GridView gravity won't change

Question

I am trying to change the gravity of a GridView whenever I click a button but the app crashes when deployed. I'm not sure why this occurs when the GridView has been declared in the XML layout.

Attempt to invoke virtual method 'android.view.ViewGroup$LayoutParams android.widget.GridView.getLayoutParams()' on a null object reference

sw600dp/activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    tools:context=".MainActivity">
    <Button
        android:id="@+id/btn_moveup"
        android:text="move up"
        android:background="@android:color/transparent"
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:layout_gravity="top"
        android:onClick="moveup_click"
        />

    <GridView
        android:id="@+id/abslistview_main"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="center_vertical"
        android:numColumns="auto_fit"
        />

    <Button
        android:id="@+id/btn_movedown"
        android:text="move down"
        android:background="@android:color/transparent"
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:layout_gravity="bottom"
        android:onClick="movedown_click"
        />
</LinearLayout>

MainActivity.java public class MainActivity extends Activity {

    Activity mActivity;
    private Button btnUp;
    private Button btnDown;
    private GridView gridMain;

    private AbsListView absListView;
    private ArrayAdapter<MainItem> adapter;
    private ArrayList<MainItem> mainArrayList;

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

        absListView = findViewById(R.id.abslistview_main);
        settingData();

        adapter = new AbsListViewAdapter(this, R.layout.item_abslistview, mainArrayList);
        absListView.setAdapter(adapter);
    }

    private void settingData() {
        mainArrayList = new ArrayList<>();

        mainArrayList.add(new MainItem(R.drawable.ic_circle, getResources().getString(R.string.item_1)));
        mainArrayList.add(new MainItem(R.drawable.ic_star, getResources().getString(R.string.item_2)));
        mainArrayList.add(new MainItem(R.drawable.ic_square, getResources().getString(R.string.item_3)));
        mainArrayList.add(new MainItem(R.drawable.ic_triangle, getResources().getString(R.string.item_4)));
        mainArrayList.add(new MainItem(R.drawable.ic_rectangle, getResources().getString(R.string.item_5)));
        mainArrayList.add(new MainItem(R.drawable.ic_crescent, getResources().getString(R.string.item_6)));
    }

    public void moveup_click(View view) {
        LinearLayout.LayoutParams params = (LinearLayout.LayoutParams) gridMain.getLayoutParams();
        gridMain = findViewById(R.id.abslistview_main);
        if (params.gravity == Gravity.BOTTOM) {
            gridMain.setGravity(Gravity.CENTER_VERTICAL);
            btnDown.setVisibility(View.VISIBLE);
        } else if (params.gravity == Gravity.CENTER_VERTICAL) {
            gridMain.setGravity(Gravity.TOP);
            btnUp.setVisibility(View.GONE);
        }
    }
}

score 1 · Accepted Answer · answered Sep 29 '17 at 18:54

You have buttons that fire the moveup_click() function. In that function, you first want LayoutParams of GridView and after that you initialise the gridview itself. This is not the right sequence.

To solve it, you can move the gridMain = findViewById(R.id.abslistview_main);the oncreate function or BEFORE LinearLayout.LayoutParams params = (LinearLayout.LayoutParams) gridMain.getLayoutParams();. -

score 0 · Answer 2 · edited Sep 29 '17 at 19:43

0

You have also made a mistake in the create method

absListView = findViewById(R.id.abslistview_main);

Remove this line with

gridMain = findViewById(R.id.abslistview_main);

edited Sep 29 '17 at 19:43

Donald Duck

8,409
22
75
99

answered Sep 29 '17 at 19:17

jessica

1,700
1
11
17

GridView gravity won't change

2 Answers2