android - 无法实例化 Activity ComponentInfo {…}:java.lang.NullPointerException和其他一些东西

我有以下代码：

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;
    }

}
});

}

}

但是启动try之后，它立即崩溃并显示错误。这是我在logCat中看到的内容：

    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

我确实在Manifest中声明了我的活动，并做了其他一些应能使我的应用程序正常运行的工作，但没有成功，所以我不知道该怎么办，请帮忙！

最佳答案

既然如此，您甚至在Layout设置为活动之前就尝试查找视图，这就是为什么跟随Exception发生的原因...

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

初始化ImageViews方法中的所有onCreate()，如下所示...

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);

}