Android ListView CheckBox repetition

i have a problem related to using checkbox in a dynamic listview. I checked similar questions and their answers and tried to adjust my code. Unfortunately, i couldn't get over it.

Briefly, at the begining my checks on the list was disappeared when i scrolled the list. Later on i made some changes on its codes and this time when i clicked on the first item on the list, checkbox repeated every 6th items without clicking on them.

Book.java

package book.lib;

public class Books {

private String title;
private String image;
private Boolean checkbox;

public Books() {

}

//Getters and setters
public String getTitle() {
    return title;
}
public void setTitle(String title) {
    this.title = title;
}
public String getImage() {
    return image;
}
public void setImage(String image) {
    this.image = image;
}

public Boolean getCheckbox() {
    return checkbox;
}
public void setCheckbox(boolean checkbox) {
    this.checkbox = checkbox;
}

}

BookAdapter.java

public class BookAdapter extends ArrayAdapter<Books> {
private final Context context;
private ArrayList<Books> bookList;

LayoutInflater vi;
int Resource;
ViewHolder holder = null;
GetBookThumb getBookThumb;

public BookAdapter(Context context, int resource, ArrayList<Books> objects) {
    super(context, resource, objects);
    vi = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    Resource = resource;
    bookList = objects;
    getBookThumb=new GetBookThumb(context);
    this.context = context;

}

public int getCount() {
    return bookList.size();
}

public Books getItem(int position) {
    return bookList.get(position);
}
@Override

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

@Override
public View getView(final int position, View convertView, ViewGroup parent) {
    // convert view = design
    View v = convertView;

    if (v == null) {
        holder = new ViewHolder();
        v = vi.inflate(Resource, null);

        holder.bookIMG = (ImageView) v.findViewById(R.id.thumb);
        holder.bookTITLE = (TextView) v.findViewById(R.id.book_title);
        holder.checkBox = (CheckBox) v.findViewById(R.id.checkBox);

        v.setTag(holder);

    } else {
        holder = (ViewHolder) v.getTag();
        getBookThumb.DisplayImage(bookList.get(position).getImage(), holder.bookIMG);
        holder.bookTITLE.setText(bookList.get(position).getTitle());
        holder.checkBox.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                Books book = bookList.get(position);
                book.setCheckbox(holder.checkBox.isChecked());
                Toast.makeText(context.getApplicationContext(),

                        "Clicked on Checkbox: " + holder.checkBox.getText() +
                                " is " + holder.checkBox.isChecked(),
                        Toast.LENGTH_LONG).show();
            }
        });
    }
        return v;

}
static class ViewHolder {
    public ImageView bookIMG;
    public TextView bookTITLE;
    public CheckBox checkBox;
}

}