java - Android Firebase - 卡未在 RecyclerView 和 NullPointerException 中显示

Question

我正在尝试构建一个应用程序，用户可以在其中发布带有图像和描述的内容，这些内容单独包含在 CardView 中的 RecyclerView 中。所以我遇到了三个问题

1.当id(userId)用作子项，然后我发布数据时，它会被旧数据替换，而不是添加新数据(如果帖子有多个具有相同ID的子项，那么我的卡片应该显示，但ID每次都会被替换时间)

添加帖子时卡片不会显示。(当帖子中有帖子时Firebase 数据库)

如果默认情况下 Firebase 数据库中没有帖子 NullPointerException 到方法 getDesc() 甚至生成尽管我已经进行了空检查。

之前，为了存储帖子，我使用的子项是 id(userId) ，但帖子被替换而不是添加新帖子，因此我使用了随机字符串生成器并尝试从中获取信息。

HomeFragment.java

    public class HomeFragment extends Fragment {

    private RecyclerView rv;
    private TextView username,date,desc;
    private CircleImageView postImage;
    private ImageView postBigImage;
    private CardView postCard;
    private List<PostActivity> postItems;
    private DatabaseReference dr;
    private StorageReference sr;
    private FirebaseUser user;
    //FirebaseRecyclerAdapter<PostActivity, userViewHolder> fra;


    private PostAdapter pa;

    public HomeFragment() {
        // Required empty public constructor
    }



    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_home, container, false);


        sr = FirebaseStorage.getInstance().getReference();
        user = FirebaseAuth.getInstance().getCurrentUser();
        String id = user.getUid();

        AddPost ap = new AddPost();

        dr = 
      FirebaseDatabase.getInstance().getReference().child("Posts")
      .child(ap.getUni());

        postItems = new ArrayList<>();
        pa = new PostAdapter(postItems);

        rv =view.findViewById(R.id.rvPostItems);

        rv.setLayoutManager(new LinearLayoutManager(getActivity()));

        rv.setAdapter(pa);



        dr.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                for (DataSnapshot ds : dataSnapshot.getChildren()){

                    //postItems.add(ds.getValue(PostActivity.class));

                    PostActivity pa =dataSnapshot.getValue(PostActivity.class);
                    postItems.add(pa);

                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });

        return view;
    }


    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        void onFragmentInteraction(Uri uri);
    }
}

PostAdapter.java

package com.pappu5.navigation;

import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import java.util.List;

public class PostAdapter extends RecyclerView.Adapter<PostAdapter.ViewHolder> {

    public List<PostActivity> postItems;

    public PostAdapter(List<PostActivity> postItems) {
        this.postItems = postItems;

    }

    @NonNull
    @Override
    public PostAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.post_items,parent,false);

        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull final PostAdapter.ViewHolder holder, int position) {
        String descTe = postItems.get(position).getDesc();
        if(descTe!=null) {

            holder.setDesc(descTe);
        }else{
            String def = "This is NULL";
            holder.setDesc(def);
        }
    }

    @Override
    public int getItemCount() {


        return postItems.size();
    }

    public class ViewHolder extends RecyclerView.ViewHolder{

        private TextView desc;
        private View mView;

        public ViewHolder(View itemView) {
            super(itemView);
            mView = itemView;
        }

        public void setDesc(String descText){
            desc = mView.findViewById(R.id.postDesc);

                desc.setText(descText);

        }
    }

}

AddPost.java

package com.pappu5.navigation;

public class AddPost extends AppCompatActivity {

    private Toolbar toolbar;
    private ImageView postImage;
    private Button postButton;
    private EditText postDesc;
    private static final int gallery_no = 1;
    private ProgressDialog pd;
    private FirebaseUser user;
    private StorageReference sr;
    private DatabaseReference dr;
    private Uri postImageUri = null;
    private Bitmap compressor;
    String date = new SimpleDateFormat("yyyy-MM-dd", Locale.getDefault()).format(new Date());



    protected String getSaltString() {
        String SALTCHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
        StringBuilder salt = new StringBuilder();
        Random rnd = new Random();
        while (salt.length() < 18) { // length of the random string.
            int index = (int) (rnd.nextFloat() * SALTCHARS.length());
            salt.append(SALTCHARS.charAt(index));
        }
        String saltStr = salt.toString();
        return saltStr;

    }

    String uni = getSaltString();

    public String getUni(){
        return this.uni;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add_post);

        toolbar = findViewById(R.id.postBar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle("Add New Post !");

        postImage = findViewById(R.id.postImage);
        postDesc = findViewById(R.id.inputDesc);
        postButton = findViewById(R.id.post);

        sr = FirebaseStorage.getInstance().getReference();
        user = FirebaseAuth.getInstance().getCurrentUser();

        String id = user.getUid();


        dr = FirebaseDatabase.getInstance().getReference().child("Posts").child(uni);
        dr.keepSynced(true);


        postImage.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent gallery = new Intent();
                gallery.setType("image/*");
                gallery.setAction(Intent.ACTION_GET_CONTENT);

                startActivityForResult(Intent.createChooser(gallery,"Select Image"),gallery_no);
            }
        });

        postButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                String desc = postDesc.getText().toString();

                if(!TextUtils.isEmpty(desc) && postImageUri!=null ){
                    pd = new ProgressDialog(AddPost.this);
                    pd.setTitle("Uploading..");
                    pd.setMessage("Please Wait ..");
                    pd.setCanceledOnTouchOutside(false);
                    pd.show();

                    final String id = user.getUid();


                    File thumb = new File(postImageUri.getPath());
                    try {
                        compressor = new Compressor(AddPost.this)
                                .setMaxWidth(200)
                                .setMaxHeight(200)
                                .setQuality(75)
                                .compressToBitmap(thumb);




                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    ByteArrayOutputStream baos = new ByteArrayOutputStream();
                    compressor.compress(Bitmap.CompressFormat.JPEG, 100, baos);

                    final byte[] thumb_byte = baos.toByteArray();



                    final StorageReference path = sr.child("Post_Images").child(id + ".jpg");

                    final StorageReference thumb_nail = sr.child("Post_Images").child("Thumb_Nails").child(id+".jpg");

                    path.putFile(postImageUri).addOnCompleteListener(new OnCompleteListener<UploadTask.TaskSnapshot>() {
                        @Override
                        public void onComplete(@NonNull Task<UploadTask.TaskSnapshot> task) {
                            if (task.isSuccessful()) {

                                path.putFile(postImageUri).addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>() {
                                    @Override
                                    public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                                        path.getDownloadUrl().addOnSuccessListener(new OnSuccessListener<Uri>() {
                                            @Override
                                            public void onSuccess(Uri uri) {
                                                final String down_url = uri.toString();

                                                UploadTask uploadTask = thumb_nail.putBytes(thumb_byte);
                                                uploadTask.addOnCompleteListener(new OnCompleteListener<UploadTask.TaskSnapshot>() {
                                                    @Override
                                                    public void onComplete(@NonNull Task<UploadTask.TaskSnapshot> task) {

                                                        thumb_nail.putFile(postImageUri).addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>() {
                                                            @Override
                                                            public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                                                                thumb_nail.getDownloadUrl().addOnSuccessListener(new OnSuccessListener<Uri>() {
                                                                    @Override
                                                                    public void onSuccess(Uri uri) {


                                                                        final String thumb_down_url = uri.toString();

                                                                        Map <String,Object> update_map = new HashMap<>();
                                                                        update_map.put("image", down_url);
                                                                        update_map.put("thumb_image", thumb_down_url);
                                                                        update_map.put("desc",desc);

                                                                        update_map.put("timestamp", date);

                                                                        dr.updateChildren(update_map).addOnCompleteListener(new OnCompleteListener<Void>() {
                                                                            @Override
                                                                            public void onComplete(@NonNull Task<Void> task) {
                                                                                if (task.isSuccessful()) {
                                                                                    Toast.makeText(AddPost.this, "Uploaded !", Toast.LENGTH_SHORT).show();
                                                                                    Intent intent = new Intent(AddPost.this,MainActivity.class);
                                                                                    startActivity(intent);
                                                                                    finish();

                                                                                    pd.dismiss();
                                                                                } else {
                                                                                    Toast.makeText(AddPost.this, "Error in uploading thumbnail!", Toast.LENGTH_SHORT).show();
                                                                                    pd.dismiss();
                                                                                }
                                                                            }
                                                                        });



                                                                    }
                                                                });
                                                            }
                                                        });
                                                    }
                                                });

                                            }
                                        });
                                    }
                                });

                            } else {
                                Toast.makeText(AddPost.this, "Error!", Toast.LENGTH_SHORT).show();
                                pd.dismiss();
                            }
                        }
                    });


                }
            }
        });

    }


    public void onActivityResult(int requestCode, int resultCode, Intent data) {

        super.onActivityResult(requestCode, resultCode, data);

        if (requestCode == gallery_no && resultCode == RESULT_OK) {
            Uri imageUri = data.getData();

            CropImage.activity(imageUri).setAspectRatio(1, 1).setMinCropWindowSize(500, 500)
                    .start(AddPost.this);
        }


        if (requestCode == CropImage.CROP_IMAGE_ACTIVITY_REQUEST_CODE) {

            CropImage.ActivityResult result = CropImage.getActivityResult(data);

            if (resultCode == RESULT_OK) {


                postImageUri = result.getUri();
                postImage.setImageURI(postImageUri);



            } else if (resultCode == CropImage.CROP_IMAGE_ACTIVITY_RESULT_ERROR_CODE) {
                Exception error = result.getError();
            }
        }
    }

}

post_items.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <android.support.v7.widget.CardView
        android:id="@+id/postCard"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginBottom="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.56"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <android.support.constraint.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content">

            <de.hdodenhof.circleimageview.CircleImageView
                android:id="@+id/postImage"
                android:layout_width="65dp"
                android:layout_height="65dp"
                android:layout_marginStart="16dp"
                android:layout_marginTop="16dp"
                android:src="@drawable/default_avatar"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toTopOf="parent" />

            <TextView
                android:id="@+id/postUsername"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="16dp"
                android:layout_marginTop="16dp"
                android:text="Username"
                android:textStyle="bold"
                app:layout_constraintStart_toEndOf="@+id/postImage"
                app:layout_constraintTop_toTopOf="parent" />

            <TextView
                android:id="@+id/postDate"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="16dp"
                android:text="Datee"
                android:textStyle="italic"
                app:layout_constraintStart_toEndOf="@+id/postImage"
                app:layout_constraintTop_toBottomOf="@+id/postUsername" />

            <ImageView
                android:id="@+id/postBigImage"
                android:layout_width="0dp"
                android:layout_height="256dp"
                android:scaleType="centerCrop"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toBottomOf="@+id/postImage"
                tools:srcCompat="@tools:sample/avatars[3]" />

            <TextView
                android:id="@+id/postDesc"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:layout_marginStart="16dp"
                android:layout_marginTop="16dp"
                android:layout_marginEnd="16dp"
                android:layout_marginBottom="16dp"
                android:text="TextViewTextViewTextViewTextViewTextViewTextViewTextViewTextViewTextView"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toBottomOf="@+id/postBigImage" />
        </android.support.constraint.ConstraintLayout>

    </android.support.v7.widget.CardView>


</android.support.constraint.ConstraintLayout>

Answer 1

要修复 NullPointerException，请检查 postItems.get(position) 的 null 值，而不是 postItems.get(position).getDesc() ，即

@Override
public void onBindViewHolder(@NonNull final PostAdapter.ViewHolder holder, int position) {
    if(postItems.get(position) != null) {
        String descTe = postItems.get(position).getDesc();

        holder.setDesc(descTe);
    } else {
        String def = "This is NULL";
        holder.setDesc(def);
    }
}

此外，您正在使用空白的 postItems 列表初始化 PostAdapter，即

postItems = new ArrayList<>();
pa = new PostAdapter(postItems);

项目随后添加到 ValueEventListener 的 onDataChange() 方法中。您必须为新更新的 List postItems 更新 PostAdapter 并通知适配器。像这样。

public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            for (DataSnapshot ds : dataSnapshot.getChildren()){

                //postItems.add(ds.getValue(PostActivity.class));

                PostActivity pa =dataSnapshot.getValue(PostActivity.class);
                postItems.add(pa);

            }
            pa.notifyDataSetChanged();
        }

对于未显示的卡片，您可以分享 post_items.xml 文件的代码吗？