我正在创建一个listview活动来从sqlite数据库中检索数据 数据库类为..
package com.example.database;
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.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
public class MyDBHandler extends SQLiteOpenHelper{
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "productDB.db";
public static final String TABLE_PRODUCTS = "products";
public static final String TABLE_STUDENT = "students";
public static final String COLUMN_ID = "_id";
public static final String COLUMN_PRODUCTNAME = "productname";
public static final String COLUMN_QUANTITY = "quantity";
public static final String COLUMN_ROLLNO = "_rollno";
public static final String COLUMN_STUDENTNAME = "studentname";
public static final String COLUMN_CONTACT = "contact";
SQLiteDatabase data=this.getWritableDatabase();
SQLiteDatabase data1 =this.getReadableDatabase();
Product product;
Detail detail;
public MyDBHandler(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
try {
// Create table Place
String CREATE_STUDENTS_TABLE = "CREATE TABLE " +
TABLE_STUDENT + "("
+ COLUMN_ROLLNO + " INTEGER PRIMARY KEY," + COLUMN_STUDENTNAME
+ " TEXT," + COLUMN_CONTACT + " INTEGER" + ");";
db.execSQL(CREATE_STUDENTS_TABLE);
// Create table Reminder
String CREATE_PRODUCTS_TABLE = "CREATE TABLE " +
TABLE_PRODUCTS + "("
+ COLUMN_ID + " INTEGER PRIMARY KEY," + COLUMN_PRODUCTNAME
+ " TEXT," + COLUMN_QUANTITY + " INTEGER" + ");";
db.execSQL(CREATE_PRODUCTS_TABLE);
} catch (SQLException se) {
Log.v("DatabaseHandler Oncreate SQLException",
Log.getStackTraceString(se));
} catch (Exception e) {
Log.v("DatabaseHandler Oncreate Exception",
Log.getStackTraceString(e));
}
Log.d("database","table created");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
try {
// Drop table Place
String placeTableDropQuery = "DROP TABLE IF EXISTS " + TABLE_PRODUCTS;
db.execSQL(placeTableDropQuery);
// Drop table Reminder
String ReminderTableDropQuery = "DROP TABLE IF EXISTS "
+ TABLE_STUDENT;
db.execSQL(ReminderTableDropQuery);
// Upgrade database
onCreate(db);
} catch (SQLException se) {
Log.v("DatabaseHandler onUpgrade SQLException",
Log.getStackTraceString(se));
} catch (Exception e) {
Log.v("DatabaseHandler onUpgrade Exception",
Log.getStackTraceString(e));
}
}
public String addProduct(Product product) {
SQLiteDatabase db = this.getWritableDatabase();
try {
ContentValues values = new ContentValues();
values.put(COLUMN_PRODUCTNAME, product.getProductName());
values.put(COLUMN_QUANTITY, product.getQuantity());
db.insert(TABLE_PRODUCTS, null, values);
db.close();
return "Record insert succussfully...";
} catch (SQLiteException se) {
Log.v("DatabaseHandler insertStudentRecord Exception",
Log.getStackTraceString(se));
return se.getMessage();
} catch (Exception e) {
Log.v("DatabaseHandler insertStudentRecord Exception",
Log.getStackTraceString(e));
return e.getMessage();
} finally{
db.close();
}
}
public String addProduct(Detail product) {
SQLiteDatabase db = this.getWritableDatabase();
try {
ContentValues values = new ContentValues();
values.put(COLUMN_STUDENTNAME, product.getName());
values.put(COLUMN_CONTACT, product.getNo());
db.insert(TABLE_STUDENT, null, values);
db.close();
return "Record insert succussfully...";
} catch (SQLiteException se) {
Log.v("DatabaseHandler insertTeacherRecord Exception",
Log.getStackTraceString(se));
return se.getMessage();
} catch (Exception e) {
Log.v("DatabaseHandler insertTeacherRecord Exception",
Log.getStackTraceString(e));
return e.getMessage();
} finally{
db.close();
}
}
public Product findProduct(String productname) {
String query = "Select * FROM " + TABLE_PRODUCTS + " WHERE " + COLUMN_PRODUCTNAME + " = \"" + productname + "\"";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(query, null);
Product product = new Product();
if (cursor.moveToFirst()) {
cursor.moveToFirst();
product.setID(Integer.parseInt(cursor.getString(0)));
product.setProductName(cursor.getString(1));
product.setQuantity(Integer.parseInt(cursor.getString(2)));
cursor.close();
} else {
product = null;
}
db.close();
return product;
}
public Detail findDetail(int studentname) {
String query = "Select * FROM " + TABLE_STUDENT + " WHERE " + COLUMN_ROLLNO + " = \"" + studentname + "\"";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(query, null);
Detail detail = new Detail();
if (cursor.moveToFirst()) {
cursor.moveToFirst();
detail.setID(Integer.parseInt(cursor.getString(0)));
detail.setName(cursor.getString(1));
detail.setNo(Integer.parseInt(cursor.getString(2)));
cursor.close();
} else {
detail= null;
}
db.close();
return detail;
}
public boolean deleteProduct(String productname) {
boolean result = false;
String query = "Select * FROM " + TABLE_PRODUCTS + " WHERE " + COLUMN_PRODUCTNAME + " = \"" + productname + "\"";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(query, null);
Product product = new Product();
if (cursor.moveToFirst()) {
product.setID(Integer.parseInt(cursor.getString(0)));
db.delete(TABLE_PRODUCTS, COLUMN_ID + " = ?",
new String[] { String.valueOf(product.getID()) });
cursor.close();
result = true;
}
db.close();
return result;
}
public boolean deleteDetail(String studentname) {
boolean result = false;
String query = "Select * FROM " + TABLE_STUDENT + " WHERE " + COLUMN_STUDENTNAME + " = \"" + studentname + "\"";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(query, null);
Detail detail = new Detail();
if (cursor.moveToFirst()) {
detail.setID(Integer.parseInt(cursor.getString(0)));
db.delete(TABLE_STUDENT, COLUMN_ROLLNO + " = ?",
new String[] { String.valueOf(detail.getID()) });
cursor.close();
result = true;
}
db.close();
return result;
}
}
创建具有按钮的活动，并在按钮上单击mylistactivity显示 活动为......
package com.example.database;
import java.util.ArrayList;
import com.example.database.R.string;
import android.app.ListActivity;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.widget.ArrayAdapter;
public class MyListActivity extends ListActivity{
ArrayAdapter<String> adapter;
MyDBHandler db;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
retrive();
setListAdapter(adapter);
db =new MyDBHandler(MyListActivity.this);
}
private void retrive() {
ArrayList<String>namelist= new ArrayList<String>();
int ii;
//ArrayList<String>studentno = new ArrayList<String>();
SQLiteDatabase sd = db.getReadableDatabase();
Cursor cursor = sd.query(db.TABLE_STUDENT, null, null, null, null, null, null);
ii=cursor.getColumnIndex("studentname");
while (cursor.moveToNext()){
namelist.add(cursor.getString(ii));
//studentno.add(cursor.getString(1));
}
ArrayList<String>combimelist=new ArrayList<String>();
for(int i = 0; i<namelist.size();i++){
combimelist.add(namelist.get(i));
}
adapter = new ArrayAdapter<String>(MyListActivity.this,R.layout.mylist, combimelist);
}
}
和布局将其显示为
main.xml中
<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:padding="15dp"
android:textSize="25dp"
android:layout_marginBottom="25dp">
</TextView>
它显示错误为..
05-21 05:08:53.823: I/Choreographer(1887): Skipped 34 frames! The application may be doing too much work on its main thread.
05-21 05:08:54.043: D/AndroidRuntime(1887): Shutting down VM
05-21 05:08:54.043: W/dalvikvm(1887): threadid=1: thread exiting with uncaught exception (group=0x41465700)
05-21 05:08:54.363: E/AndroidRuntime(1887): FATAL EXCEPTION: main
05-21 05:08:54.363: E/AndroidRuntime(1887): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.database/com.example.database.MyListActivity}: java.lang.NullPointerException
05-21 05:08:54.363: E/AndroidRuntime(1887): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
05-21 05:08:54.363: E/AndroidRuntime(1887): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
05-21 05:08:54.363: E/AndroidRuntime(1887): at android.app.ActivityThread.access$600(ActivityThread.java:141)
05-21 05:08:54.363: E/AndroidRuntime(1887): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
05-21 05:08:54.363: E/AndroidRuntime(1887): at android.os.Handler.dispatchMessage(Handler.java:99)
05-21 05:08:54.363: E/AndroidRuntime(1887): at android.os.Looper.loop(Looper.java:137)
05-21 05:08:54.363: E/AndroidRuntime(1887): at android.app.ActivityThread.main(ActivityThread.java:5103)
05-21 05:08:54.363: E/AndroidRuntime(1887): at java.lang.reflect.Method.invokeNative(Native Method)
05-21 05:08:54.363: E/AndroidRuntime(1887): at java.lang.reflect.Method.invoke(Method.java:525)
05-21 05:08:54.363: E/AndroidRuntime(1887): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
05-21 05:08:54.363: E/AndroidRuntime(1887): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
05-21 05:08:54.363: E/AndroidRuntime(1887): at dalvik.system.NativeStart.main(Native Method)
05-21 05:08:54.363: E/AndroidRuntime(1887): Caused by: java.lang.NullPointerException
05-21 05:08:54.363: E/AndroidRuntime(1887): at com.example.database.MyListActivity.retrive(MyListActivity.java:30)
05-21 05:08:54.363: E/AndroidRuntime(1887): at com.example.database.MyListActivity.onCreate(MyListActivity.java:21)
05-21 05:08:54.363: E/AndroidRuntime(1887): at android.app.Activity.performCreate(Activity.java:5133)
05-21 05:08:54.363: E/AndroidRuntime(1887): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-21 05:08:54.363: E/AndroidRuntime(1887): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
05-21 05:08:54.363: E/AndroidRuntime(1887): ... 11 more
答案 0 :(得分：2)
首先初始化 db ，然后调用 retrive（）方法：
替换：
retrive();
setListAdapter(adapter);
db =new MyDBHandler(MyListActivity.this);
要：
db =new MyDBHandler(MyListActivity.this);
retrive();
setListAdapter(adapter);
答案 1 :(得分：0)
可能有帮助
package com.src.sqlite.list;
public class Contact {
// private variables
int _id;
String _name;
byte[] _image;
// Empty constructor
public Contact() {
}
// constructor
public Contact(int keyId, String name, byte[] image) {
this._id = keyId;
this._name = name;
this._image = image;
}
// constructor
public Contact(String contactID, String name, byte[] image) {
this._name = name;
this._image = image;
}
// constructor
public Contact(String name, byte[] image) {
this._name = name;
this._image = image;
}
// getting ID
public int getID() {
return this._id;
}
// setting id
public void setID(int keyId) {
this._id = keyId;
}
// getting name
public String getName() {
return this._name;
}
// setting name
public void setName(String name) {
this._name = name;
}
// getting phone number
public byte[] getImage() {
return this._image;
}
// setting phone number
public void setImage(byte[] image) {
this._image = image;
}
}
//////////////////////////////////////////////////////////////////////////////
package com.src.sqlite.list;
import java.io.ByteArrayInputStream;
import java.util.ArrayList;
import com.manish.sqlite.R;
import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;
public class ContactImageAdapter extends ArrayAdapter<Contact>{
Context context;
int layoutResourceId;
// BcardImage data[] = null;
ArrayList<Contact> data=new ArrayList<Contact>();
public ContactImageAdapter(Context context, int layoutResourceId, ArrayList<Contact> data) {
super(context, layoutResourceId, data);
this.layoutResourceId = layoutResourceId;
this.context = context;
this.data = data;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View row = convertView;
ImageHolder holder = null;
if(row == null)
{
LayoutInflater inflater = ((Activity)context).getLayoutInflater();
row = inflater.inflate(layoutResourceId, parent, false);
holder = new ImageHolder();
holder.txtTitle = (TextView)row.findViewById(R.id.txtTitle);
holder.imgIcon = (ImageView)row.findViewById(R.id.imgIcon);
row.setTag(holder);
}
else
{
holder = (ImageHolder)row.getTag();
}
Contact picture = data.get(position);
holder.txtTitle.setText(picture._name);
//convert byte to bitmap take from contact class
byte[] outImage=picture._image;
ByteArrayInputStream imageStream = new ByteArrayInputStream(outImage);
Bitmap theImage = BitmapFactory.decodeStream(imageStream);
holder.imgIcon.setImageBitmap(theImage);
return row;
}
static class ImageHolder
{
ImageView imgIcon;
TextView txtTitle;
}
}
///////////////////////////////////////////////
package com.src.sqlite.list;
import java.util.ArrayList;
import java.util.List;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DataBaseHandler extends SQLiteOpenHelper {
// All Static variables
// Database Version
private static final int DATABASE_VERSION = 1;
// Database Name
private static final String DATABASE_NAME = "imagedb";
// Contacts table name
private static final String TABLE_CONTACTS = "contacts";
// Contacts Table Columns names
private static final String KEY_ID = "id";
private static final String KEY_NAME = "name";
private static final String KEY_IMAGE = "image";
public DataBaseHandler(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
// Creating Tables
@Override
public void onCreate(SQLiteDatabase db) {
String CREATE_CONTACTS_TABLE = "CREATE TABLE " + TABLE_CONTACTS + "("
+ KEY_ID + " INTEGER PRIMARY KEY," + KEY_NAME + " text not null UNIQUE,"
+ KEY_IMAGE + " BLOB" + ")";
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_CONTACTS);
// Create tables again
onCreate(db);
}
/**
* All CRUD(Create, Read, Update, Delete) Operations
*/
public// Adding new contact
void addContact(Contact contact) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(KEY_NAME, contact._name); // Contact Name
values.put(KEY_IMAGE, contact._image); // Contact Phone
// Inserting Row
db.insert(TABLE_CONTACTS, null, values);
db.close(); // Closing database connection
}
// Getting single contact
Contact getContact(int id) {
SQLiteDatabase db = this.getReadableDatabase();
Cursor cursor = db.query(TABLE_CONTACTS, new String[] { KEY_ID,
KEY_NAME, KEY_IMAGE }, KEY_ID + "=?",
new String[] { String.valueOf(id) }, null, null, null, null);
if (cursor != null)
cursor.moveToFirst();
Contact contact = new Contact(Integer.parseInt(cursor.getString(0)),
cursor.getString(1), cursor.getBlob(1));
// return contact
return contact;
}
// Getting All Contacts
public List<Contact> getAllContacts() {
List<Contact> contactList = new ArrayList<Contact>();
// Select All Query
String selectQuery = "SELECT * FROM contacts ORDER BY name";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(selectQuery, null);
// looping through all rows and adding to list
if (cursor.moveToFirst()) {
do {
Contact contact = new Contact();
contact.setID(Integer.parseInt(cursor.getString(0)));
contact.setName(cursor.getString(1));
contact.setImage(cursor.getBlob(2));
// Adding contact to list
contactList.add(contact);
} while (cursor.moveToNext());
}
// close inserting data from database
db.close();
// return contact list
return contactList;
}
// Updating single contact
public int updateContact(Contact contact) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(KEY_NAME, contact.getName());
values.put(KEY_IMAGE, contact.getImage());
// updating row
return db.update(TABLE_CONTACTS, values, KEY_ID + " = ?",
new String[] { String.valueOf(contact.getID()) });
}
// Deleting single contact
public void deleteContact(Contact contact) {
SQLiteDatabase db = this.getWritableDatabase();
db.delete(TABLE_CONTACTS, KEY_ID + " = ?",
new String[] { String.valueOf(contact.getID()) });
db.close();
}
// Getting contacts Count
public int getContactsCount() {
String countQuery = "SELECT * FROM " + TABLE_CONTACTS;
SQLiteDatabase db = this.getReadableDatabase();
Cursor cursor = db.rawQuery(countQuery, null);
cursor.close();
// return count
return cursor.getCount();
}
}
/////////////////////////////////////////////
package com.src.sqlite.list;
import java.io.ByteArrayOutputStream;
import java.util.ArrayList;
import java.util.List;
import com.manish.sqlite.R;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.util.Log;
import android.widget.ListView;
public class SQLiteDemoActivity extends Activity {
ArrayList<Contact> imageArry = new ArrayList<Contact>();
ContactImageAdapter adapter;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
DataBaseHandler db = new DataBaseHandler(this);
// get image from drawable
Bitmap image = BitmapFactory.decodeResource(getResources(),
R.drawable.facebook);
// convert bitmap to byte
ByteArrayOutputStream stream = new ByteArrayOutputStream();
image.compress(Bitmap.CompressFormat.JPEG, 100, stream);
byte imageInByte[] = stream.toByteArray();
/**
* CRUD Operations
* */
// Inserting Contacts
Log.d("Insert: ", "Inserting ..");
db.addContact(new Contact("FaceBook", imageInByte));
// display main List view bcard and contact name
// Reading all contacts from database
List<Contact> contacts = db.getAllContacts();
for (Contact cn : contacts) {
String log = "ID:" + cn.getID() + " Name: " + cn.getName()
+ " ,Image: " + cn.getImage();
// Writing Contacts to log
Log.d("Result: ", log);
//add contacts data in arrayList
imageArry.add(cn);
}
adapter = new ContactImageAdapter(this, R.layout.screen_list,
imageArry);
ListView dataList = (ListView) findViewById(R.id.list);
dataList.setAdapter(adapter);
}
}
//////////////////////////////////
main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<ListView
android:id="@+id/list"
android:layout_width="fill_parent"
android:layout_height="0dp"
android:layout_weight="0.55" >
</ListView>
</LinearLayout>
//////////////////////////////////
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="horizontal"
android:padding="10dp" >
<ImageView
android:id="@+id/imgIcon"
android:layout_width="0dp"
android:layout_height="100dp"
android:layout_weight="0.71"
android:gravity="center_vertical" />
<TextView
android:id="@+id/txtTitle"
android:layout_width="80dp"
android:layout_height="fill_parent"
android:gravity="center_vertical"
android:textSize="14dp"
android:layout_marginLeft="7dp" />
</LinearLayout>