如何从sqlite数据库检索数据到android片段？

Question

传入的通知数据保存在sqlite中，在sqlite.how中检索它。我已将数据从 sqlite 传输到片段。我没有任何数据。我尝试使用 sqlite 来片段错误 this-> java.lang.NullPointerException: 尝试调用虚拟方法 'java.util.在空对象引用上列出 com.police.exam.databases.sqlite.DbHandler.getAllbell()'

sqlite 游标

 public List<bell> getAllbell() {
        List<bell> dataList = new ArrayList<bell>();
        // Select All Query
        String selectQuery = "SELECT  * FROM " + TABLE_NOTIFICATION + " ORDER BY id DESC";

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to list
        if (cursor.moveToFirst()) {
            do {
                bell values = new bell();
                values.setId(Integer.parseInt(cursor.getString(0)));
                values.setunique_id(Integer.parseInt(cursor.getString(1)));
                values.settitle(cursor.getString(1));
                values.setmessage(cursor.getString(2));
                values.setbig_image(cursor.getString(3));
                values.setimage(cursor.getString(4));
                values.setcontent_type(cursor.getString(5));
                values.setpost_id(Long.parseLong(cursor.getString(6)));
                dataList.add(values);
            } while (cursor.moveToNext());
        }

        // return contact list
        return dataList;
    }`

数据传递sqlite到fragment

`public class Fragmentbell extends Fragment {

private View root_view;
private RecyclerView recyclerView;
private SwipeRefreshLayout swipeRefreshLayout;
private Adapterbell adapterbell;
public static final String EXTRA_OBJC = "key.EXTRA_OBJC";
DbHandler databaseHandler;

private ShimmerFrameLayout lyt_shimmer;
SharedPref sharedPref;
private List<bell> data = new ArrayList<>();

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    root_view = inflater.inflate(R.layout.fragment_category, container, false);

    sharedPref = new SharedPref(getActivity());

    lyt_shimmer = root_view.findViewById(R.id.shimmer_view_container);
    swipeRefreshLayout = root_view.findViewById(R.id.swipe_refresh_layout_category);
    swipeRefreshLayout.setColorSchemeResources(R.color.colorPrimary);

    recyclerView = root_view.findViewById(R.id.recyclerViewCategory);
    recyclerView.setHasFixedSize(true);

    recyclerView.setLayoutManager(new StaggeredGridLayoutManager(1, LinearLayoutManager.VERTICAL));
    recyclerView.addItemDecoration(new EqualSpacingItemDecoration(0));
    recyclerView.setHasFixedSize(true);

    //set data and list adapter
    adapterbell = new Adapterbell(getActivity(), recyclerView,data);
    recyclerView.setAdapter(adapterbell);

    // on item list clicked
    adapterbell.setOnItemClickListener((v, obj, position) -> {
        Intent intent = new Intent(getActivity(), ActivityCategoryDetail.class);
        intent.putExtra(EXTRA_OBJC, obj);
        startActivity(intent);
        ((MainActivity) getActivity()).showInterstitialAd();
    });

    loadDataFromDatabase();


    //initShimmerLayout();

    return root_view;
}




private void loadDataFromDatabase() {
    data = databaseHandler.getAllbell();


}`

适配器

`公共类 Adapterbell 扩展了 RecyclerView.Adapter {

private final int VIEW_ITEM = 1;
private final int VIEW_PROG = 0;

private List<bell> items;

private boolean loading;
private OnLoadMoreListener onLoadMoreListener;

private Context context;
private OnItemClickListener mOnItemClickListener;
boolean scrolling = false;

private SharedPref sharedPref;



public interface OnItemClickListener {
    void onItemClick(View view, bell obj, int position);
}

public void setOnItemClickListener(final OnItemClickListener mItemClickListener) {
    this.mOnItemClickListener = mItemClickListener;
}

public Adapterbell(Context context, RecyclerView view, List<bell> items) {
    this.items = items;
    this.context = context;
    this.sharedPref = new SharedPref(context);
    lastItemViewDetector(view);
}

public class OriginalViewHolder extends RecyclerView.ViewHolder {

    public TextView category_name;
    public TextView recipe_title;
    public ImageView recipe_image;
    public ImageView thumbnail_video;
    public MaterialRippleLayout lyt_parent;

    public OriginalViewHolder(View v) {
        super(v);
        category_name = v.findViewById(R.id.category_name);
        recipe_title = v.findViewById(R.id.recipe_title);
        recipe_image = v.findViewById(R.id.recipe_image);
        thumbnail_video = v.findViewById(R.id.thumbnail_video);
        lyt_parent = v.findViewById(R.id.lyt_parent);
    }

}

public static class ProgressViewHolder extends RecyclerView.ViewHolder {
    public ProgressBar progressBar;

    public ProgressViewHolder(View v) {
        super(v);
        progressBar = v.findViewById(R.id.load_more);
    }
}

@NonNull
@Override
public RecyclerView.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    RecyclerView.ViewHolder vh;
    if (viewType == VIEW_ITEM) {
        View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_home_recipe, parent, false);
        vh = new OriginalViewHolder(v);
    } else {
        View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_load_more, parent, false);
        vh = new ProgressViewHolder(v);
    }
    return vh;
}

// Replace the contents of a view (invoked by the layout manager)
@Override
public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, final int position) {
    if (holder instanceof OriginalViewHolder) {
        final bell b = items.get(position);
        final OriginalViewHolder vItem = (OriginalViewHolder) holder;


        vItem.recipe_title.setText(b.title);

        if (b.content_type != null && b.content_type.equals("youtube")) {
            Picasso.get()
                    .load(Constant.YOUTUBE_IMAGE_FRONT + b.video_id + Constant.YOUTUBE_IMAGE_BACK_MQ)
                    .placeholder(R.drawable.ic_thumbnail)
                    .into(vItem.recipe_image);
        } else {
            Picasso.get()
                    .load(sharedPref.getApiUrl() + "/upload/" + b.image)
                    .placeholder(R.drawable.ic_thumbnail)
                    .into(vItem.recipe_image);
        }

        if (b.content_type != null && b.content_type.equals("Post")) {
            vItem.thumbnail_video.setVisibility(View.GONE);
        } else {
            vItem.thumbnail_video.setVisibility(View.VISIBLE);
        }

        vItem.lyt_parent.setOnClickListener(view -> {
            if (mOnItemClickListener != null) {
                mOnItemClickListener.onItemClick(view, b, position);
            }
        });

    } else {
        ((ProgressViewHolder) holder).progressBar.setIndeterminate(true);
    }

    if (getItemViewType(position) == VIEW_PROG) {
        StaggeredGridLayoutManager.LayoutParams layoutParams = (StaggeredGridLayoutManager.LayoutParams) holder.itemView.getLayoutParams();
        layoutParams.setFullSpan(true);
    } else {
        StaggeredGridLayoutManager.LayoutParams layoutParams = (StaggeredGridLayoutManager.LayoutParams) holder.itemView.getLayoutParams();
        layoutParams.setFullSpan(false);
    }

}

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

@Override
public int getItemViewType(int position) {
    if (items.get(position) != null) {
        return VIEW_ITEM;
    } else {
        return VIEW_PROG;
    }
}

public void insertData(List<bell> items) {
    setLoaded();
    int positionStart = getItemCount();
    int itemCount = items.size();
    this.items.addAll(items);
    notifyItemRangeInserted(positionStart, itemCount);
}

public void setLoaded() {
    loading = false;
    for (int i = 0; i < getItemCount(); i++) {
        if (items.get(i) == null) {
            items.remove(i);
            notifyItemRemoved(i);
        }
    }
}

public void setLoading() {
    if (getItemCount() != 0) {
        this.items.add(null);
        notifyItemInserted(getItemCount() - 1);
        loading = true;
    }
}

public void resetListData() {
    this.items = new ArrayList<>();
    notifyDataSetChanged();
}

public void setOnLoadMoreListener(OnLoadMoreListener onLoadMoreListener) {
    this.onLoadMoreListener = onLoadMoreListener;
}

private void lastItemViewDetector(RecyclerView recyclerView) {

    if (recyclerView.getLayoutManager() instanceof StaggeredGridLayoutManager) {
        final StaggeredGridLayoutManager layoutManager = (StaggeredGridLayoutManager) recyclerView.getLayoutManager();
        recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
            @Override
            public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
                super.onScrolled(recyclerView, dx, dy);
                int lastPos = getLastVisibleItem(layoutManager.findLastVisibleItemPositions(null));
                if (!loading && lastPos == getItemCount() - 1 && onLoadMoreListener != null) {
                    int current_page = getItemCount() / AppConfig.POST_PER_PAGE;
                    onLoadMoreListener.onLoadMore(current_page);
                    loading = true;
                }
            }
        });
    }
}

public interface OnLoadMoreListener {
    void onLoadMore(int current_page);
}

private int getLastVisibleItem(int[] into) {
    int last_idx = into[0];
    for (int i : into) {
        if (last_idx < i) last_idx = i;
    }
    return last_idx;
}

}`

型号

`导入java.io.Serialized;

公共类响铃实现了可序列化{

public int id;

public long unique_id;
public String title;
public String message;

public String big_image;

public String image;

public String content_type;

public String video_id;

public String link;

public long post_id;

public bell(long unique_id, String title, String message, String big_image, String image, String content_type, String video_id, String link, long post_id) {
    this.unique_id = unique_id;
    this.title = title;
    this.message = message;
    this.big_image = big_image;
    this.image = image;
    this.content_type = content_type;
    this.video_id = video_id;
    this.link = link;
    this.post_id = post_id;


}

public bell() {

}


public int getId() {
    return id;
}

public void setId(int id) {
    this.id = id;
}



public long getunique_id() {
    return unique_id;
}

public void setunique_id(long unique_id) {
    this.unique_id = unique_id;
}



public String gettitle() {
    return title;
}

public void settitle(String title) {
    this.title = title;
}

public String getmessage() {
    return message;
}

public void setmessage(String message) {
    this.message = message;
}



public String getbig_image() {
    return big_image;
}

public void setbig_image(String big_image) {
    this.big_image = big_image;
}


public String getimage() {
    return image;
}

public void setimage(String image) {
    this.image = image;
}

public String getcontent_type() {
    return content_type;
}


public String getvideo_id() {
    return video_id;
}

public void setvideo_id(String video_id) {
    this.video_id = video_id;
}
public void setcontent_type(String content_type) {
    this.content_type = content_type;
}


public long getpost_id() {
    return post_id;
}

public void setpost_id(long post_id) {
    this.post_id = post_id;
}

} `

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“java.util.List com.police.exam.databases.sqlite.DbHandler.getAllbell()”如何处理

Answer 1

在您的片段中，您需要初始化

DbHandler

 @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    //Add this in onCreateView
    databaseHandler = new DbHandler();
}