Java:如何在每次删除项目时更新回收者视图?

Question

我似乎遇到了这个问题，我想实时更新回收者视图。这意味着我希望回收器视图更新其列表时，用户滑动删除一行。正如你可以在下面的代码中看到的，我试图在DatabaseAdapter java类中使用NotifyDataSetChange()，但我只得到错误"RecyclerViewAdapter.notifyDataSetChanged()对一个空对象引用"

我都用谷歌搜索了上面的错误以及如何实时更新回收视图，但我发现的解决方案都没有工作。请帮帮我，这个问题把我弄糊涂了。

NamesAdapter.Java

package com.example.myapplication.ui;
import android.content.Context;
import android.database.Cursor;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.widget.Toast;
import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
import com.example.myapplication.DatabaseAdapter;
import com.example.myapplication.R;
import org.w3c.dom.NameList;
import java.util.ArrayList;
import java.util.List;
public class NamesAdapter extends RecyclerView.Adapter<NamesAdapter.ViewHolder> {

Context context;
List<Names> namesList;
RecyclerView rvPrograms;
final View.OnClickListener onClickListener = new MyOnClickListner();
public static class ViewHolder extends RecyclerView.ViewHolder{
TextView rowID;
TextView rowName;
TextView rowAmount;
TextView rowDate;

public ViewHolder(@NonNull View itemView) {
super(itemView);
//Store the item sub-views in member variables
rowID = itemView.findViewById(R.id.idTextView);
rowName = itemView.findViewById(R.id.nameTextView);
rowAmount = itemView.findViewById(R.id.amountEditText);
rowDate = itemView.findViewById(R.id.dateTextView);
}
}
public NamesAdapter(Context context, List<Names> namesList, RecyclerView rvPrograms){
this.context = context;
this.namesList= namesList;
this.rvPrograms = rvPrograms;
}
@NonNull
@Override
public NamesAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int viewType) {
LayoutInflater inflater = LayoutInflater.from(context);
View view = inflater.inflate(R.layout.recycler_list, viewGroup, false);
view.setOnClickListener(onClickListener);
ViewHolder viewHolder = new ViewHolder(view);
return viewHolder;
}
@Override
public void onBindViewHolder(@NonNull NamesAdapter.ViewHolder holder, int i) {
Names name = namesList.get(i);
holder.rowID.setText(""+name.getId());
holder.rowName.setText(name.getName());
holder.rowAmount.setText(name.getAmount());
holder.rowDate.setText(name.getDate());
holder.itemView.setTag(name.getId());
}
@Override
public int getItemCount() {
return namesList.size();
}

private class MyOnClickListner implements View.OnClickListener {
@Override
public void onClick(View v) {
int itemPosition = rvPrograms.getChildLayoutPosition(v);
String item = namesList.get(itemPosition).getName();
Toast.makeText(context, item, Toast.LENGTH_SHORT).show();
}
}
}

DatabaseAdapter.Java

package com.example.myapplication;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.widget.Toast;
import androidx.annotation.Nullable;
import com.example.myapplication.ui.Names;
import com.example.myapplication.ui.NamesAdapter;
import java.util.ArrayList;
import java.util.List;
public class DatabaseAdapter {
DatabaseHelper helper;
SQLiteDatabase db;
List<Names> namesList = new ArrayList<>();
NamesAdapter na;
public DatabaseAdapter(Context context) {
helper = new DatabaseHelper(context);
db = helper.getWritableDatabase();
}
public int deleteData(String name){
String whereArgs[] = {name};
int count = db.delete(DatabaseHelper.Table_Name, DatabaseHelper.Key_Name+ "=?", whereArgs);
return count;
}
public int removeItem (long id){
int count = db.delete(DatabaseHelper.Table_Name, DatabaseHelper.Key_ID+ "="+ id, null);
//na.notifyDataSetChanged();

return count;
}
public int updateAmount(String name, String amount){
ContentValues contentValues = new ContentValues();
contentValues.put(DatabaseHelper.Key_Amount, amount);
String whereArgs[] = {name};
int count = db.update(DatabaseHelper.Table_Name, contentValues, DatabaseHelper.Key_Name + "=?", whereArgs);
return count;
}
public  String searchData(String sname){
String columns[] = {DatabaseHelper.Key_Name, DatabaseHelper.Key_Amount};
String selectionArgs[] = {sname};
Cursor cursor = db.query(DatabaseHelper.Table_Name, columns, DatabaseHelper.Key_Name + "=?", selectionArgs, null, null, null);
StringBuffer buffer = new StringBuffer();
while (cursor.moveToNext()){
int index1 = cursor.getColumnIndex(DatabaseHelper.Key_Name);
int index2 = cursor.getColumnIndex(DatabaseHelper.Key_Amount);
String name = cursor.getString(index1);
String amount = cursor.getString(index2);
buffer.append(name + " " + amount + "n");
}
return buffer.toString();
}
//    This is for the Toast dispaly
public String getAllData(){
String columns[] = {DatabaseHelper.Key_ID, DatabaseHelper.Key_Name, DatabaseHelper.Key_Amount};
Cursor cursor = db.query(DatabaseHelper.Table_Name, columns, null, null, null, null, null, null );
StringBuffer buffer = new StringBuffer();
while (cursor.moveToNext()){
int index1 = cursor.getColumnIndex(DatabaseHelper.Key_ID);
int rowid = cursor.getInt(index1);
int index2 = cursor.getColumnIndex(DatabaseHelper.Key_Name);
String name = cursor.getString(index2);
int index3 = cursor.getColumnIndex(DatabaseHelper.Key_Amount);
int amount = cursor.getInt(index3);
buffer.append(rowid + " " + name + " " + amount + "n");
}
return buffer.toString();
}
public List<Names> getAllNames(){
String columns[] = {DatabaseHelper.Key_ID, DatabaseHelper.Key_Name, DatabaseHelper.Key_Amount, DatabaseHelper.Key_Date};
Cursor cursor = db.query(DatabaseHelper.Table_Name, columns, null, null, null, null, DatabaseHelper.Key_ID+" DESC",null );
while (cursor.moveToNext()){
int index1 = cursor.getColumnIndex(DatabaseHelper.Key_ID);
int rowid = cursor.getInt(index1);
int index2 = cursor.getColumnIndex(DatabaseHelper.Key_Name);
String name = cursor.getString(index2);
int index3 = cursor.getColumnIndex(DatabaseHelper.Key_Amount);
Double amount = cursor.getDouble(index3);
int index4 = cursor.getColumnIndex(DatabaseHelper.Key_Date);
String date = cursor.getString(index4);
Names names = new Names(rowid, name, amount, date);
namesList.add(names);
}
return namesList;
}
public long insertData (String name, Double amount, String date){
ContentValues contentValues = new ContentValues();
contentValues.put(DatabaseHelper.Key_Name, name);
contentValues.put(DatabaseHelper.Key_Amount, amount);
contentValues.put(DatabaseHelper.Key_Date, date);
long id = db.insert(DatabaseHelper.Table_Name, null ,contentValues);
return id;
}

private static class DatabaseHelper extends SQLiteOpenHelper {
private static final String Database_Name = "Demoze.db";
private static final String Table_Name = "salary";
private static final int Database_Version= 10;
private static final String Key_ID = "_id";
private static final String Key_Name = "name";
private static final String Key_Amount = "amount";
private static final String Key_Date = "date";
private static final String Create_table = "create table "+Table_Name+
" ("+Key_ID+" integer primary key autoincrement, "+Key_Name+
" text, "+Key_Amount+ " real, "+Key_Date+" text)";
private static final String Table_Drop = "drop table if exists "+Table_Name;
private Context context;
public DatabaseHelper(@Nullable Context context) {
super(context, Database_Name, null, Database_Version);
this.context = context;
Toast.makeText(context, "Constructor Called", Toast.LENGTH_SHORT).show();
}
@Override
public void onCreate(SQLiteDatabase db) {
try {
db.execSQL(Create_table);
Toast.makeText(context, "OnCreate Called", Toast.LENGTH_SHORT).show();
} catch (SQLException e){
Toast.makeText(context, ""+e, Toast.LENGTH_SHORT).show();
}
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
try {
Toast.makeText(context, "OnUpgrade Called", Toast.LENGTH_SHORT).show();
db.execSQL(Table_Drop);
onCreate(db);
} catch (SQLException e) {
Toast.makeText(context, "" + e, Toast.LENGTH_SHORT).show();
}
}
}
}

SpendingActivity.Java

package com.example.myapplication;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.EditText;
import android.widget.Toast;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.ItemTouchHelper;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import com.example.myapplication.ui.Names;
import com.example.myapplication.ui.NamesAdapter;
import java.util.ArrayList;
import java.util.List;
public class SpendingActivity extends AppCompatActivity {
DatabaseAdapter databaseAdapter;
EditText crud;
RecyclerView rvPrograms;
NamesAdapter namesAdapter;
RecyclerView.LayoutManager layoutManager;
List<Names> namesList = new ArrayList<>();

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.spending_list);
databaseAdapter = new DatabaseAdapter(this);
namesList = databaseAdapter.getAllNames();
rvPrograms = findViewById(R.id.rvDBdata);
rvPrograms.setHasFixedSize(true);
layoutManager = new LinearLayoutManager(this);
rvPrograms.setLayoutManager(layoutManager);
namesAdapter = new NamesAdapter(this, namesList, rvPrograms);
rvPrograms.setAdapter(namesAdapter);
new ItemTouchHelper(new ItemTouchHelper.SimpleCallback(0, ItemTouchHelper.LEFT | ItemTouchHelper.RIGHT) {
@Override
public boolean onMove(@NonNull RecyclerView recyclerView, @NonNull RecyclerView.ViewHolder viewHolder, @NonNull RecyclerView.ViewHolder target) {
return false;
}
@Override
public void onSwiped(@NonNull RecyclerView.ViewHolder viewHolder, int direction) {
databaseAdapter.removeItem((long) viewHolder.itemView.getTag());
}
}).attachToRecyclerView(rvPrograms);
crud = findViewById(R.id.crudEditText);
String data = databaseAdapter.getAllData();
Toast.makeText(this, data, Toast.LENGTH_LONG).show();
}
public void Home (View view) {
Intent intent = new Intent(this, MainActivity.class);
startActivity(intent);
}
public void Search (View view){
String name = crud.getText().toString().trim();
String result = databaseAdapter.searchData(name);
Toast.makeText(this, result, Toast.LENGTH_SHORT).show();
}

public void Update (View view) {
String amount = crud.getText().toString().trim();
databaseAdapter.updateAmount("taxi", amount);
}
public void Delete (View view) {
String name = crud.getText().toString().trim();
databaseAdapter.deleteData(name);
}
}

Answer 1

在您的NamesAdapter中，您可以公开一个名为deleteItem的方法:

public class NamesAdapter extends RecyclerView.Adapter<NamesAdapter.ViewHolder> {
...
public void deleteItem(int position){
namesList.remove(position);
notifyItemRemoved(position);
}
}

在onSwiped()方法中，您需要调用NameAdapter的deleteItem方法:

@Override
public void onSwiped(@NonNull RecyclerView.ViewHolder viewHolder, int direction) {
int position = viewHolder.getAdapterPosition();
namesAdapter.deleteItem(position);
databaseAdapter.removeItem((long) viewHolder.itemView.getTag());
}

Answer 2

Recycleview adapterclass

public class MyRecyclerViewAdapter extends RecyclerView.Adapter<MyRecyclerViewAdapter.ViewHolder> {
private List<String> mData;
private LayoutInflater mInflater;
private ItemClickListener mClickListener;
// data is passed into the constructor
MyRecyclerViewAdapter(Context context, List<String> data) {
this.mInflater = LayoutInflater.from(context);
this.mData = data;
}
// inflates the row layout from xml when needed
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = mInflater.inflate(R.layout.recyclerview_row, parent, false);
return new ViewHolder(view);
}
// binds the data to the TextView in each row
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
String animal = mData.get(position);
holder.myTextView.setText(animal);
}
// total number of rows
@Override
public int getItemCount() {
return mData.size();
}

// stores and recycles views as they are scrolled off screen
public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
TextView myTextView;
ViewHolder(View itemView) {
super(itemView);
myTextView = itemView.findViewById(R.id.tvAnimalName);
itemView.setOnClickListener(this);
}
@Override
public void onClick(View view) {
if (mClickListener != null) mClickListener.onItemClick(view, getAdapterPosition());
}
}
// convenience method for getting data at click position
String getItem(int id) {
return mData.get(id);
}
// allows clicks events to be caught
void setClickListener(ItemClickListener itemClickListener) {
this.mClickListener = itemClickListener;
}
// parent activity will implement this method to respond to click events
public interface ItemClickListener {
void onItemClick(View view, int position);
}
}

MainActivity

public class MainActivity extends AppCompatActivity implements MyRecyclerViewAdapter.ItemClickListener {
MyRecyclerViewAdapter adapter;
ArrayList<String> animalNames
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// data to populate the RecyclerView with
animalNames = new ArrayList<>();
animalNames.add("Horse");
animalNames.add("Cow");
animalNames.add("Camel");
animalNames.add("Sheep");
animalNames.add("Goat");
// set up the RecyclerView
RecyclerView recyclerView = findViewById(R.id.rvAnimals);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
adapter = new MyRecyclerViewAdapter(this, animalNames);
adapter.setClickListener(this);
recyclerView.setAdapter(adapter);
}
@Override
public void onItemClick(View view, int position) {
Toast.makeText(this, "You clicked " + adapter.getItem(position) + " on row number " + position, Toast.LENGTH_SHORT).show();
animalNames.remove(position);
adapter.notifyItemRemoved(position);
}
}