Question

我知道什么是空指针异常，但我的问题是我无法弄清楚如何处理它并向用户提供一个toast消息，说明以下代码中的列表为空。我正在为我的应用创建一个图库，我正在指定系统应该将所有文件和文件夹加载到我的Android应用程序中的路径。

当路径中没有文件或文件夹时，应用程序崩溃说：

Attempting to invoke length on a null array.

GalleryFragment.java

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_gallery, container, false);
    toolbar = (Toolbar) getActivity().findViewById(R.id.toolbar);
    ImageView icon = (ImageView) getActivity().findViewById(R.id.toolbarIcon);
    icon.setImageResource(R.drawable.ic_perm_media_black_24dp);
    icon.setColorFilter(getResources().getColor(R.color.Gallery));
    TextView title = (TextView) getActivity().findViewById(R.id.toolbarTitle);
    title.setText(getString(R.string.galleryLabel));
    toolbar.setBackground(getResources().getDrawable(R.drawable.tile_green));
    toolbar.setNavigationIcon(getResources().getDrawable(R.drawable.ic_menu_green_24dp));

    gridView = (GridView) view.findViewById(R.id.gridView);

    setGridAdapter(Environment.getExternalStorageDirectory().getPath());

    return view;
}
private void setGridAdapter(String path) {
    // Create a new grid adapter

gridItems = createGridItems(path);

MyGridAdapter adapter = new MyGridAdapter(getContext(), gridItems);

// Set the grid adapter
gridView.setAdapter(adapter);

// Set the onClickListener
gridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        if (gridItems.get(position).isDirectory()) {
            setGridAdapter(gridItems.get(position).getPath());
        }
        else {
            gridItems.get(position).getImage();
        }
    }
});

gridView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
    @Override
    public boolean onItemLongClick(AdapterView<?> parent, View view, int position, long id) {
        if (gridItems.get(position).isDirectory()) {
            Dialog dialog = new Dialog(getContext());
            dialog.setTitle("Test");
            dialog.setContentView(R.layout.activity_recognition);
            dialog.show();
            return false;
        } else {
            Dialog dialog = new Dialog(getContext());
            dialog.setTitle("Test");
            dialog.setContentView(R.layout.activity_reset_password);
            dialog.show();
            return false;
        }
    }
});
}

private List<GridViewItem> createGridItems(String directoryPath) {
    List<GridViewItem> items = new ArrayList<>();

    // List all the items within the folder.
    File[]  files = new File(directoryPath).listFiles(new ImageFileFilter());
        for (File file : files) {

            // Add the directories containing images or sub-directories
            if (file.isDirectory()
                    && file.listFiles(new ImageFileFilter()).length > 0) {

                items.add(new GridViewItem(file.getAbsolutePath(), true, null));
            }
            // Add the images
            else {
                Bitmap image = BitmapHelper.decodeBitmapFromFile(file.getAbsolutePath(),
                        50,
                        50);
                items.add(new GridViewItem(file.getAbsolutePath(), false, image));
            }
        }
    return items;
}



/**
 * Checks the file to see if it has a compatible extension.
 */
private boolean isImageFile(String filePath) {
    if (filePath.endsWith(".jpg") || filePath.endsWith(".png"))
    // Add other formats as desired
    {
        return true;
    }
    return false;
}

/**
 * This can be used to filter files.
 */
private class ImageFileFilter implements FileFilter {

    @Override
    public boolean accept(File file) {
        if (file.isDirectory()) {
            return true;
        }
        else if (isImageFile(file.getAbsolutePath())) {
            return true;
        }
        return false;
    }
}

GalleryAdapter.java

public class MyGridAdapter extends BaseAdapter {

    LayoutInflater inflater;
    List < GridViewItem > items;
    private Context mContext;


    public MyGridAdapter(Context context, List < GridViewItem > items) {
        mContext = context;
        this.items = items;
        inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }


    @Override
    public int getCount() {
        return items.size();
    }


    @Override
    public Object getItem(int position) {
        return items.get(position);
    }


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


    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        if (convertView == null) {
            convertView = inflater.inflate(R.layout.grid_item, null);
        }

        TextView text = (TextView) convertView.findViewById(R.id.textView);
        text.setText(items.get(position).getPath());

        ImageView imageView = (ImageView) convertView.findViewById(R.id.imageView);
        Bitmap image = items.get(position).getImage();

        if (image != null) {
            imageView.setImageBitmap(image);
        } else {
            // If no image is provided, display a folder icon.
            imageView.setImageResource(R.drawable.ic_folder_grey_24dp);
        }

        return convertView;
    }

}

我想只在数组的长度大于零时才列出文件。我不知道在哪里检查。

Answer 1

如果您尝试迭代for数组，则增强的null循环将失败。因此，这里的一个选择就是检查这种可能性，并仅在存在非空数组文件时进行迭代。

private List<GridViewItem> createGridItems(String directoryPath) {
    List<GridViewItem> items = new ArrayList<>();

    // List all the items within the folder.
    File[]  files = new File(directoryPath).listFiles(new ImageFileFilter());
    if (files != null) {  // check here for null
        for (File file : files) {
            // Add the directories containing images or sub-directories
            if (file.isDirectory()
                    && file.listFiles(new ImageFileFilter()).length > 0) {

                items.add(new GridViewItem(file.getAbsolutePath(), true, null));
            }
            // Add the images
            else {
                Bitmap image = BitmapHelper.decodeBitmapFromFile(file.getAbsolutePath(),
                        50,
                        50);
                items.add(new GridViewItem(file.getAbsolutePath(), false, image));
            }
        }
    }

    return items;
}

Answer 2

第一次更改在下面的循环之前进行空检查

private List<GridViewItem> createGridItems(String directoryPath) {
List<GridViewItem> items = new ArrayList<>();

// List all the items within the folder.
File[]  files = new File(directoryPath).listFiles(new ImageFileFilter());

       if(files != null && files.length != 0) {

       for (File file : files) {

        // Add the directories containing images or sub-directories
        if (file.isDirectory()
                && file.listFiles(new ImageFileFilter()).length > 0) {

            items.add(new GridViewItem(file.getAbsolutePath(), true, null));
        }
        // Add the images
        else {
            Bitmap image = BitmapHelper.decodeBitmapFromFile(file.getAbsolutePath(),
                    50,
                    50);
            items.add(new GridViewItem(file.getAbsolutePath(), false, image));
        }
    }
 }
return items;
}

接下来在myGridAdapter的getItemCount中为空列表编写逻辑，如下所示

 @Override
 public int getCount() {

     if(item.size == 0) {
        Log.d("List Size :", "List is empty");
        //If you want to toast message accepts context in your constructor
        //Then toast message 
     }

     return items.size();
}

无法弄清楚检查长度数组的位置

2 个答案: