無法實例活動ComponentInfo {...}：顯示java.lang.NullPointerException和一些其他的東西

Question

我有以下代碼：

import android.media.AudioManager; 
import android.media.MediaPlayer; 
import android.os.Bundle; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 
import android.widget.ImageView; 

import android.app.Activity; 


public class MainActivity extends Activity 
{ 
MediaPlayer m3; 
MediaPlayer m2; 
MediaPlayer m1; 
MediaPlayer m4; 

ImageView im2 = (ImageView) findViewById(R.id.im2); 
ImageView im3 = (ImageView) findViewById(R.id.im3); 
ImageView im4 = (ImageView) findViewById(R.id.im4); 
ImageView im5 = (ImageView) findViewById(R.id.im5); 
Button button; 
int a = 0; 

@Override 
protected void onCreate(Bundle savedInstanceState) 
{ 
super.onCreate(savedInstanceState); 
setContentView(R.layout.activity_main); 


    m1 = new MediaPlayer(); 
m1 = MediaPlayer.create(this, R.raw.m1); 
m1.setAudioStreamType(AudioManager.STREAM_MUSIC); 

m2 = new MediaPlayer(); 
m2 = MediaPlayer.create(this, R.raw.m2); 
m2.setAudioStreamType(AudioManager.STREAM_MUSIC); 

m3 = new MediaPlayer(); 
m3 = MediaPlayer.create(this, R.raw.m3); 
m3.setAudioStreamType(AudioManager.STREAM_MUSIC); 

m4 = new MediaPlayer(); 
m4 = MediaPlayer.create(this, R.raw.m4); 
m4.setAudioStreamType(AudioManager.STREAM_MUSIC); 

button = (Button) findViewById(R.id.button); 
button.setOnClickListener(new OnClickListener() { 

@Override 
public void onClick(View v) { 
    // TODO Auto-generated method stub 

    if (a == 0) 
    { 
     m1.start(); 
     im2.setImageResource(R.drawable.i2); 
     a = 1; 
    } 
    else if (a == 1) 
    { m2.start(); 
     im3.setImageResource(R.drawable.i3); 
     a = 2; 
    } 
    else if (a == 2) 
    { m3.start(); 
     im4.setImageResource(R.drawable.i4); 
     a = 3; 
    } 
    else if (a == 3) 
    { m4.start(); 
     im5.setImageResource(R.drawable.i5); 
     a = 4; 
    } 

} 
}); 

} 

} 

但它有一個錯誤發射嘗試後馬上崩潰。以下是我在logcat中看到：

03-19 22:14:58.232: E/AndroidRuntime(29846): FATAL EXCEPTION: main 
    03-19 22:14:58.232: E/AndroidRuntime(29846): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.luckyyou.justin/com.luckyyou.justin.MainActivity}: java.lang.NullPointerException 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2269) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2395) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at android.app.ActivityThread.access$600(ActivityThread.java:162) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at android.os.Handler.dispatchMessage(Handler.java:107) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at android.os.Looper.loop(Looper.java:194) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at android.app.ActivityThread.main(ActivityThread.java:5371) 
    03-19 22:14:58.232: E/AndroidRuntime(29846):  at java.lang.reflect.Method.invokeNative(Native Method) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at java.lang.reflect.Method.invoke(Method.java:525) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at dalvik.system.NativeStart.main(Native Method) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): Caused by: java.lang.NullPointerException 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at android.app.Activity.findViewById(Activity.java:1839) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at com.luckyyou.justin.MainActivity.<init>(MainActivity.java:21) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at java.lang.Class.newInstanceImpl(Native Method) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at java.lang.Class.newInstance(Class.java:1319) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at android.app.Instrumentation.newActivity(Instrumentation.java:1054) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): at     android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2260) 
    03-19 22:14:58.232: E/AndroidRuntime(29846): ... 11 more 
    03-19 22:14:58.292: E/AppErrorDialog(497): Failed to get ILowStorageHandle instance 

我沒宣佈我的清單活動，確實應該讓我的應用程序工作，其他的東西，但它沒有，所以我不知道該怎麼辦，請大家幫忙！

Answer 1

，因爲你正在試圖找出連Layout設置爲，這就是爲什麼以下Exception發生的活動之前的觀點...

java.lang.NullPointerException at android.app.Activity.findViewById(Activity.java:1839) 

初始化所有ImageViews，如下onCreate()方法內...

ImageView im2; 
ImageView im3; 
ImageView im4; 
ImageView im5; 


@Override 
protected void onCreate(Bundle savedInstanceState) 
{ 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    im2 = (ImageView) findViewById(R.id.im2); 
    im3 = (ImageView) findViewById(R.id.im3); 
    im4 = (ImageView) findViewById(R.id.im4); 
    im5 = (ImageView) findViewById(R.id.im5); 

}