java.lang.NullPointerException：尝试在null对象引用上调用虚拟方法'int java.lang.String.length（）'在News API Android中

Question

我还是Android编程的新手，我一直在尝试制作一个简单的应用程序进行练习。我想获取API新闻，并且我的代码抛出NullPointerException。这是代码

NewsAdapter.java

public class NewsAdapter extends RecyclerView.Adapter<NewsAdapter.MyViewHolder> {

private List<Article> articles;
private Context context;
private OnItemClickListener onItemClickListener;


public NewsAdapter(List<Article> articles, Context context) {
    this.articles = articles;
    this.context = context;
}

@NonNull
@Override
public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    View view = LayoutInflater.from(context).inflate(R.layout.item,parent,false);
    return new MyViewHolder(view, onItemClickListener);
}

@Override
public void onBindViewHolder(@NonNull MyViewHolder holders, int position) {
    final MyViewHolder holder2 = holders;
    Article model = articles.get(position);

    RequestOptions requestOptions = new RequestOptions();
    requestOptions.placeholder(Utils.getRandomDrawbleColor());
    requestOptions.error(Utils.getRandomDrawbleColor());
    requestOptions.diskCacheStrategy(DiskCacheStrategy.ALL);
    requestOptions.centerCrop();

    Glide.with(context)
            .load(model.getUrlToImage())
            .apply(requestOptions)
            .listener(new RequestListener<Drawable>() {
                @Override
                public boolean onLoadFailed(@Nullable GlideException e, Object model, Target<Drawable> target, boolean isFirstResource) {
                    holder2.progressBar.setVisibility(View.GONE);
                    return false;
                }

                @Override
                public boolean onResourceReady(Drawable resource, Object model, Target<Drawable> target, DataSource dataSource, boolean isFirstResource) {
                    holder2.progressBar.setVisibility(View.GONE);
                    return false;
                }
            })
            .transition(DrawableTransitionOptions.withCrossFade())
            .into(holder2.imageView);

    holder2.title.setText(model.getTitle());
    holder2.desc.setText(model.getDescription());
    holder2.source.setText(model.getSource().getName());
    holder2.time.setText(" \u2022 "+Utils.DateToTimeFormat(model.getPublishedAt()));
    holder2.published_at.setText(Utils.DateFormat(model.getPublishedAt()));
    holder2.author.setText(model.getAuthor());
}

@Override
public int getItemCount() {
    return articles.size();
}
public void setOnItemClickListener(OnItemClickListener onItemClickListener){
    this.onItemClickListener = onItemClickListener;
}
public interface OnItemClickListener{
    void onItemClick(View view, int position);
}

public class MyViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {

    TextView title, desc, author, published_at, source, time;
    ImageView imageView;
    ProgressBar progressBar;
    OnItemClickListener onItemClickListener;

    public MyViewHolder(View itemView, OnItemClickListener onItemClickListener) {
        super(itemView);

        itemView.setOnClickListener(this);
        title =  itemView.findViewById(R.id.title);
        desc = itemView.findViewById(R.id.desc);
        author = itemView.findViewById(R.id.author);
        published_at = itemView.findViewById(R.id.publishedAt);
        source = itemView.findViewById(R.id.source);
        time = itemView.findViewById(R.id.time);
        imageView = itemView.findViewById(R.id.img_news);
        progressBar = itemView.findViewById(R.id.load_photo);

        this.onItemClickListener = onItemClickListener;
    }

    @Override
    public void onClick(View v) {
        onItemClickListener.onItemClick(v, getAdapterPosition());
    }
}

这是代码Utils.java

public class Utils {

public static ColorDrawable[] vibrantLightColorList =
        {
                new ColorDrawable(Color.parseColor("#ffeead")),
                new ColorDrawable(Color.parseColor("#93cfb3")),
                new ColorDrawable(Color.parseColor("#fd7a7a")),
                new ColorDrawable(Color.parseColor("#faca5f")),
                new ColorDrawable(Color.parseColor("#1ba798")),
                new ColorDrawable(Color.parseColor("#6aa9ae")),
                new ColorDrawable(Color.parseColor("#ffbf27")),
                new ColorDrawable(Color.parseColor("#d93947"))
        };

public static ColorDrawable getRandomDrawbleColor() {
    int idx = new Random().nextInt(vibrantLightColorList.length);
    return vibrantLightColorList[idx];
}

public static String DateToTimeFormat(String oldstringDate){
    PrettyTime p = new PrettyTime(new Locale(getCountry()));
    String isTime = null;
    try {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'",
                Locale.ENGLISH);
        Date date = sdf.parse(oldstringDate);
        isTime = p.format(date);
    } catch (ParseException e) {
        e.printStackTrace();
    }

    return isTime;
}

public static String DateFormat(String oldstringDate){
    String newDate;
    SimpleDateFormat dateFormat = new SimpleDateFormat("E, d MMM yyyy", new Locale(getCountry()));
    try {
        Date date = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'").parse(oldstringDate);
        newDate = dateFormat.format(date);
    } catch (ParseException e) {
        e.printStackTrace();
        newDate = oldstringDate;
    }

    return newDate;
}

public static String getCountry(){
    Locale locale = Locale.getDefault();
    String country = String.valueOf(locale.getCountry());
    return country.toLowerCase();
}

在此代码中，我得到

java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
        at com.example.android.likeeatapplication.Utils.DateToTimeFormat(Utils.java:39)
        at com.example.android.likeeatapplication.Adapter.NewsAdapter.onBindViewHolder(NewsAdapter.java:81)
        at com.example.android.likeeatapplication.Adapter.NewsAdapter.onBindViewHolder(NewsAdapter.java:29)

那我该如何解决呢？