidRuntime(620): at java.lang.reflect.Method.invokeNative(Native Method) 
05-30 13:52:49.644: E/AndroidRuntime(620): at java.lang.reflect.Method.invoke(Method.java:511) 
05-30 13:52:49.644: E/AndroidRuntime(620): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 
05-30 13:52:49.644: E/AndroidRuntime(620): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 
05-30 13:52:49.644: E/AndroidRuntime(620): at dalvik.system.NativeStart.main(Native Method) 
05-30 13:52:49.644: E/AndroidRuntime(620): Caused by: java.lang.NullPointerException 
05-30 13:52:49.644: E/AndroidRuntime(620): at android.app.Activity.findViewById(Activity.java:1794) 
05-30 13:52:49.644: E/AndroidRuntime(620): at com.icmpping.Joe.PingAppActivity.<init>(PingAppActivity.java:15) 
05-30 13:52:49.644: E/AndroidRuntime(620): at java.lang.Class.newInstanceImpl(Native Method) 
05-30 13:52:49.644: E/AndroidRuntime(620): at java.lang.Class.newInstance(Class.java:1319) 
05-30 13:52:49.644: E/AndroidRuntime(620): at android.app.Instrumentation.newActivity(Instrumentation.java:1023) 
05-30 13:52:49.644: E/AndroidRuntime(620): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1871) 
05-30 13:52:49.644: E/AndroidRuntime(620): ... 11 more 

我很確定這與AsyncTask有關。 ICMP_Ping.ping方法來自另一個類，它返回一個String對象並接受一個String參數。它使用java.net.InetAddress和System.currentTimeMillis（）。總之，這裏是我的主類：把後

////

package com.icmpping.Joe; 

import android.app.Activity; 
import android.os.AsyncTask; 
import android.os.Bundle; 
import android.util.Log; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.TextView; 

public class PingAppActivity extends Activity { 

private TextView console = (TextView) findViewById(R.id.TEXT_STATUS_ID); 

private OnClickListener listener = new OnClickListener() { 
    public void onClick(View v) { 

     Log.d("pingactivity", "Hey LOGCAT2"); 

     EditText urlbox = (EditText) findViewById(R.id.url_edittext); 
     String url = urlbox.getText().toString(); 

     Log.d("pingactivity", "Hey LOGCAT3"); 
     new AsyncPing().execute(url); 


     Log.d("pingactivity", "Hey LOGCAT4"); 
    } 
}; 


public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.main); 

    Log.d("pingactivity", "Hey LOGCAT0"); 

    Button pingbutton = (Button) findViewById(R.id.ping_button); 

    pingbutton.setOnClickListener(listener); 
    Log.d("pingactivity", "Hey LOGCAT1"); 

} 


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

    @Override 
    protected String doInBackground(String... url) { 
     return ICMP_Ping.ping(url[0]); 
    } 

    //protected void onProgressUpdate(Integer... progress) {} 

    protected void onPostExecute(String pingresult) { 
     console.append(pingresult); 

    } 
} 

}

Answer 1

此行TextView console = (TextView) findViewById(R.id.yourtextid);

setContentView(R.layout.main);

TextView console; 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.main); 

     Log.d("pingactivity", "Hey LOGCAT0"); 

     Button pingbutton = (Button) findViewById(R.id.ping_button); 
     console = (TextView) findViewById(R.id.yourtextid); 
    }