如何從超類重用代碼Intent intent = new Intent（context，Activity.class）到子類？

Question

假設子類具有不同的上下文以及在單擊監聽器之后要啟動的不同活動，那么超類中的代碼Intent intent=new Intent(context,Activity.class)可以如何被其子類重用。 可能嗎？

這是超類：

public class CommonPost extends AppCompatActivity {

    public void on_create(final Context context, final Class aclass) {
        post.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                CommonPost commonPost = new CommonPost();
                MyTask task = commonPost.new MyTask(context, aclass);
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB)
                    task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
                else
                    task.execute();
            }
        });
    }


class MyTask extends AsyncTask<Void,Void,Void> {
          Context context;
          Class aclass;

        public MyTask(Context context,Class aclass){
             this.context=context;
             this.aclass=aclass;
        }

        @Override
        protected void onPreExecute() {
            // do something
        }

        @Override
        protected Void doInBackground(Void... voids) {
            // do something
            return null;
        }

        @Override
        protected void onPostExecute(Void aVoid) {
            Intent intent = new Intent(context, aclass);
            startActivity(intent);
        }
}

子類之一：

public class PlacementPost extends CommonPost {
            Context context=PlacementPost.this;
            Class aclass=Placements.class;

            @Override
            protected void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                on_create(context,aclass);
            }
        }

我嘗試上面的代碼時出現以下錯誤：

java.lang.NullPointerException: Attempt to invoke virtual method 'android.app.ActivityThread$ApplicationThread android.app.ActivityThread.getApplicationThread()' on a null object reference
       at android.app.Activity.startActivityForResult(Activity.java:4266)
       at android.support.v4.app.BaseFragmentActivityJB.startActivityForResult(BaseFragmentActivityJB.java:50)



at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:79)
   at android.app.Activity.startActivityForResult(Activity.java:4224)
   at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:859)
   at android.app.Activity.startActivity(Activity.java:4548)
   at android.app.Activity.startActivity(Activity.java:4516)
   at studentapp.notefi.CommonPost$PlaceTask.onPostExecute(CommonPost.java:240)
   at studentapp.notefi.CommonPost$PlaceTask.onPostExecute(CommonPost.java:177)
   at android.os.AsyncTask.finish(AsyncTask.java:660)
   at android.os.AsyncTask.-wrap1(AsyncTask.java)
   at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:677)
   at android.os.Handler.dispatchMessage(Handler.java:102)
   at android.os.Looper.loop(Looper.java:154)
   at android.app.ActivityThread.main(ActivityThread.java:6077)
   at java.lang.reflect.Method.invoke(Native Method)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

我不確定我錯了什么或我錯過了什么。 我只是在邏輯上嘗試了我認為正確的方法。 請糾正我錯誤的地方！

Answer 1

對於初學者，您永遠不要使用new初始化Activity，它具有自己的生命周期，上下文應屬於您實際開始意圖的類，將代碼更改為

on_create

 public void on_create(final Context context, final Class aclass) {
            post.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    MyTask task = new MyTask(stor_root, mProgress, editTextplace, post, ninfo, imageUri,
                            mstorage, mDatabase, context, aclass);
                    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB)
                        task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
                    else
                        task.execute();
                }
            });
        }

MyTask-停止從子級傳遞context

class MyTask extends AsyncTask<Void,Void,Void> {
        Class aclass;

        public MyTask(Class aclass){
             this.aclass=aclass;
        }

        @Override
        protected void onPreExecute() {
            // do something
        }

        @Override
        protected Void doInBackground(Void... voids) {
            // do something
            return null;
        }

        @Override
        protected void onPostExecute(Void aVoid) {
            Intent intent = new Intent(CommonPost.this, aclass);
            startActivity(intent);
        }
}

子類

public class PlacementPost extends CommonPost {
            Context context=PlacementPost.this;
            Class aclass=Placements.class;

            @Override
            protected void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                on_create(aclass);
            }
        }

Answer 2

您需要使變量成為類范圍而不是內部類范圍 。

這是一個內部類作用域變量：

public class CommonPost extends AppCompatActivity {

  ...
  class MyTask extends AsyncTask<Void,Void,Void> {
     Context context;
     Class aclass;

     ...

  }
}

您無法訪問Context context和Class aclass; 來自兒童班。

您需要使其成為類范圍：

public class CommonPost extends AppCompatActivity {

  // set to protected to only allow child class access.
  protected Context context;
  protected Class aclass;

  ...
  class MyTask extends AsyncTask<Void,Void,Void> {

  }
}

然后，在您的子類中，將變量更改為：

public class PlacementPost extends CommonPost {

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // set the value to the base class.
    context = PlacementPost.this;
    aclass = Placements.class;

    on_create(context,aclass);
  }
}