为什么此应用程序显示错误空指针异常。在此应用程序中，我想检索“student_login”表中的条目编号并显示相同的编号。出勤表.xml 中的复选框
logcat 显示以下错误：
08-11 20:44:52.662: W/System.err(333): java.lang.NullPointerException
08-11 20:44:52.672: W/System.err(333): at com.example.attendancesystem.TeacherLoggedInPage.retrieveStringRoll(TeacherLoggedInPage.java:115)
08-11 20:44:52.672: W/System.err(333): at com.example.attendancesystem.TeacherLoggedInPage.getRollnoList(TeacherLoggedInPage.java:98)
08-11 20:44:52.672: W/System.err(333): at com.example.attendancesystem.TeacherLoggedInPage.onCreate(TeacherLoggedInPage.java:56)
08-11 20:44:52.682: W/System.err(333): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
08-11 20:44:52.694: W/System.err(333): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
08-11 20:44:52.702: W/System.err(333): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
08-11 20:44:52.702: W/System.err(333): at android.app.ActivityThread.access$2300(ActivityThread.java:125)
08-11 20:44:52.712: W/System.err(333): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
08-11 20:44:52.712: W/System.err(333): at android.os.Handler.dispatchMessage(Handler.java:99)
08-11 20:44:52.712: W/System.err(333): at android.os.Looper.loop(Looper.java:123)
08-11 20:44:52.722: W/System.err(333): at android.app.ActivityThread.main(ActivityThread.java:4627)
08-11 20:44:52.722: W/System.err(333): at java.lang.reflect.Method.invokeNative(Native Method)
08-11 20:44:52.732: W/System.err(333): at java.lang.reflect.Method.invoke(Method.java:521)
08-11 20:44:52.732: W/System.err(333): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
08-11 20:44:52.732: W/System.err(333): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
08-11 20:44:52.732: W/System.err(333): at dalvik.system.NativeStart.main(Native Method)
08-11 20:44:52.742: D/AndroidRuntime(333): Shutting down VM
08-11 20:44:52.752: W/dalvikvm(333): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
08-11 20:44:52.772: E/AndroidRuntime(333): FATAL EXCEPTION: main
08-11 20:44:52.772: E/AndroidRuntime(333): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.attendancesystem/com.example.attendancesystem.TeacherLoggedInPage}: java.lang.NullPointerException
08-11 20:44:52.772: E/AndroidRuntime(333): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
08-11 20:44:52.772: E/AndroidRuntime(333): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
08-11 20:44:52.772: E/AndroidRuntime(333): at android.app.ActivityThread.access$2300(ActivityThread.java:125)
08-11 20:44:52.772: E/AndroidRuntime(333): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
08-11 20:44:52.772: E/AndroidRuntime(333): at android.os.Handler.dispatchMessage(Handler.java:99)
08-11 20:44:52.772: E/AndroidRuntime(333): at android.os.Looper.loop(Looper.java:123)
08-11 20:44:52.772: E/AndroidRuntime(333): at android.app.ActivityThread.main(ActivityThread.java:4627)
08-11 20:44:52.772: E/AndroidRuntime(333): at java.lang.reflect.Method.invokeNative(Native Method)
08-11 20:44:52.772: E/AndroidRuntime(333): at java.lang.reflect.Method.invoke(Method.java:521)
这是TeacherLoggedIn.class的代码
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.attendancesheet);
Bundle extras = getIntent().getExtras();
if (savedInstanceState == null) {
extras = getIntent().getExtras();
if (extras == null) {
t_id = (String) null;
} else {
t_id = extras.getString("key");
}
} else {
t_id = (String) savedInstanceState.getSerializable("key");
}
try {
Roll_array = getRollnoList(TeacherLoggedInPage.this);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Array_Count = Roll_array.size();
LinearLayout my_layout = (LinearLayout) findViewById(R.id.layout_checklist);
for (int i = 0; i < Array_Count; i++) {
TableRow row = new TableRow(this);
row.setId(i);
row.setLayoutParams(new LayoutParams(
android.widget.TableRow.LayoutParams.FILL_PARENT,
android.widget.TableRow.LayoutParams.WRAP_CONTENT));
CheckBox checkBox = new CheckBox(this);
checkBox.setId(i);
checkBox.setText(Roll_array.get(i));
row.addView(checkBox);
my_layout.addView(row);
}
UpdateAttendanceButton = (Button) findViewById(R.id.bUpdateAttendance);
UpdateAttendanceButton.setOnClickListener(this);
}
private ArrayList<String> getRollnoList(Context context) {
// TODO Auto-generated method stub
try {
SQLiteDatabase db = myDb.getReadableDatabase();
String[] columns = { "s_roll_no" };
Cursor cursor = db.query(DbHelper.STUDENT_TABLE, columns, null,
null, null, null, null);
if (cursor != null) {
System.out.println("fething roll nos");
startManagingCursor(cursor);
ArrayList<String> temp = retrieveStringRoll(cursor);
return temp;
}
System.out.println("Cursor NuLL");
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
private ArrayList<String> retrieveStringRoll(Cursor cursor) {
// TODO Auto-generated method stub
while (cursor != null && cursor.moveToNext()) {
String roll = cursor.getString(0);
temp1.add(roll);
}
if (!cursor.isClosed()) {
cursor.close();
}
return temp1;
}
这是我创建表 studentLogin.class 的 DbHelper.class
private static final int DATABASE_VERSION = 1;
public static final String ATTENDANCE_TABLE = "Student_attendance";
public static final String TEACHER_TABLE = "Teacher_login";
public static final String STUDENT_TABLE = "Student_login";
public static final String T_ROW_ID = "teacher_id";
private static final String ATTENDANCE_TABLE_CREATE = "CREATE TABLE "
+ ATTENDANCE_TABLE + "(" + "_id INTEGER PRIMARY KEY AUTOINCREMENT,"
+ "teacher_id INTEGER , " + "time TEXT NOT NULL, "
+ "rl_one INTEGER , rl_two INTEGER , rl_three INTEGER ,"
+ "rl_four INTEGER , rl_five INTEGER , rl_six INTEGER); ";
private static final String TEACHER_TABLE_CREATE = "CREATE TABLE "
+ TEACHER_TABLE
+ "( teacher_id INTEGER PRIMARY KEY AUTOINCREMENT , "
+ "t_name TEXT NOT NULL , t_pass TEXT NOT NULL , t_email TEXT NOT NULL);";
private static final String STUDENT_TABLE_CREATE = "CREATE TABLE "
+ STUDENT_TABLE + "( s_id INTEGER PRIMARY KEY AUTOINCREMENT , "
+ "s_name TEXT NOT NULL , s_pass TEXT NOT NULL , "
+ "s_roll_no TEXT NOT NULL , " + "s_email TEXT NOT NULL);";
public DbHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
System.out.println("In constructor");
}
@Override
public void onCreate(SQLiteDatabase db) {
// TODO Auto-generated method stub
try {
db.execSQL(TEACHER_TABLE_CREATE);
db.execSQL(ATTENDANCE_TABLE_CREATE);
db.execSQL(STUDENT_TABLE_CREATE);
System.out.println("In onCreate");
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}