我一直在寻找如何使用MySQLite数据库中的信息来填充列表视图。现在当我想到我终于找到了一些信息时，我想出了NullPointers。
这是我的代码（HomeFragment.java）：
package zygs.com.seniorproject.fragments;
import android.app.Fragment;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import java.util.ArrayList;
import java.util.List;
import zygs.com.seniorproject.R;
import zygs.com.seniorproject.database.DatabaseHandler;
import zygs.com.seniorproject.database.Inventory;
public class HomeFragment extends Fragment {
// Initializing Variables
Button Item_Button;
Button Display_Button;
EditText Box_Item_Des;
EditText Box_Item_Quant;
EditText Box_Item_ID;
final DatabaseHandler db = new DatabaseHandler(getActivity());
public HomeFragment(){}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
setHasOptionsMenu(true);
final View rootView = inflater.inflate(R.layout.fragment_home, container, false);
ListView listContent = (ListView)rootView.findViewById(R.id.inventory_list);
Item_Button = (Button)rootView.findViewById(R.id.add_item);
Display_Button = (Button)rootView.findViewById(R.id.display_button);
Box_Item_Des = (EditText)rootView.findViewById(R.id.Item_Des);
Box_Item_ID = (EditText)rootView.findViewById(R.id.Item_ID);
Box_Item_Quant = (EditText)rootView.findViewById(R.id.Item_Quant);
Item_Button.setOnClickListener(
new View.OnClickListener() {
public void onClick(View view) {
// Grab Information from Text Boxes
String Item_Des = Box_Item_Des.getText().toString();
int Item_Quantity = Integer.parseInt(Box_Item_Quant.getText().toString());
int Item_ID = Integer.parseInt(Box_Item_ID.getText().toString());
db.addItem(new Inventory(Item_ID, Item_Des, Item_Quantity));
}
}
);
Display_Button.setOnClickListener(
new OnClickListener() {
public void onClick(View view){
Log.d("Reading: ", "Reading all Inventory");
List<Inventory> inventoryList = db.getAllItems();
for (Inventory in : inventoryList) {
String log = "Id: " + in.getItemID() + ", Name: " + in.getItemName() + " , Quantity: " + in.getItemQuant();
//Write List to Log
Log.d("Name:", log);
}
}
}
);
List<Inventory> inv_list = getAllInventory();
ArrayAdapter adapter = new ArrayAdapter(getActivity(), android.R.layout.simple_list_item_1, inv_list);
listContent.setAdapter(adapter);
return rootView;
}
public List<Inventory> getAllInventory() {
List<Inventory> contactList = new ArrayList<Inventory>();
String selectQuery = "SELECT * FROM Inventory";
SQLiteDatabase data = db.getWritableDatabase();
Cursor cursor = data.rawQuery(selectQuery, null);
if (cursor.moveToFirst()) {
do {
Inventory inventory = new Inventory();
inventory.setItemID(Integer.parseInt(cursor.getString(0)));
inventory.setItemName(cursor.getString(1));
inventory.setItemQuant(Integer.parseInt(cursor.getString(2)));
} while (cursor.moveToNext());
}
return contactList;
}
}
我正在使用AndroidHive
中的数据库处理程序以下是代码：
package zygs.com.seniorproject.database;
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;
public class DatabaseHandler extends SQLiteOpenHelper {
//Logcat Tag
private static final String LOG = "DatabaseHelper";
//Database Version
private static final int DATABASE_VERSION = 2;
//Database Name
private static final String DATABASE_NAME = "InventoryManager";
//Tables
private static final String TABLE_INVENTORY = "Inventory";
private static final String TABLE_KITS = "Kits";
// Common Keys
private static final String KEY_ITEM_ID = "item_id";
//Kits Table Columns Name
private static final String KEY_ID = "_id";
private static final String KEY_KIT_ID = "kit_id";
private static final String KEY_KIT_NAME = "kit_name";
//Inventory Tables Columns Names
private static final String KEY_ITEM_NAME = "item_name";
private static final String KEY_ITEM_QUANT = "item_quant";
public DatabaseHandler(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
//Creating Tables Strings
private static final String CREATE_INVENTORY_TABLE = "CREATE TABLE " + TABLE_INVENTORY + "(" + KEY_ITEM_ID + " INTEGER PRIMARY KEY," + KEY_ITEM_NAME + " TEXT," + KEY_ITEM_QUANT + " INTEGER" + ");";
private static final String CREATE_KIT_TABLE = "CREATE TABLE " + TABLE_KITS + "(" + KEY_ID + " AUTOINCREMENT INTEGER PRIMARY KEY," + KEY_KIT_ID + " INTEGER," + KEY_KIT_NAME + " TEXT," + KEY_ITEM_ID + " INTEGER" + ");";
//Create Tables
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(CREATE_INVENTORY_TABLE);
db.execSQL(CREATE_KIT_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_INVENTORY);
// Create Tables again;
onCreate(db);
}
/////////////////////////////////////////
/////// CREATE READ UPDATE DELETE //////
///////////////////////////////////////
//Add a new record
public void addItem(Inventory inventory) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(KEY_ITEM_NAME, inventory.getItemName()); // Item Name
values.put(KEY_ITEM_ID, inventory.getItemID()); // Item ID
values.put(KEY_ITEM_QUANT, inventory.getItemQuant()); //Item Quantity
//Insert Row
db.insert(TABLE_INVENTORY, null, values);
db.close();
}
// Query a single Item
//public Inventory getItemID(int id) {}
//Query All Items
public List<Inventory> getAllItems() {
List<Inventory> inventoryList = new ArrayList<Inventory>();
// Select All Query
String selectQuery = "SELECT * FROM " + TABLE_INVENTORY;
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(selectQuery, null);
//Looping through all rows and adding to list
if (cursor.moveToFirst()) {
do {
Inventory inventory = new Inventory();
inventory.setItemID(Integer.parseInt(cursor.getString(0)));
inventory.setItemName(cursor.getString(1));
inventory.setItemQuant(Integer.parseInt(cursor.getString(2)));
//Adding Inventory to List
inventoryList.add(inventory);
} while (cursor.moveToNext());
}
//Return Inventory List
return inventoryList;
}
// Query Item Count
//public int getItemCount() {}
//Update Single Item
//public int updateItem(Inventory inventory) {}
//Delete Single Item
//public void deleteItem(Inventory inventory){}
}
我得到的错误日志如下：
04-09 13:34:54.187 6393-6393/zygs.com.seniorproject E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: zygs.com.seniorproject, PID: 6393
java.lang.RuntimeException: Unable to start activity ComponentInfo{zygs.com.seniorproject/zygs.com.seniorproject.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2693)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
at android.app.ActivityThread.access$900(ActivityThread.java:177)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:5942)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:223)
at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:163)
at zygs.com.seniorproject.fragments.HomeFragment.getAllInventory(HomeFragment.java:97)
at zygs.com.seniorproject.fragments.HomeFragment.onCreateView(HomeFragment.java:83)
at android.app.Fragment.performCreateView(Fragment.java:2114)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:904)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1082)
at android.app.BackStackRecord.run(BackStackRecord.java:833)
at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1467)
at android.app.Activity.performStart(Activity.java:6328)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2656)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
at android.app.ActivityThread.access$900(ActivityThread.java:177)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:5942)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
感谢您提供的任何帮助。
答案 0 :(得分：9)
行final DatabaseHandler db = new DatabaseHandler(getActivity());在类的顶部声明，因此创建Fragment getActivity（）将返回一个空引用。
如果你在onCreate（）方法中移动它，你将有一个db处理程序可以使用的Activity引用。