我有一个需要登录和注册SQLite的应用程序。我有数据库，用户可以登录和注册。但我希望用户的用户名显示在textview中的下一个活动上。当我尝试从数据库中收集用户名时，应用程序崩溃，并说我为textview所做的变量中没有任何内容。
这是主要活动（登录）：
public class MainActivity extends AppCompatActivity {
Button btnSignIn,btnSignUp;
LoginDataBaseAdapter loginDataBaseAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
loginDataBaseAdapter=new LoginDataBaseAdapter(this);
try {
loginDataBaseAdapter=loginDataBaseAdapter.open();
} catch (SQLException e) {
e.printStackTrace();
}
btnSignIn=(Button)findViewById(R.id.buttonSignIn);
btnSignUp=(Button)findViewById(R.id.buttonSignUP);
btnSignUp.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
// TODO Auto-generated method stub
/// Create Intent for SignUpActivity abd Start The Activity
Intent intentSignUP=new Intent(getApplicationContext(),SignUp.class);
startActivity(intentSignUP);
}
});
}
public void signIn(View V)
{
// get the Refferences of views
final EditText editTextUserName=(EditText)findViewById(R.id.editTextUserNameToLogin);
final EditText editTextPassword=(EditText)findViewById(R.id.editTextPasswordToLogin);
Button btnSignIn=(Button)findViewById(R.id.buttonSignIn);
// Set On ClickListener
btnSignIn.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
// get The User name and Password
String userName = editTextUserName.getText().toString();
String password = editTextPassword.getText().toString();
// fetch the Password form database for respective user name
String storedPassword = loginDataBaseAdapter.getSinlgeEntry(userName);
// check if the Stored password matches with Password entered by user
if (password.equals(storedPassword)) {
Toast.makeText(MainActivity.this, "Congrats: Login Successfull" + userName, Toast.LENGTH_LONG).show();
startProfileActivity();
} else {
Toast.makeText(MainActivity.this, "User Name or Password does not match", Toast.LENGTH_LONG).show();
}
}
});
}
@Override
protected void onDestroy() {
super.onDestroy();
// Close The Database
loginDataBaseAdapter.close();
}
public void startProfileActivity() {
Intent intent = new Intent(this, ProfileActivity.class);
startActivity(intent);
}
这是第二个活动（textview应更改为用户名）：
public class ProfileActivity extends AppCompatActivity {
LoginDataBaseAdapter loginDataBaseAdapter;
//TextView profileName;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_profile);
TextView profileName = (TextView) findViewById(R.id.profileNameTextView);
String userName = loginDataBaseAdapter.getUserName("USERNAME");
String user = loginDataBaseAdapter.getUserName(userName);
profileName.setText("hello" + user);
loginDataBaseAdapter=new LoginDataBaseAdapter(this);
try {
loginDataBaseAdapter=loginDataBaseAdapter.open();
} catch (SQLException e) {
e.printStackTrace();
}
}
这是数据库助手：
public class LoginDataBaseAdapter {
static final String DATABASE_NAME = "login.db";
static final int DATABASE_VERSION = 1;
public static final int NAME_COLUMN = 1;
// TODO: Create public field for each column in your table.
// SQL Statement to create a new database.
static final String DATABASE_CREATE = "create table "+"LOGIN"+
"( " +"ID"+" integer primary key autoincrement,"+ "USERNAME text,PASSWORD text); ";
// Variable to hold the database instance
public SQLiteDatabase db;
// Context of the application using the database.
private final Context context;
// Database open/upgrade helper
private DataBaseHelper dbHelper;
public LoginDataBaseAdapter(Context _context)
{
context = _context;
dbHelper = new DataBaseHelper(context, DATABASE_NAME, null, DATABASE_VERSION);
}
public LoginDataBaseAdapter open() throws SQLException
{
db = dbHelper.getWritableDatabase();
return this;
}
public void close()
{
db.close();
}
public SQLiteDatabase getDatabaseInstance()
{
return db;
}
public void insertEntry(String userName,String password)
{
ContentValues newValues = new ContentValues();
// Assign values for each row.
newValues.put("USERNAME", userName);
newValues.put("PASSWORD",password);
// Insert the row into your table
db.insert("LOGIN", null, newValues);
///Toast.makeText(context, "Reminder Is Successfully Saved", Toast.LENGTH_LONG).show();
}
public int deleteEntry(String UserName)
{
//String id=String.valueOf(ID);
String where="USERNAME=?";
int numberOFEntriesDeleted= db.delete("LOGIN", where, new String[]{UserName}) ;
// Toast.makeText(context, "Number fo Entry Deleted Successfully : "+numberOFEntriesDeleted, Toast.LENGTH_LONG).show();
return numberOFEntriesDeleted;
}
public String getSinlgeEntry(String userName)
{
Cursor cursor=db.query("LOGIN", null, " USERNAME=?", new String[]{userName}, null, null, null);
if(cursor.getCount()<1) // UserName Not Exist
{
cursor.close();
return "NOT EXIST";
}
cursor.moveToFirst();
String password= cursor.getString(cursor.getColumnIndex("PASSWORD"));
cursor.close();
return password;
}
public void updateEntry(String userName,String password)
{
// Define the updated row content.
ContentValues updatedValues = new ContentValues();
// Assign values for each row.
updatedValues.put("USERNAME", userName);
updatedValues.put("PASSWORD", password);
String where="USERNAME = ?";
db.update("LOGIN", updatedValues, where, new String[]{userName});
}
public String getUserName(String userName) {
Cursor cursor=db.query("LOGIN", new String[]{userName}, null, null, null, null, null);
cursor.moveToFirst();
String user = cursor.getString(cursor.getColumnIndex("USERNAME"));
cursor.close();
return user;
}
}
这是错误：
java.lang.RuntimeException: Unable to start activity ComponentInfo{be.ehb.bookme/be.ehb.bookme.ProfileActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String be.ehb.bookme.LoginDataBaseAdapter.getUserName(java.lang.String)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2406)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2466)
at android.app.ActivityThread.access$1200(ActivityThread.java:152)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1341)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5538)
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:960)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String be.ehb.bookme.LoginDataBaseAdapter.getUserName(java.lang.String)' on a null object reference
at be.ehb.bookme.ProfileActivity.onCreate(ProfileActivity.java:27)
at android.app.Activity.performCreate(Activity.java:6013)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2359)
答案 0 :(得分：0)
在致电LoginDataBaseAdapter之前，您应该在ProfileActivity创建getUsername的实例。
public class ProfileActivity extends AppCompatActivity {
LoginDataBaseAdapter loginDataBaseAdapter;
//TextView profileName;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_profile);
loginDataBaseAdapter=new LoginDataBaseAdapter(this);
try {
loginDataBaseAdapter=loginDataBaseAdapter.open();
} catch (SQLException e) {
e.printStackTrace();
}
TextView profileName = (TextView) findViewById(R.id.profileNameTextView);
String userName = loginDataBaseAdapter.getUserName("USERNAME");
String user = loginDataBaseAdapter.getUserName(userName);
profileName.setText("hello" + user);
}