Android- I can't make the ListView on second activity

Question

The problem is about when I want to open a second activity. I can't open it, it crashes. But on first activity everything works fine. I tried my best to solve it, but I failed. Here's a code:

Inside 1st Java - onCreate:

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    String [] listaDziedzin = { getResources().getString(R.string.astronomia),
            getResources().getString(R.string.dynamika), getResources().getString(R.string.hydrostatyka),
            getResources().getString(R.string.kinematyka), getResources().getString(R.string.optyka),
            getResources().getString(R.string.termodynamika)};


    ListAdapter adapterListDziedzin = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, listaDziedzin);

    listaD = (ListView) findViewById(R.id.listView);

    listaD.setAdapter(adapterListDziedzin);

    listaD.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {


            switch (position) {
                case 0:
                    Intent x = new Intent(MainActivity.this, SecondActivityA.class);
                    startActivity(x);
                    break;
                case 1:
                    break;
                case 2:
                    break;
                case 3:
                    break;
                case 4:
                    break;
                case 5:
                    break;
            }

        }
    });
}

Inside the second one:

 String [] listaKategorii = { getResources().getString(R.string.wzor_newtona),getResources().getString(R.string.trzecie_prawo_keplera),
            getResources().getString(R.string.wzor_ciolkowskiego)};

    ListAdapter v = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, listaKategorii);

    listax = (ListView) findViewById(R.id.listView2);

    listax.setAdapter(v);

    final TextView a = (TextView) findViewById(R.id.textView);



    listax.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {


           switch (position) {
                case 0:
                    a.setText(R.string.wzor_newtona_wzor);
                    break;
                case 1:
                    a.setText(R.string.trzecie_prawo_keplera_wzor);
                    break;
                case 2:
                    a.setText(R.string.wzor_ciolkowskiego_wzor);
                    break;}

        }

    });
}

What I noticed is that when I open the second activity without the ListView everything works alright. So probably it is something about creating the ListView . But what? :/

Thank you for your help.

PS. I found an error:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.blogspot.mikeusz.physicsformulas/com.blogspot.mikeusz.physicsformulas.SecondActivityA}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

Answer 1

That NullPointerException should tell you that you are trying to assign an adapter to a non existing list. Make sure you have set the right id to the second list view and also make sure you set setContentView() in your second activity.