运行截图

错误信息:

错误代码:

 public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {       // Log.d("size",bookList.size()+"");        Book book=getItem(position);        View view;        ViewHolder viewHolder;        if(convertView==null){            view= LayoutInflater.from(getContext()).inflate(resourceId,parent,false);            viewHolder=new ViewHolder();            viewHolder.bookAuthor=view.findViewById(R.id.book_item_author);            viewHolder.bookTitle=view.findViewById(R.id.book_item_title);            viewHolder.bookLastChapter=view.findViewById(R.id.book_item_lastChapter);        } else{           view=convertView;           viewHolder=(ViewHolder) view.getTag();        }        viewHolder.bookAuthor.setText(book.getAuthor());        viewHolder.bookTitle.setText(book.getTitle());        viewHolder.bookLastChapter.setText(book.getLastChapter());        return view;    }

错误原因:
ListView滑动后，viewHolder内的变量不能找到了。
解决方法:
convertView为null时候，设置 view.setTag(viewHolder);

 public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {       // Log.d("size",bookList.size()+"");        Book book=getItem(position);        View view;        ViewHolder viewHolder;        if(convertView==null){            view= LayoutInflater.from(getContext()).inflate(resourceId,parent,false);            viewHolder=new ViewHolder();            viewHolder.bookAuthor=view.findViewById(R.id.book_item_author);            viewHolder.bookTitle=view.findViewById(R.id.book_item_title);            viewHolder.bookLastChapter=view.findViewById(R.id.book_item_lastChapter);            view.setTag(viewHolder);        } else{           view=convertView;           viewHolder=(ViewHolder) view.getTag();        }        viewHolder.bookAuthor.setText(book.getAuthor());        viewHolder.bookTitle.setText(book.getTitle());        viewHolder.bookLastChapter.setText(book.getLastChapter());        return view;    }

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