如何在安卓工作室中使用recyclerview显示我的数据库

Question

我不知道自己做错了什么。我准备从listview迁移到recyclerview，并遵循语法，然后它给了我的错误

"元素==空"；从我的构建输出

这是我的Databasehelper文件：

package com.revise;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import androidx.annotation.Nullable;
import java.util.ArrayList;
import java.util.List;
public class DataBaseHelper extends SQLiteOpenHelper {
public static final String  NAME_TABLE = "NAME_TABLE";
public static final String  COL_ID = "COLUMN_ID";
public static final String  COLUMN_NAME = "COLUMN_NAME";
public static final String  COLUMN_PRICE = "COLUMN_PRICE";
public static final String  COLUMN_QUANTITY = "COLUMN_QUANTITY";
public static final String  COLUMN_TOTAL = "COLUMN_TOTAL";
//private static final int DATABASE_VERSION = 2;

public DataBaseHelper(@Nullable Context context) {
super(context, "item.db", null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
String createTableStatement = "CREATE TABLE " + NAME_TABLE + "("
+ COL_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
+ COLUMN_NAME + " TEXT, "
+ COLUMN_PRICE + " FLOAT, "
+ COLUMN_QUANTITY + " INT, "
+ COLUMN_TOTAL + " FLOAT)";
db.execSQL(createTableStatement);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS "+NAME_TABLE);
onCreate(db);
}
public boolean addOne(Product product){
SQLiteDatabase db = this.getWritableDatabase();
ContentValues cv = new ContentValues();
cv.put(COLUMN_NAME, product.getName());
cv.put(COLUMN_PRICE, product.getPrice());
cv.put(COLUMN_QUANTITY, product.getQuantity());
cv.put(COLUMN_TOTAL, product.getTotal());
long insert = db.insert(NAME_TABLE, null, cv);
if(insert==-1){
return false;
}
else{ return true; }
}
public boolean deleteOne(Product product){
SQLiteDatabase db = this.getWritableDatabase();
String queryString = "DELETE FROM "+NAME_TABLE+" WHERE " +COLUMN_NAME+ "=" +product.getName();
Cursor cursor = db.rawQuery(queryString, null);
if(cursor.moveToFirst()){
return true;
}else { return false; }
}
public List<Product> getEveryone(){
List<Product> returnList = new ArrayList<>();
String queryString = "SELECT * FROM " + NAME_TABLE;
SQLiteDatabase db = this.getReadableDatabase();
Cursor cursor = db.rawQuery(queryString, null);
if(cursor.moveToFirst()){
do{
String name = cursor.getString(1);
float price = cursor.getFloat(2);
int quantity = cursor.getInt(3);
Product newProduct = new Product(name, price, quantity);
returnList.add(newProduct);
}while(cursor.moveToNext());
}
cursor.close();
db.close();
return returnList;
}
}

这是我的RecyclerViewAdapter:

package com.revise;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
import java.util.List;
public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.ViewHolder>{
List<Product> productList;
Context context;
public RecyclerViewAdapter(List<Product> productList, Context context) {
this.productList = productList;
this.context = context;
}
@NonNull
@Override
public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
//itemlayout is the name of the layout that we are gonna be using
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.itemlayout,parent,false);
ViewHolder holder = new ViewHolder(view);
return holder;
}
@Override
public void onBindViewHolder(@NonNull RecyclerViewAdapter.ViewHolder holder, int position) {
holder.tv_name.setText(productList.get(position).getName());//tv_name
holder.tv_price.setText(String.valueOf(productList.get(position).getPrice()));
holder.tv_qty.setText(String.valueOf(productList.get(position).getQuantity()));
holder.tv_total.setText(String.valueOf(productList.get(position).getTotal()));
}
@Override
public int getItemCount() {
return productList.size();
}
public class ViewHolder extends  RecyclerView.ViewHolder{
//variables from my itemlayout
TextView tv_name, tv_price, tv_qty, tv_total;
public ViewHolder(@NonNull View itemView) {
super(itemView);
tv_name = itemView.findViewById(R.id.tv_name);
tv_price = itemView.findViewById(R.id.tv_price);
tv_qty = itemView.findViewById(R.id.tv_qty);
tv_total = itemView.findViewById(R.id.tv_total);
}
}
}

配置按钮的活动：

package com.revise;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class DataActivity extends AppCompatActivity {
Button bt_add;
EditText et_name, et_price, et_qty;
DataBaseHelper myDb;
List<Product> productList = new ArrayList<Product>();
private RecyclerView recyclerView;
private RecyclerView.Adapter mAdapter;
private RecyclerView.LayoutManager layoutManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

bt_add = findViewById(R.id.bt_add);
et_name = findViewById(R.id.et_name);
et_price = findViewById(R.id.et_price);
et_qty = findViewById(R.id.et_qty);
//lv_itemlist = findViewById(R.id.lv_itemlist);

myDb = new DataBaseHelper(DataActivity.this);
fillProductList(myDb);
bt_add.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Product p = new Product();
try {
p.setName(et_name.getText().toString());
p.setPrice(Float.parseFloat(et_price.getText().toString()));
p.setQuantity(Integer.parseInt(et_qty.getText().toString()));
} catch (Exception e){
Toast.makeText(DataActivity.this,"Exception:"+e,Toast.LENGTH_SHORT).show();
}
DataBaseHelper dataBaseHelper = new DataBaseHelper(DataActivity.this);
dataBaseHelper.addOne(p);
}
});
recyclerView = findViewById(R.id.lv_itemlist);
recyclerView.setHasFixedSize(true);
layoutManager = new LinearLayoutManager(DataActivity.this);
recyclerView.setLayoutManager(layoutManager);
mAdapter = new RecyclerViewAdapter(productList, DataActivity.this);
recyclerView.setAdapter(mAdapter);

}
private void fillProductList(DataBaseHelper myDb) {
productList.addAll(myDb.getEveryone());
}
}

它对应的xml文件：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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="match_parent"
tools:context=".DataActivity">
<EditText
android:id="@+id/et_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="32dp"
android:ems="10"
android:inputType="textPersonName"
android:text="Name"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<EditText
android:id="@+id/et_price"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:ems="10"
android:inputType="numberDecimal"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/et_name" />
<EditText
android:id="@+id/et_qty"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:ems="10"
android:inputType="number"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/et_price" />
<Button
android:id="@+id/bt_add"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="12dp"
android:text="Button"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.189"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/et_qty" />
<androidx.recyclerview.widget.RecyclerView
android:id="@+id/lv_itemlist"
android:layout_width="384dp"
android:layout_height="456dp"
android:layout_marginTop="16dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/bt_add" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

我注意到的一件事是，您的mAdapter的类型是RecyclerView.Adapter而不是RecyclerView Adapter。我建议你看一堆关于与回收商合作的视频和文章。同时开始使用Androids Room库。这是使用SQLite的推荐方法。它有一个代码实验室，我相信叫做"看得见风景的房间"。