Why doesn't calling a method from another class work?

Question

Java

The activity class contains a method:
.....

public void openListView(){

        FragmentManager manager = getSupportFragmentManager();
        androidx.fragment.app.FragmentTransaction transaction = manager.beginTransaction();

        ListViewFragment listViewFragment = new ListViewFragment();

        transaction.replace(R.id.fragmentFrameLayout, listViewFragment);

        transaction.commit();
    }

.....
When you call this method from the activity, everything is ok, the fragment appears.
There is also an adapter class for recyclerview. In this class, when the recyclerview element is clicked, the above method should be called. Calling a method in an adapter:

ProductsActivity productsActivity = new ProductsActivity();
productsActivity.openListView();

When pressed, the application crashes, when debugging, it determined that the crash occurs when transaction.commit () is called.
Error:

9913-9913/ru.retail.app E/AndroidRuntime: FATAL EXCEPTION: main
    Process: ru.retail.app, PID: 9913
    java.lang.IllegalStateException: Activity has been destroyed
        at androidx.fragment.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:2114)
        at androidx.fragment.app.BackStackRecord.commitInternal(BackStackRecord.java:683)
        at androidx.fragment.app.BackStackRecord.commit(BackStackRecord.java:637)
        at ru.retail.app.Activity.ProductsActivity.openListView(ProductsActivity.java:203)
        at ru.retail.app.adapter.ProductsActivity_CategoriesRecyclerViewAdapter.openCategory(ProductsActivity_CategoriesRecyclerViewAdapter.java:121)
        at ru.retail.app.adapter.ProductsActivity_CategoriesRecyclerViewAdapter$categoriesButtonListener.onClick(ProductsActivity_CategoriesRecyclerViewAdapter.java:198)
        at android.view.View.performClick(View.java:5675)
        at android.view.View$PerformClick.run(View.java:22641)
        at android.os.Handler.handleCallback(Handler.java:836)
        at android.os.Handler.dispatchMessage(Handler.java:103)
        at android.os.Looper.loop(Looper.java:203)
        at android.app.ActivityThread.main(ActivityThread.java:6251)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1063)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:924)

Answer 1

You can't do that, and it's not just about classes and methods. Activity in android is one of the fundamental components. They are always created by the OS, you can only do something in the lifecycle callback methods.
Also, in a fragment , you can get an instance of the Activity to which this fragment is added (getActivity()). True, this can not always be done, but only between onAttach / onDetach calls.
After reading the info on the links you should understand something. Not 100%, but some understanding should arise.

Answer 2

Hellish gesture.
You need to do the following:

1. Take textbook
   
2. Read about classes and objects