如何从超类重用代码Intent intent = new Intent（context，Activity.class）到子类？

Question

假设子类具有不同的上下文以及在单击监听器之后要启动的不同活动，那么超类中的代码Intent intent=new Intent(context,Activity.class)可以如何被其子类重用。 可能吗？

这是超类：

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);
        }
}

子类之一：

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);
            }
        }

我尝试上面的代码时出现以下错误：

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)

我不确定我错了什么或我错过了什么。 我只是在逻辑上尝试了我认为正确的方法。 请纠正我错误的地方！

Answer 1

对于初学者，您永远不要使用new初始化Activity，它具有自己的生命周期，上下文应属于您实际开始意图的类，将代码更改为

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-停止从子级传递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);
        }
}

子类

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

您需要使变量成为类范围而不是内部类范围 。

这是一个内部类作用域变量：

public class CommonPost extends AppCompatActivity {

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

     ...

  }
}

您无法访问Context context和Class aclass; 来自儿童班。

您需要使其成为类范围：

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，在您的子类中，将变量更改为：

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);
  }
}