java - Android/Java-来自类的通信返回java.lang.nullpointerException

我在一个Android项目中创建了扩展活动的主类，并创建了一个扩展主类的主类。我想在主类中使用在第二个类中声明的方法。但是我总是收到nullpointerexception。我把两个简单的类放在这里。
主班：

package com.example.androidserverapp;

import android.os.Bundle;
import android.app.Activity;
import android.content.pm.ActivityInfo;
import android.view.Menu;
import android.widget.ImageView;
public class AndroidServer extends Activity {

  public ImageView main_image;


  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_android_server);
    this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
    Utilities utilities = new Utilities();
    utilities.initialize();
  }


}

第二类：

package com.example.androidserverapp;

import android.widget.ImageView;;

public class Utilities extends AndroidServer{

  public ImageView main_image;

  public void initialize(){
      main_image.findViewById(R.id.main_image);
      main_image.setImageResource(R.drawable.image3);
  }

}

我看到错误是指行main_image =（ImageView）findViewById（R.id.main_image）;但我不明白我在做什么错。有什么帮助吗？
这是日志猫：

07-12 11:35:04.473: E/AndroidRuntime(14954): FATAL EXCEPTION: main
07-12 11:35:04.473: E/AndroidRuntime(14954): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.androidserverapp/com.example.androidserverapp.AndroidServer}: java.lang.NullPointerException
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2351)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread.access$600(ActivityThread.java:151)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1331)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.os.Handler.dispatchMessage(Handler.java:99)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.os.Looper.loop(Looper.java:155)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread.main(ActivityThread.java:5454)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at java.lang.reflect.Method.invokeNative(Native Method)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at java.lang.reflect.Method.invoke(Method.java:511)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1029)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:796)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at dalvik.system.NativeStart.main(Native Method)
07-12 11:35:04.473: E/AndroidRuntime(14954): Caused by: java.lang.NullPointerException
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.Activity.findViewById(Activity.java:1870)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at com.example.androidserverapp.Utilities.initialize(Utilities.java:8)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at com.example.androidserverapp.AndroidServer.onCreate(AndroidServer.java:20)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.Activity.performCreate(Activity.java:5066)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1101)
07-12 11:35:04.473: E/AndroidRuntime(14954):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2307)
07-12 11:35:04.473: E/AndroidRuntime(14954):    ... 11 more

谢谢

最佳答案

崩溃的原因是您创建了扩展了Utilities的类AndroidServer的对象，但未调用onCreate()方法，因为它只是新的Java对象，并且未启动活动。因此，在initialize()方法中，您可以找到视图，但是只有在开始该活动后才能这样做。希望你明白我的意思。

编辑：因此您无需在Utilities类中扩展Activity，并且可以进行如下操作：

public class Utilities {

   public void initialize(View mainHolder){
      ImageView imageView = mainHolder.findViewById(R.id.main_image);
      imageView.setImageResource(R.drawable.image3);
   }
}

@Override
protected void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   LayoutInflater inflater = LayoutInflater.from(this);
   View contentView = infler.inflate(R.layout.activity_android_server, null);
   setContentView(contentView);
   this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

   Utilities utilities = new Utilities();
   utilities.initialize(contentView);
}