Android - Listview with external class

Question

Layout->

activity_main.xml
content_main.xml

Logic ->

ActivityMain.java
MainLogic.java

2 Issues: (second issue is the actual issue)

1) java.lang.RuntimeException: Unable to start activity ComponentInfo{com.test.app./com.test.app.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object referenc

2) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference at android.app.Activity.findViewById(Activity.java:2124) at com.test.app.MainLogic.CreateListView(MainLogic.java:51)

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>

<android.support.design.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:fitsSystemWindows="true"
    tools:context=".MainActivity">

<android.support.design.widget.AppBarLayout android:layout_height="wrap_content"
    android:layout_width="match_parent" android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar android:id="@+id/toolbar"
        android:layout_width="match_parent" android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary" app:popupTheme="@style/AppTheme.PopupOverlay" />

</android.support.design.widget.AppBarLayout>

<include layout="@layout/content_main" />


<android.support.design.widget.FloatingActionButton android:id="@+id/fab"
    android:layout_width="wrap_content" android:layout_height="wrap_content"
    android:layout_gravity="bottom|end"     android:layout_margin="@dimen/fab_margin"
    android:src="@android:drawable/ic_dialog_email" />

</android.support.design.widget.CoordinatorLayout>

content_main.xml

<?xml version="1.0" encoding="utf-8"?>

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context=".MainActivity"
    tools:showIn="@layout/activity_main"
    android:id="@+id/main_layout">

<LinearLayout
    android:id="@+id/linLayout"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:layout_centerHorizontal="true">
    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/origin_string"
        android:textSize="20sp"/>
    <TextView
        android:id="@+id/textView2"
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:text="@string/random_string_sample"
        android:textSize="20sp"
        />
</LinearLayout>
    <Button
        android:id="@+id/btnClick1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/linLayout"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="61dp"
        android:text="@string/app_name" />
<ListView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/listview1"
    android:layout_below="@+id/btnClick1">
</ListView>

</RelativeLayout>

ActivityMain.java->

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setIcon(R.drawable.logo);
        start();
    }
    void start(){
        ListView listView = (ListView) findViewById(R.id.listview1); //ADDED
        MainLogic ml = new MainLogic();
        ml.CreateListView(>ADDED: listview<);
    }
}

MainLogic.java-> EDIT: added class line below Removed extends Activity

public class MainLogic {

public void CreateListView() {
    final ListView list_View1 = (ListView) findViewById(R.id.listview1);
    final String[] listViewStrings = new String[]{"first_item", "second_item", "third_item", "fouth_item", "fifth_item", "sixth_item", "seventh_item", "eigth_item"};
    ArrayAdapter list_adapter = new ArrayAdapter<String>(`<s>`getApplicationContext()`</s>`, android.R.layout.simple_list_item_1,listViewStrings);
    //list_adapter.addAll(>REMOVED: listViewStrings.toString()<);
    list_View1.setAdapter(list_adapter); // Use data, pass the adapter with the strings, numbers, etc.
    }
}

Whom ever does answer this if you can explain why I am doing things wrong and also possibly show me how to fix the code, that would be great. I have read over these links:

http://www.vogella.com/tutorials/AndroidListView/article.html

http://developer.android.com/guide/topics/ui/layout/listview.html

http://www.tutorialspoint.com/android/android_list_view.htm

I do not see the inherent flaw in my code except that i am implementing the logic in an external class then creating an object to call the method.

SOLUTION ->

ActivityMain.java->

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setIcon(R.drawable.logo);
        start();
    }

    public void start(){
        MainLogic ml = new MainLogic();
        ml.CreateListView(this);
    }

}

MainLogic.java->

public class MainLogic {

public void CreateListView(final MainActivity newActivity) {

    final ListView list_View1 = (ListView) newActivity.findViewById(R.id.listview1);
        final String[] listViewStrings = new String[]{"first_item", "second_item", "third_item", "fouth_item", "fifth_item", "sixth_item", "seventh_item", "eigth_item"};
//      This will adapt array that listview can work with i.e. generate all rows
        ArrayAdapter list_adapter = new ArrayAdapter<>(newActivity.getApplicationContext(), android.R.layout.simple_list_item_1, listViewStrings);
        list_adapter.addAll();
        list_View1.setAdapter(list_adapter); // Use data, pass the adapter with the strings, numbers, etc.

        list_View1.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapter, View view, int position_in_array, long id) {
                // Logic for what happens when clicked
                // Context => http://stackoverflow.com/questions/3572463/what-is-context-in-android
                int position = position_in_array;
                String item_value = (String) adapter.getItemAtPosition(position);
                String item = "Item: " + listViewStrings.toString() + " / value " + item_value + " ." ;
                Toast.makeText(newActivity.getApplicationContext(), item, Toast.LENGTH_SHORT).show();
            }
        });  

} }

Answer 1

The error Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference indicates that the ListView could not be found.

The findViewById you are calling is of MainLogic instead of MainActivity. And since your ListView is in MainActivity, that's why it could not find the ListView. Try to pass the MainActivity.this from MainActivity to MainLogic. Also, I think that there is no need to use list_adapter.addAll since you have passed the array in the constructor of ArrayAdapter.

So here's an example:

MainActivity:

ml.CreateListView(this);

MainLogic:

public void CreateListView(Activity activity) {
    ListView list_View1 = (ListView) activity.findViewById(R.id.listview1);
    final String[] listViewStrings = new String[]{"first_item", "second_item", "third_item", "fouth_item", "fifth_item", "sixth_item", "seventh_item", "eigth_item"};
    ArrayAdapter list_adapter = new ArrayAdapter<String>(getApplicationContext(), android.R.layout.simple_list_item_1,listViewStrings);
    list_View1.setAdapter(list_adapter); // Use data, pass the adapter with the strings, numbers, etc.
}