Null pointer exception occurs when you call findViewById() on Android.

Could you tell me what the problem is in the program below? In the onCreate() method, findViewById() is called, and the return value is null even if you try all the id. I don't know why, but does anyone know?

Main Code:

public class MainActivity extends Activity {

    ViewPager pager;
    MyPagerAdapter adapter;
    LinearLayout layout1, layout2, layout3;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        layout1 = (LinearLayout) findViewById(R.id.first_View);
        layout2 = (LinearLayout) findViewById(R.id.second_View);
        layout3 = (LinearLayout) findViewById(R.id.third_View);

        adapter = new MyPagerAdapter();
        pager = (ViewPager) findViewById(R.id.main_pager);
        pager.setAdapter(adapter);
    }

    private class MyPagerAdapter extends PagerAdapter
    {

        @Override
        public int getCount() { 
            return 3;
        }

        @Override
        public Object instantiateItem(ViewGroup collection, int position) {

            LinearLayout l = null;

            if (position == 0 )
            {
                l = layout1;
            }
            if (position == 1)
            {
                l = layout2;
            }

            if (position == 2)
            {
                l = layout3;
            }
                collection.addView(l, position);
                return l;
        }

        @Override
        public boolean isViewFromObject(View view, Object object) {
            return (view==object);
        }

         @Override
         public void destroyItem(ViewGroup collection, int position, Object view) {
                 collection.removeView((View) view);
         }
    }
}

Associated xml code

activity_main layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
                android:orientation="vertical"
                android:layout_width="fill_parent"
                android:layout_height="fill_parent"
                android:background="#a4c639">


    <android.support.v4.view.ViewPager
                        android:layout_width="match_parent" 
                        android:layout_height="match_parent" 
                        android:id="@+id/main_pager"/>
</LinearLayout>

activity_first layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/first_View">

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />

<Button
    android:id="@+id/button1"
    style="?android:attr/buttonStyleSmall"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Button" />

</LinearLayout>

activity_second layout:

 <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/second_View">

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />

</LinearLayout>

activity_third layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/third_View">

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />

</LinearLayout>

android nullpointerexception oncreate findviewbyid

2022-09-22 21:36

1 Answers

findViewById() returns View. However, setContentView() will be returned only when the layout is present. Otherwise, it returns null as it is now.

For example, setContentView(R.layout.activity_first); call in this way and then Call findViewById (R.id.first_View);

The View will be returned.

of course findViewById (R.id.second_View); Calling the layout of activity_second.xml in this way will return null. Activity_first.xml does not have @+id/second_View.

2022-09-22 21:36

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