我正在關注本教程（https://www.youtube.com/watch?v=hDSVInZ2JCs&t=3615s&ab_channel=SoftwareandProgrammingwithProfessorSluiter）
一切都很好，直到我嘗試這行代碼：
lv_productList.setAdapter(productArrayAdapter);
然后應用程式崩潰。我在網上查看并嘗試了很多東西，但我被卡住了
這是問題區域
btn_viewAll.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
DataBaseHelper dataBaseHelper = new DataBaseHelper(MainActivity.this);
List<ProductModel> all = dataBaseHelper.getAll();
ArrayAdapter productArrayAdapter = new ArrayAdapter<ProductModel>(MainActivity.this, android.R.layout.simple_list_item_1, all);
lv_productList.setAdapter(productArrayAdapter);
Toast.makeText(MainActivity.this, all.toString(), Toast.LENGTH_SHORT).show();
}
});
這是我的完整代碼
主活動.java
package com.example.sqldemo;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import java.util.List;
public class MainActivity extends AppCompatActivity {
//reference to all buttons and controls on the layout
Button btn_add, btn_viewAll;
EditText et_name, et_number;
ListView lv_productList;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btn_add = findViewById(R.id.btn_add);
btn_viewAll = findViewById(R.id.btn_viewAll);
et_name = findViewById(R.id.et_name);
et_number = findViewById(R.id.et_number);
btn_add.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
ProductModel productModel;
try {
productModel = new ProductModel(-1, et_name.getText().toString(), Integer.parseInt(et_number.getText().toString()));
Toast.makeText(MainActivity.this, productModel.toString(), Toast.LENGTH_SHORT).show();
}
catch(Exception err) {
Toast.makeText(MainActivity.this, "Error creating product", Toast.LENGTH_SHORT).show();
productModel = new ProductModel(-1, "error", 0);
}
DataBaseHelper dataBaseHelper = new DataBaseHelper(MainActivity.this);
boolean success = dataBaseHelper.addOne(productModel);
Toast.makeText(MainActivity.this, "Success! Product has been added to the database :)", Toast.LENGTH_SHORT).show();
}
});
btn_viewAll.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
DataBaseHelper dataBaseHelper = new DataBaseHelper(MainActivity.this);
List<ProductModel> all = dataBaseHelper.getAll();
ArrayAdapter productArrayAdapter = new ArrayAdapter<ProductModel>(MainActivity.this, android.R.layout.simple_list_item_1, all);
lv_productList.setAdapter(productArrayAdapter);
Toast.makeText(MainActivity.this, all.toString(), Toast.LENGTH_SHORT).show();
}
});
}
}
資料庫助手
package com.example.sqldemo;
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 PRODUCT_TABLE = "PRODUCT_TABLE";
public static final String COLUMN_PRODUCT_NAME = "PRODUCT_NAME";
public static final String COLUMN_PRODUCT_NUMBER = "PRODUCT_AGE";
public static final String COLUMN_ID = "ID";
public DataBaseHelper(@Nullable Context context) {
super(context, "Products.db", null, 1);
}
//this is called the first item a database is accessed. used to create a new database
@Override
public void onCreate(SQLiteDatabase db) {
String createTableStatement = "CREATE TABLE " PRODUCT_TABLE " (" COLUMN_ID " INTEGER PRIMARY KEY AUTOINCREMENT, " COLUMN_PRODUCT_NAME " TEXT, " COLUMN_PRODUCT_NUMBER " INT)";
db.execSQL(createTableStatement);
}
//this is called if the database version number changes. It prevents previous users apps from breaking when the database is updated
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
public boolean addOne(ProductModel productModel) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues cv = new ContentValues();
cv.put(COLUMN_PRODUCT_NAME, productModel.getName());
cv.put(COLUMN_PRODUCT_NUMBER, productModel.getNumber());
//long insert returns positive in data insertion was successful, and negative if it was not
long insert = db.insert(PRODUCT_TABLE, null, cv);
if (insert == -1) {
return false;
}
else {
return true;
}
}
public List<ProductModel> getAll() {
List<ProductModel> returnList = new ArrayList<>();
// get data from the database
String queryString = "SELECT * FROM " PRODUCT_TABLE;
SQLiteDatabase db = this.getReadableDatabase();
Cursor cursor = db.rawQuery(queryString, null);
//move the cursor to the first set of data and check if anything was returned
if (cursor.moveToFirst()) {
// loop through the cursor (result set) and create new product objects. Put them into the return list (defined above)
do {
int productID = cursor.getInt(0);
String productName = cursor.getString(1);
int productNumber = cursor.getInt(2);
ProductModel newProduct = new ProductModel(productID, productName, productNumber);
returnList.add(newProduct);
} while (cursor.moveToNext());
}
else {
//if no results from database, don't add anything to the list.
}
cursor.close();
db.close();
return returnList;
}
}
產品模型.java
package com.example.sqldemo;
//declare a new public class/object with 3 variables
public class ProductModel {
private int id;
private String name;
private int number;
//constructor
public ProductModel(int id, String name, int number) {
this.id = id;
this.name = name;
this.number = number;
}
public ProductModel() {
}
// toString is necessary for printing the contents of a class object
@Override
public String toString() {
return "ProductModel{"
"id=" id
", name='" name '\''
", number=" number
'}';
}
//getters and setters
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getNumber() {
return number;
}
public void setNumber(int number) {
this.number = number;
}
}
> Blockquote
uj5u.com熱心網友回復：
您可以簡單地將 RecyclerView 更改為 ListView。
或者
如果你想使用 RecyclerView，你必須創建一個自定義的 Recyclerview Adapter。這里是初學者友好指南：https : //medium.com/@suhanshupatel/recyclerview-in-android-for-beginner-cfbc132a0dec
轉載請註明出處，本文鏈接：https://www.uj5u.com/ruanti/360111.html