Android:无法将Firebase中的URL提取到Arraylist中

我试图将Firebase存储中的图像显示到一个recyclerview中，URL存储在Firebase数据库中，但看起来Firebase没有返回任何内容

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
HashMap<String,String> m;
ArrayList<String> links;
private FirebaseStorage storage;
private StorageReference storagereference;
private FirebaseDatabase database;
private DatabaseReference myRef;

MyAdapter(){
    database = FirebaseDatabase.getInstance();
    myRef = database.getReference("Links");
    myRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            m=(HashMap<String, String>) dataSnapshot.getValue();
            links=new ArrayList<String>(m.values());
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}

public static class ViewHolder extends RecyclerView.ViewHolder {
    CardView cv;
    ImageView iv;

    public ViewHolder(View itemView) {
        super(itemView);
        cv = (CardView)itemView.findViewById(R.id.cardView);
        iv = (ImageView)itemView.findViewById(R.id.imageView);
    }
}

@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View v = LayoutInflater.from(parent.getContext())
            .inflate(R.layout.ground_recycler_view, parent, false);
    //do some modifications such as set the view's size, margins, paddings and layout parameters if required
    ViewHolder vh=new ViewHolder(v);
    return vh;
}

@Override
public void onBindViewHolder(ViewHolder holder, int position) {
    Glide.with(holder.iv.getContext()).load(links.get(position)).centerCrop().thumbnail(0.1f).crossFade()
            .diskCacheStrategy(DiskCacheStrategy.ALL).into(holder.iv);
}

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

---

可能还有其他问题，但我突然想到，

链接

将为空，直到从数据库返回值，并且这里可能存在竞争条件

为getItemCount中的链接添加空检查。 或者更好，使用Glide和Firebase Storage查看此示例。

FATAL EXCEPTION: main
                  Process: idk.cricsters, PID: 22958
                  java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
                      at idk.cricsters.MyAdapter.getItemCount(MyAdapter.java:87)
                      at android.support.v7.widget.RecyclerView.dispatchLayoutStep1(RecyclerView.java:2938)
                      at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:2904)
                      at android.support.v7.widget.RecyclerView.onLayout(RecyclerView.java:3283)
                      at android.view.View.layout(View.java:16630)
                      at android.view.ViewGroup.layout(ViewGroup.java:5437)
                      at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1079)
                      at android.view.View.layout(View.java:16630)
                      at android.view.ViewGroup.layout(ViewGroup.java:5437)
                      at android.widget.FrameLayout.layoutChildren(FrameLayout.java:336)
                      at android.widget.FrameLayout.onLayout(FrameLayout.java:273)
                      at android.view.View.layout(View.java:16630)
                      at android.view.ViewGroup.layout(ViewGroup.java:5437)
                      at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1743)
                      at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1586)
                      at android.widget.LinearLayout.onLayout(LinearLayout.java:1495)
                      at android.view.View.layout(View.java:16630)
                      at android.view.ViewGroup.layout(ViewGroup.java:5437)
                      at android.widget.FrameLayout.layoutChildren(FrameLayout.java:336)
                      at android.widget.FrameLayout.onLayout(FrameLayout.java:273)
                      at android.view.View.layout(View.java:16630)
                      at android.view.ViewGroup.layout(ViewGroup.java:5437)
                      at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1743)
                      at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1586)
                      at android.widget.LinearLayout.onLayout(LinearLayout.java:1495)
                      at android.view.View.layout(View.java:16630)
                      at android.view.ViewGroup.layout(ViewGroup.java:5437)
                      at android.widget.FrameLayout.layoutChildren(FrameLayout.java:336)
                      at android.widget.FrameLayout.onLayout(FrameLayout.java:273)
                      at com.android.internal.policy.PhoneWindow$DecorView.onLayout(PhoneWindow.java:2678)
                      at android.view.View.layout(View.java:16630)
                      at android.view.ViewGroup.layout(ViewGroup.java:5437)
                      at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2171)
                      at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1931)
                      at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107)
                      at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013)
                      at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
                      at android.view.Choreographer.doCallbacks(Choreographer.java:670)
                      at android.view.Choreographer.doFrame(Choreographer.java:606)
                      at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
                      at android.os.Handler.handleCallback(Handler.java:739)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:148)
                      at android.app.ActivityThread.main(ActivityThread.java:5417)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)