java - 如何在 android 中使用 SQLite 扩展类在数据库中插入值?
问题描述
我正在尝试制作一辆购物车来研究 SQLite 数据库。当我通过调用函数将数据插入数据库时addProduct()
,我的应用程序将关闭。我不知道为什么应用程序正在关闭,我认为问题出在我的函数中addProduct()
。我正在从片段中调用此函数。任何帮助,将不胜感激!
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import java.util.ArrayList;
import java.util.List;
import androidx.annotation.Nullable;
//database functions class
public class db_helper extends SQLiteOpenHelper {
private static final String product_name = "product_name";
private static final String id = "id";
private static final String price = "price";
private static final String count = "count";
private static final String total = "total";
private static final String img = "img";
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "user";
private static final String TABLE_ = "cart";
public db_helper(@Nullable Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
// Creating Tables
@Override
public void onCreate(SQLiteDatabase db) {
String CREATE_CONTACTS_TABLE = "CREATE TABLE " + TABLE_ + "("
+ id + " TEXT," + product_name + " TEXT,"
+ price + " TEXT," +count + " INTEGER," +total + " TEXT," + img + " TEXT" +")";
db.execSQL(CREATE_CONTACTS_TABLE);
}
// Upgrading database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// Drop older table if existed
db.execSQL("DROP TABLE IF EXISTS " + TABLE_);
// Create tables again
onCreate(db);
}
// code to add the new contact
public void addProduct(cartmodal modal) {
SQLiteDatabase db = getWritableDatabase();
ContentValues values = new ContentValues();
values.put(id, modal.getId());
values.put(product_name, modal.getProduct_name());
values.put(count, modal.getCount());
values.put(total, modal.getTotal());
values.put(price, modal.getPrice());
values.put(img, modal.getImg());
// Inserting Row
db.insert(TABLE_, null, values);
//2nd argument is String containing nullColumnHack
db.close(); // Closing database connection
}
public List<cartmodal> getAlldata() {
SQLiteDatabase db = this.getReadableDatabase();
List<cartmodal> List = new ArrayList<cartmodal>();
// Select All Query
String selectQuery = "SELECT * FROM " + TABLE_;
// SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(selectQuery, null);
// looping through all rows and adding to list
if (cursor.moveToFirst()) {
do {
cartmodal modal = new cartmodal();
modal.setId(cursor.getString(0));
modal.setProduct_name(cursor.getString(1));
modal.setPrice(cursor.getString(2));
modal.setCount(Integer.parseInt(cursor.getString(3)));
modal.setTotal(cursor.getString(4));
modal.setImg(cursor.getString(5));
// Adding contact to list
List.add(modal);
} while (cursor.moveToNext());
}
// return contact list
return List;
}
public void updateCount(cartmodal modal) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(id, modal.getId());
values.put(product_name, modal.getProduct_name());
values.put(count, modal.getCount());
values.put(total, modal.getTotal());
values.put(price, modal.getPrice());
values.put(img, modal.getImg());
// updating row
db.update(TABLE_, values, id + " = ?",
new String[] { String.valueOf(modal.getId()) });
}
public void deleteproduct(String id_){
SQLiteDatabase db = this.getWritableDatabase();
db.delete(TABLE_,id+" =?",new String[]{id_});
}
}
错误日志
V/FA: Inactivity, disconnecting from the service
D/ViewRootImpl@d62e4b3[Home]: ViewPostIme pointer 0
D/ViewRootImpl@d62e4b3[Home]: ViewPostIme pointer 1
D/ViewRootImpl@d62e4b3[Home]: ViewPostIme pointer 0
D/ViewRootImpl@d62e4b3[Home]: ViewPostIme pointer 1
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.firstprinciple.white, PID: 17749
java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getDatabasePath(java.lang.String)' on a null object reference
at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:445)
at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:391)
at com.firstprinciple.white.database_sql.db_helper.addProduct(db_helper.java:55)
at com.firstprinciple.white.ui.home.HomeFragment$12$4.onClick(HomeFragment.java:622)
at android.view.View.performClick(View.java:7870)
at android.widget.TextView.performClick(TextView.java:14970)
at android.view.View.performClickInternal(View.java:7839)
at android.view.View.access$3600(View.java:886)
at android.view.View$PerformClick.run(View.java:29363)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:237)
at android.app.ActivityThread.main(ActivityThread.java:7814)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1075)
I/Process: Sending signal. PID: 17749 SIG: 9
以上是error stacktrace
我似乎无法解决这个问题。
解决方案
交换这条线
public db_helper(@Nullable Context context)
对此
public db_helper(@NotNull Context context)
并适当地传递Context
给它。Context
不能为空SQLiteDatabase
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