Android應用程序將關閉，並無法實例活動ComponentInfo

我工作的一個簡單的應用程序，以瞭解Android，但每當我嘗試運行它，我得到這個錯誤：Android應用程序將關閉，並無法實例活動ComponentInfo

11-28 14:51:34.536: E/AndroidRuntime(272): FATAL EXCEPTION: main 
11-28 14:51:34.536: E/AndroidRuntime(272): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{hu.mdpr.wk1/hu.mdpr.wk1.MainActivity}: java.lang.NullPointerException 
11-28 14:51:34.536: E/AndroidRuntime(272): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585) 
11-28 14:51:34.536: E/AndroidRuntime(272): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679) 
11-28 14:51:34.536: E/AndroidRuntime(272): at android.app.ActivityThread.access$2300(ActivityThread.java:125) 
11-28 14:51:34.536: E/AndroidRuntime(272): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033) 
11-28 14:51:34.536: E/AndroidRuntime(272): at android.os.Handler.dispatchMessage(Handler.java:99) 
11-28 14:51:34.536: E/AndroidRuntime(272): at android.os.Looper.loop(Looper.java:123) 
11-28 14:51:34.536: E/AndroidRuntime(272): at android.app.ActivityThread.main(ActivityThread.java:4627) 
11-28 14:51:34.536: E/AndroidRuntime(272): at java.lang.reflect.Method.invokeNative(Native Method) 
11-28 14:51:34.536: E/AndroidRuntime(272): at java.lang.reflect.Method.invoke(Method.java:521) 
11-28 14:51:34.536: E/AndroidRuntime(272): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) 
11-28 14:51:34.536: E/AndroidRuntime(272): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) 
11-28 14:51:34.536: E/AndroidRuntime(272): at dalvik.system.NativeStart.main(Native Method) 
11-28 14:51:34.536: E/AndroidRuntime(272): Caused by: java.lang.NullPointerException 
11-28 14:51:34.536: E/AndroidRuntime(272): at android.app.Activity.findViewById(Activity.java:1637) 
11-28 14:51:34.536: E/AndroidRuntime(272): at hu.mdpr.wk1.MainActivity.<init>(MainActivity.java:12) 
11-28 14:51:34.536: E/AndroidRuntime(272): at java.lang.Class.newInstanceImpl(Native Method) 
11-28 14:51:34.536: E/AndroidRuntime(272): at java.lang.Class.newInstance(Class.java:1429) 
11-28 14:51:34.536: E/AndroidRuntime(272): at android.app.Instrumentation.newActivity(Instrumentation.java:1021) 
11-28 14:51:34.536: E/AndroidRuntime(272): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577) 
11-28 14:51:34.536: E/AndroidRuntime(272): ... 11 more

根據大多數線程在這個問題上，這通常意味着我的Manifest存在問題，但我一直無法找到任何東西。

我的清單：

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
    package="hu.mdpr.wk1" 
    android:versionCode="1" 
    android:versionName="1.0" > 

    <uses-sdk 
     android:minSdkVersion="8" 
     android:targetSdkVersion="18" /> 

    <application 
     android:allowBackup="true" 
     android:icon="@drawable/ic_launcher" 
     android:label="@string/app_name" 
     android:theme="@style/AppTheme" > 
     <activity 
      android:name=".MainActivity" 
      android:label="@string/app_name" > 
      <intent-filter> 
       <action android:name="android.intent.action.MAIN" /> 

       <category android:name="android.intent.category.LAUNCHER" /> 
      </intent-filter> 
     </activity> 
    </application> 

</manifest>

我的活動：

package hu.mdpr.wk1; 

import android.os.Bundle; 
import android.app.Activity; 
import android.view.Menu; 
import android.view.View; 
import android.widget.Button; 
import android.widget.TextView; 

public class MainActivity extends Activity { 
    private int presses = 0; 
    private final Button b = (Button)findViewById(R.id.button1); 
    private final TextView v = (TextView)findViewById(R.id.textView1); 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     b.setOnClickListener(new ButtonClickHandler()); 
    } 


    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.main, menu); 
     return true; 
    } 

    public class ButtonClickHandler implements View.OnClickListener 
    { 
     @Override 
     public void onClick(View source) 
     { 
      presses++; 
      v.setText("button pressed "+presses+" times."); 
     } 
    } 

}

我已經試圖改變出口秩序，淨化工程和更改API級別，其中沒有固定的問題。請幫幫我。

編輯：事實證明，使按鈕最終，並試圖填充變量以外的onCreate導致的問題。感謝所有回答的人。

來源

2013-11-28 Wouter van den Heuvel

變化

private int presses = 0; 
private final Button b = (Button)findViewById(R.id.button1); 
private final TextView v = (TextView)findViewById(R.id.textView1); 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    b.setOnClickListener(new ButtonClickHandler()); 
}

到

private int presses = 0; 
private Button b; 
private TextView v; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    b = (Button)findViewById(R.id.button1); 
    v = (TextView)findViewById(R.id.textView1); 

    b.setOnClickListener(new ButtonClickHandler()); 
}

來源

2013-11-28 15:12:36 JoeC

private final Button b = (Button)findViewById(R.id.button1); 
private final TextView v = (TextView)findViewById(R.id.textView1);

您嘗試在未創建anctivity時查找視圖。
呼叫findViewById()後setContentView()

來源

2013-11-28 15:12:07 Dima

你正在創建視圖時尚未創建活動：

做這樣的：

package hu.mdpr.wk1; 

import android.os.Bundle; 
import android.app.Activity; 
import android.view.Menu; 
import android.view.View; 
import android.widget.Button; 
import android.widget.TextView; 

public class MainActivity extends Activity { 
private int presses = 0; 
private Button b; 
private TextView v ; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
     b = (Button)findViewById(R.id.button1); 
     v = (TextView)findViewById(R.id.textView1); 
    b.setOnClickListener(new ButtonClickHandler()); 
} 


@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    // Inflate the menu; this adds items to the action bar if it is present. 
    getMenuInflater().inflate(R.menu.main, menu); 
    return true; 
} 

public class ButtonClickHandler implements View.OnClickListener 
{ 
    @Override 
    public void onClick(View source) 
    { 
     presses++; 
     v.setText("button pressed "+presses+" times."); 
    } 
} 

}

來源

2013-11-28 15:15:54

在頂部

private Button b; 
private TextView v;

在onCreate（）方法參考全局創建按鈕聲明

b = (Button)findViewById(R.id.button1); 
v = (TextView)findViewById(R.id.textView1);

來源

2013-11-28 15:20:01

Android應用程序將關閉，並無法實例活動ComponentInfo

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