App crash when using Custom ListView to receive data from SQLite

Greenhorn

Posts: 1

posted 2 years ago

Number of slices to send:

Optional 'thank-you' note:

Send

Sorry for the noob question. I'm a beginner. I'm trying to display data from SQLite database to custom listview but the app crash and got a NullPointerException at SubjectListActivity.java line 30 . What does that mean and how do I solve this issue?

SubjectListActivity.java

public class SubjectListActivity extends AppCompatActivity {
        private ListView lv_SubjectList;
        private FloatingActionButton btn_subjectAdd, btn_subjectView;
        SQLiteDatabase sqLiteDatabase;
        DatabaseHelper databaseHelper;
        Cursor cursor;
        SubjectListAdapter subjectListAdapter;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_subject_list);
    
            databaseHelper = new DatabaseHelper(getApplicationContext());
            sqLiteDatabase = databaseHelper.getReadableDatabase();
            cursor = databaseHelper.getAllSubjectListData();
            subjectListAdapter = new SubjectListAdapter(getApplicationContext(), R.layout.subject_list_listview_layout);
            lv_SubjectList.setAdapter(subjectListAdapter);
    
            if (cursor.moveToFirst()){
    
                do {
                    String subjectCode, subjectName;
                    int creditHour;
                    subjectCode = cursor.getString(0);
                    subjectName = cursor.getString(1);
                    creditHour = cursor.getInt(2);
                    SubjectListModel subjectListModel=new SubjectListModel(subjectCode,subjectName,creditHour);
                    subjectListAdapter.add(subjectListModel);
    
                }while (cursor.moveToNext());
            } else {
                //failure. Nothing add to database.
            }
    
            btn_subjectAdd.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    openDialog();
                }
            });
    
            btn_subjectView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
    
                }
            });
        }
    
        private void intViews() {
            lv_SubjectList = findViewById(R.id.lv_SubjectList);
            btn_subjectView = findViewById(R.id.btn_subjectDelete);
            btn_subjectAdd = findViewById(R.id.btn_subjectAdd);
        }
    
        public void openDialog () {
            SubjectListDialog subjectListDialog = new SubjectListDialog();
            subjectListDialog.show(getSupportFragmentManager(), "subject list dialog");
        }
    }

DatabaseHelper.java

public class DatabaseHelper extends SQLiteOpenHelper {
        public static final String SUBJECT_LIST_TABLE = "SUBJECT_LIST_TABLE";
        public static final String COLUMN_SUBJECT_CODE = "SUBJECT_CODE";
        public static final String COLUMN_SUBJECT_NAME = "SUBJECT_NAME";
        public static final String COLUMN_CREDIT_HOUR = "CREDIT_HOUR";
    
        public DatabaseHelper(@Nullable Context context) {
            super(context, "SubjectList.db", null, 1);
        }
    
        @Override
        public void onCreate(SQLiteDatabase db) {
            String createSubjectListTable = "CREATE TABLE " + SUBJECT_LIST_TABLE + " (" + COLUMN_SUBJECT_CODE + " STRING PRIMARY KEY, " + COLUMN_SUBJECT_NAME + " TEXT, " + COLUMN_CREDIT_HOUR + " INT)";
    
            db.execSQL(createSubjectListTable);
        }
    
        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    
        }
    
        public boolean addSubjectListTable (SubjectListModel subjectListModel) {
            SQLiteDatabase db = this.getWritableDatabase();
            ContentValues contentValues = new ContentValues();
    
            contentValues.put(COLUMN_SUBJECT_CODE, subjectListModel.getSubjectCode());
            contentValues.put(COLUMN_SUBJECT_NAME, subjectListModel.getSubjectName());
            contentValues.put(COLUMN_CREDIT_HOUR, subjectListModel.getCreditHour());
    
            long insert = db.insert(SUBJECT_LIST_TABLE, null, contentValues);
            if (insert == -1) {
                return false;
            } else {
                return true;
            }
    
        }
    
        public Cursor getAllSubjectListData () {
            SQLiteDatabase db = this.getWritableDatabase();
            Cursor cursor = db.rawQuery("Select * from "+SUBJECT_LIST_TABLE, null);
            return cursor;
        }
    }

SubjectListModel.java

public class SubjectListModel {
            private String subjectCode, subjectName;
            private int creditHour;
        
            //constructors
        
            public SubjectListModel(String subjectCode, String subjectName, int creditHour) {
                this.subjectCode = subjectCode;
                this.subjectName = subjectName;
                this.creditHour = creditHour;
            }
        
            public SubjectListModel() {
            }
        
            //toString for printing contents of class object
            @Override
            public String toString() {
                return "SubjectListModel{" +
                        "subjectCode='" + subjectCode + '\'' +
                        ", subjectName='" + subjectName + '\'' +
                        ", creditHour=" + creditHour +
                        '}';
            }
        
            //getter and setters
            public String getSubjectCode() {
                return subjectCode;
            }
        
            public void setSubjectCode(String subjectCode) {
                this.subjectCode = subjectCode;
            }
        
            public String getSubjectName() {
                return subjectName;
            }
        
            public void setSubjectName(String subjectName) {
                this.subjectName = subjectName;
            }
        
            public int getCreditHour() {
                return creditHour;
            }
        
            public void setCreditHour(int creditHour) {
                this.creditHour = creditHour;
            }
        }

SubjectListAdapter.java

public class SubjectListAdapter extends ArrayAdapter {
        List list = new ArrayList();
        public SubjectListAdapter(Context context, int resource) {
            super(context, resource);
        }
    
        static class LayoutHandler {
            TextView SUBJECTCODE, SUBJECTNAME, CREDITHOUR;
        }
    
        @Override
        public void add(Object object) {
            super.add(object);
            list.add(object);
        }
    
        @Override
        public int getCount() {
            return list.size();
        }
    
        @Nullable
        @Override
        public Object getItem(int position) {
            return list.get(position);
        }
    
        @NonNull
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            View row = convertView;
            LayoutHandler layoutHandler;
            if (row == null) {
                LayoutInflater layoutInflater = (LayoutInflater) this.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                row = layoutInflater.inflate(R.layout.subject_list_listview_layout,parent,false);
                layoutHandler = new LayoutHandler();
                layoutHandler.SUBJECTCODE = (TextView)row.findViewById(R.id.tv_subjectCode);
                layoutHandler.SUBJECTNAME = (TextView)row.findViewById(R.id.tv_subjectName);
                layoutHandler.CREDITHOUR = (TextView)row.findViewById(R.id.tv_subjectCreditHour);
                row.setTag(layoutHandler);
            } else {
                layoutHandler = (LayoutHandler)row.getTag();
            }
    
            SubjectListModel subjectListModel = (SubjectListModel) this.getItem(position);
            layoutHandler.SUBJECTCODE.setText(subjectListModel.getSubjectCode());
            layoutHandler.SUBJECTNAME.setText(subjectListModel.getSubjectName());
            layoutHandler.CREDITHOUR.setText(subjectListModel.getCreditHour());
    
            return row;
        }
    }

Randy Tong

Rancher

Posts: 665

I like...

posted 2 years ago

Number of slices to send:

Optional 'thank-you' note:

Send

line 30 is which line? You referring to your IDE or your post?

Life is but a BREATH

Tim Moores

Saloon Keeper

Posts: 7597

177

posted 2 years ago

Number of slices to send:

Optional 'thank-you' note:

Send

Post the full stack trace of the exception (which is in the logcat).

And post the exact source code you're working with, including imports and everything else, otherwise line numbers are meaningless due to missing lines.