java - void android.widget.GridView.setAdapter(android.widget.ListAdapter)

Question

LogCat

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.GridView.setAdapter(android.widget.ListAdapter)' on a null object reference
        at ye.ansarallah.al_ansar.HomeActivity.onCreate(HomeActivity.java:37)

首页 Activity :

 public class HomeActivity extends AppCompatActivity {


        Integer[] MyImageArray = {
                R.drawable.w1, R.drawable.w2, R.drawable.w3
        };
        GridView MyGridView;
        ImageView MyCurrentWallpaper;
        Drawable MyDrawable;
        WallpaperManager MyWallmanger;


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


            MyGridView = findViewById(R.id.mygridview);
            MyCurrentWallpaper = findViewById(R.id.myImageView);
            MyGridView.setAdapter(new ImageAdapter(getApplicationContext()));
            UpdateMyWallpaper();


        }

        private void UpdateMyWallpaper() {

            MyWallmanger = WallpaperManager.getInstance(getApplicationContext());
            MyDrawable = MyWallmanger.getDrawable();
            MyCurrentWallpaper.setImageDrawable(MyDrawable);
        }

        public class ImageAdapter extends BaseAdapter {

            Context myContext;

            ImageAdapter(Context applicationContext) {

                myContext = applicationContext;
            }

            @Override
            public int getCount() {
                return MyImageArray.length;
            }

            @Override
            public Object getItem(int position) {
                return null;
            }

            @Override
            public long getItemId(int position) {
                return 0;
            }

            @Override
            public View getView(final int i, View view, ViewGroup viewGroup) {
                ImageView GridImageView;


                if (view == null) {

                    GridImageView = new ImageView(myContext);
                    GridImageView.setLayoutParams(new GridView.LayoutParams(512, 512));
                    GridImageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
                } else {
                    GridImageView = (ImageView) view;

                }
                GridImageView.setImageResource(MyImageArray[i]);
                GridImageView.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        try {
                            MyWallmanger.setResource(MyImageArray[i]);
                        } catch (IOException e) {
                            e.printStackTrace();
                        }

                        UpdateMyWallpaper();
                    }
                });

                return GridImageView;


            }
        }
    }

activity_home.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_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".HomeActivity">

    <ImageView
        android:id="@+id/myImageView"
        android:layout_width="match_parent"
        android:layout_height="300dp"
        android:scaleType="centerCrop"

        android:src="@drawable/w1" />


    <GridView
        android:id="@+id/mygridview"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:columnWidth="100dp"
        android:numColumns="auto_fit"
        android:stretchMode="spacingWidth"
        android:verticalSpacing="1dp" />

</LinearLayout>

Answer 1

MyGridView 为空。说明layout中没有id=R.id.mygridview的view。

可能是因为您使用了 setContentView(R.layout.activity_ma​​in)，但为 Activity_home.XML 提供了源 xml。