尝试在我的Android屏幕上登录时遇到问题。我有两个字段要求用户输入用户名和密码，然后它应该去检查sqlite db如果正确它们将导航到其他屏幕（用户登录）如果不正确他们将通过Toast消息通知。我的问题是当我点击登录按钮我的应用程序显示＆＃34;应用程序数据库意外停止。请再试一次＆＃34;！请有人帮忙!!我的代码发布在下面
UserLogin.java：
public class UserLogin extends Activity {
public static String getName;
public static String getPassword;
EditText name,pwd;
Button login,signup;
DataBaseHandler dataBaseHandler;
SQLiteDatabase db;
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.ulogin);
name = (EditText) findViewById(R.id.name);
pwd = (EditText) findViewById(R.id.pwd);
login = (Button) findViewById(R.id.login);
signup = (Button) findViewById(R.id.signup);
login.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
dataBaseHandler = new DataBaseHandler(getBaseContext());
dataBaseHandler.open();
String getName = name.getText().toString();
String getPassword = pwd.getText().toString();
System.out.println("print something");
if(getName.equals("")||getName==null)
{
Toast.makeText(getBaseContext(), "User Name Empty", Toast.LENGTH_SHORT).show();
System.out.println("User name empty");
}
else if(getPassword.equals("")||getPassword==null)
{
Toast.makeText(getBaseContext(), "Password Empty", Toast.LENGTH_SHORT).show();
System.out.println("Password empty");
}
else
{
try
{
Cursor c = db.rawQuery("SELECT txtUserName,txtPassword FROM mytable WHERE txtUserName= ? AND txtPassword= ?", new String[] {getName, getPassword});
if(c.moveToFirst())
{
Toast.makeText(getBaseContext(), "User Name and Password is Correct", Toast.LENGTH_SHORT).show();
Intent i = new Intent(UserLogin.this, LoggedIn.class);
startActivity(i);
System.out.println("User name and Password correct correct");
}
else
{
Toast.makeText(getBaseContext(), "User Name and Password is not Correct", Toast.LENGTH_SHORT).show();
System.out.println("User name and password is not correct");
}
}
catch(SQLException e)
{
e.printStackTrace();
}
}
dataBaseHandler.close();
}
});
DataBaseHandler.java：
公共类DataBaseHandler {
public static final String NAME = "txtUserName";
public static final String PASSWORD = "txtPassword";
public static final String EMAIL = "email";
public static final String TABLE_NAME = "mytable";
public static final String DATA_BASE_NAME = "mydatabase";
public static final int DATABASE_VERSION = 1;
public static final String TABLE_CREATE = "create table mytable(txtUserName text primary key not null, txtPassword password not null, email text not null);";
private DataBaseHelper dbHelper;
Context ctx;
SQLiteDatabase db;
public DataBaseHandler(Context ctx){
this.ctx = ctx;
dbHelper = new DataBaseHelper(ctx);
}
private static class DataBaseHelper extends SQLiteOpenHelper
{
public DataBaseHelper(Context ctx){
super(ctx,DATA_BASE_NAME,null,DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
try
{
db.execSQL(TABLE_CREATE);
}
catch(SQLException e)
{
e.printStackTrace();
}
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
db.execSQL("DROP TABLE IF EXISTS mytable");
onCreate(db);
}
}
public DataBaseHandler open(){
db = dbHelper.getWritableDatabase();
return this;
}
public void close(){
dbHelper.close();
}
public long insertData(String txtUserName, String txtPassword, String email){
ContentValues content = new ContentValues();
content.put(NAME,txtUserName);
content.put(PASSWORD,txtPassword);
content.put(EMAIL,email);
return db.insertOrThrow(TABLE_NAME, null, content);
}
public String getAllData()
{
db = dbHelper.getReadableDatabase();
Cursor cursor = db.query(TABLE_NAME, new String[] {NAME, PASSWORD, EMAIL}, null, null, null, null, null);
StringBuffer buffer = new StringBuffer();
while(cursor.moveToNext())
{
String name = cursor.getString(0);
String pwd = cursor.getString(1);
String email = cursor.getString(2);
buffer.append(name+" "+pwd+" "+email+""+"\n");
}
return buffer.toString();
}
public Cursor returnData(){
return db.query(TABLE_NAME, new String[] {NAME,PASSWORD,EMAIL}, null, null, null, null, null);
}
}
logcat的：
08-05 10:40:43.165: E/AndroidRuntime(373): FATAL EXCEPTION: main
08-05 10:40:43.165: E/AndroidRuntime(373): java.lang.NullPointerException
08-05 10:40:43.165: E/AndroidRuntime(373): at com.example.databasesplash.UserLogin$1.onClick(UserLogin.java:67)
08-05 10:40:43.165: E/AndroidRuntime(373): at android.view.View.performClick(View.java:2408)
08-05 10:40:43.165: E/AndroidRuntime(373): at android.view.View$PerformClick.run(View.java:8816)
08-05 10:40:43.165: E/AndroidRuntime(373): at android.os.Handler.handleCallback(Handler.java:587)
08-05 10:40:43.165: E/AndroidRuntime(373): at android.os.Handler.dispatchMessage(Handler.java:92)
08-05 10:40:43.165: E/AndroidRuntime(373): at android.os.Looper.loop(Looper.java:123)
08-05 10:40:43.165: E/AndroidRuntime(373): at android.app.ActivityThread.main(ActivityThread.java:4627)
08-05 10:40:43.165: E/AndroidRuntime(373): at java.lang.reflect.Method.invokeNative(Native Method)
08-05 10:40:43.165: E/AndroidRuntime(373): at java.lang.reflect.Method.invoke(Method.java:521)
08-05 10:40:43.165: E/AndroidRuntime(373): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
08-05 10:40:43.165: E/AndroidRuntime(373): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
08-05 10:40:43.165: E/AndroidRuntime(373): at dalvik.system.NativeStart.main(Native Method)
答案 0 :(得分：0)
如果您查看代码，请使用db variable这样的
Cursor c = db.rawQuery("SELECT txtUserName,txtPassword FROM mytable WHERE txtUserName= ? AND txtPassword= ?", new String[] {getName, getPassword});
但你声明它就像
SQLiteDatabase db;
所以db当前值为null，导致应用程序中的NPE
在你的dbhelper class中
public DataBaseHandler open(){
db = dbHelper.getWritableDatabase();
return this;
}
我认为应该是
public SQLiteDatabase open(){
db = dbHelper.getWritableDatabase();
return db;
}
但在你UserLogin Activity的{{1}}中，你可以像这样打电话给
onClick method而不是
dataBaseHandler.open();