Java 在Android Studio中以字符串形式从SQLite数据库检索数据

我希望在SQLite数据库中保存该名称，稍后在另一个活动中，我希望将该名称作为字符串值检索。但我无法从数据库中获取数据。请帮帮我。我的代码如下所示

数据库帮助程序：

    private static final String TAG = "DatabaseHelper";
    private static final String TABLE_NAME = "user";
    private static final String COL1 = "ID";
    private static final String COL2 = "name";

    public void onCreate(SQLiteDatabase db) {String createTable = 
    "CREATE TABLE " + TABLE_NAME + " (ID INTEGER PRIMARY KEY 
    AUTOINCREMENT, " + COL2 +" TEXT)"; 
    db.execSQL(createTable);}

    public boolean addData(String item) {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(COL2, item);

            Log.d(TAG, "addData: Adding " + item + " to " + TABLE_NAME);

            long result = db.insert(TABLE_NAME, null, contentValues);

            if (result == -1) {
                return false;
            } else {
                return true;
            }
        }

    private static final String TAG = "DatabaseHelper";
    private static final String TABLE_NAME = "user";
    private static final String COL1 = "ID";
    private static final String COL2 = "name";



private static final String createTable = "CREATE TABLE " + TABLE_NAME + " (" + COL1 +" INTEGER PRIMARY KEY AUTOINCREMENT, " + COL2 +" TEXT)"; // You have written ID inplace of COL1






 public  DatabaseHelper(Context context)
    {

        super(context,DB_NAME,null,1);
    }

 public void onCreate(SQLiteDatabase db)
    {
        db.execSQL(createTable);

    }

public boolean insertData(String name) {

    SQLiteDatabase sqLiteDatabase = this.getWritableDatabase();
    ContentValues contentValues1 =  new ContentValues();
    contentValues1.put(COL2,name);


    long result1 = sqLiteDatabase.insert(TABLE_NAME,null,contentValues1);
    return result1 != -1;
}
public Cursor viewData()
{
    SQLiteDatabase sqLiteDatabase = this.getReadableDatabase();
    Cursor cursor ;

    String query = "Select * from " +TABLE_NAME;
    cursor= sqLiteDatabase.rawQuery(query, null);


    return cursor;
}

主要活动：

    mDatabaseHelper=new DatabaseHelper(this);
    Username=(EditText)findViewById(R.id.user);
    saveBtn=(Button)findViewById(R.id.button);

    saveBtn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
    String newEntry = Username.getText().toString();
                            AddData(newEntry);
                            Username.setText("");

    Intent intent = new Intent(MainActivity.this, HomeActivity.class);
                        startActivity(intent);
                    }
                });

            }
            public void AddData(String newEntry) {
                boolean insertData = mDatabaseHelper.addData(newEntry);
        }
        }

 mDatabaseHelper=new DatabaseHelper(this);
    Username=(EditText)findViewById(R.id.user);
    saveBtn=(Button)findViewById(R.id.button);

    saveBtn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
    String newEntry = Username.getText().toString();
                            mDatabaseHelper.insertData(newEntry);
                            Username.setText("");

    Intent intent = new Intent(MainActivity.this, HomeActivity.class);
                        startActivity(intent);
                    }
                });

            }

        }

public class HomeActivity extends AppCompatActivity {

    DatabaseHelper mDatabaseHelper;
    String Username;


ArrayList<String> listItem;
    ArrayAdapter adapter;
    ListView listView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);

        mDatabaseHelper = new DatabaseHelper(this);
listView = (ListView) findViewById(R.id.listView);
        listItem = new ArrayList<>();
viewData1();

    }

private void viewData1() {

        Cursor cursor = mDatabaseHelper.viewData();

        if (cursor.getCount() == 0) {

            Toast.makeText(this, "No data to show", Toast.LENGTH_SHORT).show();
        } else {
            while (cursor.moveToNext()) {
                Log.i("message","Data got");

                listItem.add(cursor.getString(1)); // Adding data received to a Listview
            }
            adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, listItem);
            listView.setAdapter(adapter);

        }
    }

    }

家庭活动：

public class HomeActivity extends AppCompatActivity {

    DatabaseHelper mDatabaseHelper;
    String Username;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);

        mDatabaseHelper = new DatabaseHelper(this);

    }

}

 mDatabaseHelper=new DatabaseHelper(this);
    Username=(EditText)findViewById(R.id.user);
    saveBtn=(Button)findViewById(R.id.button);

    saveBtn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
    String newEntry = Username.getText().toString();
                            mDatabaseHelper.insertData(newEntry);
                            Username.setText("");

    Intent intent = new Intent(MainActivity.this, HomeActivity.class);
                        startActivity(intent);
                    }
                });

            }

        }

public class HomeActivity extends AppCompatActivity {

    DatabaseHelper mDatabaseHelper;
    String Username;


ArrayList<String> listItem;
    ArrayAdapter adapter;
    ListView listView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);

        mDatabaseHelper = new DatabaseHelper(this);
listView = (ListView) findViewById(R.id.listView);
        listItem = new ArrayList<>();
viewData1();

    }

private void viewData1() {

        Cursor cursor = mDatabaseHelper.viewData();

        if (cursor.getCount() == 0) {

            Toast.makeText(this, "No data to show", Toast.LENGTH_SHORT).show();
        } else {
            while (cursor.moveToNext()) {
                Log.i("message","Data got");

                listItem.add(cursor.getString(1)); // Adding data received to a Listview
            }
            adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, listItem);
            listView.setAdapter(adapter);

        }
    }

    }

---

在这里，在这个家庭活动中，我想把用户名作为一个字符串，但我不知道怎么做。请帮帮我…

要检索数据，请运行一个查询，将数据提取到光标中。游标将由0-n行组成，并且将具有查询中定义的相同数量的列。然后在行中移动，并使用适当的游标get？？？？？方法获取实际数据

例如，您可以将以下方法添加到DatabaseHelper：-

public String getName(long id) {

    String rv = "not found";
    SqliteDatabase db = this.getWritableDatabase();
    String whereclause = "ID=?";
    String[] whereargs = new String[]{String.valueOf(id)};
    Cursor csr = db.query(TABLE_NAME,null,whereclause,whereargs,null,null,null);
    if (csr.moveToFirst()) {
        rv = csr.getString(csr.getColumnIndex(COL2));
    }
    return rv;
}

然后可以由

String name=mDatabaseHelper.getName（1）使用上述内容


请注意，这假定存在ID为1的行
查看此库

它很容易实现

您的数据库助手：

    private static final String TAG = "DatabaseHelper";
    private static final String TABLE_NAME = "user";
    private static final String COL1 = "ID";
    private static final String COL2 = "name";

    public void onCreate(SQLiteDatabase db) {String createTable = 
    "CREATE TABLE " + TABLE_NAME + " (ID INTEGER PRIMARY KEY 
    AUTOINCREMENT, " + COL2 +" TEXT)"; 
    db.execSQL(createTable);}

    public boolean addData(String item) {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(COL2, item);

            Log.d(TAG, "addData: Adding " + item + " to " + TABLE_NAME);

            long result = db.insert(TABLE_NAME, null, contentValues);

            if (result == -1) {
                return false;
            } else {
                return true;
            }
        }

    private static final String TAG = "DatabaseHelper";
    private static final String TABLE_NAME = "user";
    private static final String COL1 = "ID";
    private static final String COL2 = "name";



private static final String createTable = "CREATE TABLE " + TABLE_NAME + " (" + COL1 +" INTEGER PRIMARY KEY AUTOINCREMENT, " + COL2 +" TEXT)"; // You have written ID inplace of COL1






 public  DatabaseHelper(Context context)
    {

        super(context,DB_NAME,null,1);
    }

 public void onCreate(SQLiteDatabase db)
    {
        db.execSQL(createTable);

    }

public boolean insertData(String name) {

    SQLiteDatabase sqLiteDatabase = this.getWritableDatabase();
    ContentValues contentValues1 =  new ContentValues();
    contentValues1.put(COL2,name);


    long result1 = sqLiteDatabase.insert(TABLE_NAME,null,contentValues1);
    return result1 != -1;
}
public Cursor viewData()
{
    SQLiteDatabase sqLiteDatabase = this.getReadableDatabase();
    Cursor cursor ;

    String query = "Select * from " +TABLE_NAME;
    cursor= sqLiteDatabase.rawQuery(query, null);


    return cursor;
}

您的主要活动：

 mDatabaseHelper=new DatabaseHelper(this);
    Username=(EditText)findViewById(R.id.user);
    saveBtn=(Button)findViewById(R.id.button);

    saveBtn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
    String newEntry = Username.getText().toString();
                            mDatabaseHelper.insertData(newEntry);
                            Username.setText("");

    Intent intent = new Intent(MainActivity.this, HomeActivity.class);
                        startActivity(intent);
                    }
                });

            }

        }

public class HomeActivity extends AppCompatActivity {

    DatabaseHelper mDatabaseHelper;
    String Username;


ArrayList<String> listItem;
    ArrayAdapter adapter;
    ListView listView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);

        mDatabaseHelper = new DatabaseHelper(this);
listView = (ListView) findViewById(R.id.listView);
        listItem = new ArrayList<>();
viewData1();

    }

private void viewData1() {

        Cursor cursor = mDatabaseHelper.viewData();

        if (cursor.getCount() == 0) {

            Toast.makeText(this, "No data to show", Toast.LENGTH_SHORT).show();
        } else {
            while (cursor.moveToNext()) {
                Log.i("message","Data got");

                listItem.add(cursor.getString(1)); // Adding data received to a Listview
            }
            adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, listItem);
            listView.setAdapter(adapter);

        }
    }

    }

您的家庭活动：

 mDatabaseHelper=new DatabaseHelper(this);
    Username=(EditText)findViewById(R.id.user);
    saveBtn=(Button)findViewById(R.id.button);

    saveBtn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
    String newEntry = Username.getText().toString();
                            mDatabaseHelper.insertData(newEntry);
                            Username.setText("");

    Intent intent = new Intent(MainActivity.this, HomeActivity.class);
                        startActivity(intent);
                    }
                });

            }

        }

public class HomeActivity extends AppCompatActivity {

    DatabaseHelper mDatabaseHelper;
    String Username;


ArrayList<String> listItem;
    ArrayAdapter adapter;
    ListView listView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);

        mDatabaseHelper = new DatabaseHelper(this);
listView = (ListView) findViewById(R.id.listView);
        listItem = new ArrayList<>();
viewData1();

    }

private void viewData1() {

        Cursor cursor = mDatabaseHelper.viewData();

        if (cursor.getCount() == 0) {

            Toast.makeText(this, "No data to show", Toast.LENGTH_SHORT).show();
        } else {
            while (cursor.moveToNext()) {
                Log.i("message","Data got");

                listItem.add(cursor.getString(1)); // Adding data received to a Listview
            }
            adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, listItem);
            listView.setAdapter(adapter);

        }
    }

    }

公共类HomeActivity扩展了AppCompatActivity{
数据库助手mDatabaseHelper；
字符串用户名；
ArrayList列表项；
阵列适配器；
列表视图列表视图；
@凌驾
创建时受保护的void（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）；
setContentView（R.layout.activity_home）；
mDatabaseHelper=newdatabasehelper（此）；
listView=（listView）findViewById（R.id.listView）；
listItem=新的ArrayList（）；
viewData1（）；
}
私有void viewData1（）{
Cursor Cursor=mDatabaseHelper.viewData（）；
if（cursor.getCount（）==0）{
Toast.makeText（这是“无数据显示”，Toast.LENGTH_SHORT.show（）；
}否则{
while（cursor.moveToNext（））{
Log.i（“消息”、“获取数据”）；
listItem.add（cursor.getString（1））；//将收到的数据添加到Listview
}
adapter=new ArrayAdapter（这是android.R.layout.simple\u list\u item\u 1，listItem）；
setAdapter（适配器）；
}
}
}
请修复代码，使其成为有效的Java代码（奇怪的颜色编码是一个提示），并请修复代码格式，即缩进，以便于人类阅读。我是按照您的代码进行操作的。我已经在我的数据库助手中添加了这个getname方法，之后在我的HomeActivity中，我只是将它作为字符串调用。String name=mdatabasehelper.getName（1）；多谢各位@MikeT@RehanSarwar很好，如果你认为答案回答了你的问题，你现在应该在其中一个答案上打勾。谢谢你的建议，我会检查它。