向SQLite数据库添加“用户名不可用”

Question

我已经通过SQLite database创建了一个注册和登录页面。我正在尝试创建一个函数，如果要注册的新用户的用户名与另一个创建的帐户的用户名相似，则该新用户将无法注册。

我还需要在用户尝试时提示您"Username not available"。我确实尝试在数据库帮助器中对其进行编码，但它导致我的android-studio意外崩溃，因此我将其删除。 我已经尝试在线搜索教程，但无济于事。

活动数据库助手：

public class DatabaseHelper extends SQLiteOpenHelper {
    public static final String DATABASE_NAME = "register.db";
    public static final String TABLE_NAME = "registeruser";

    //Column 1 is for ID
    public static final String COL_1 = "ID";

    //Column 2 is for username
    public static final String COL_2 = "username";

    //Column 3 is for password
    public static final String COL_3 = "password";

    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null , 1);
    }


    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        sqLiteDatabase.execSQL("CREATE TABLE registeruser (ID INTEGER PRIMARY KEY AUTOINCREMENT,username TEXT, password TEXT)");

    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int ii) {
        sqLiteDatabase.execSQL(" DROP TABLE IF EXISTS " + TABLE_NAME);
        onCreate(sqLiteDatabase);

    }

    //function to add new users when registering
    public long adduser (String user, String password)
    {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put("username", user);
        contentValues.put("password", password);
        long res = db.insert("registeruser",null,contentValues);
        db.close();
        return res;

    }

    //function to check if user exists
    public boolean checkUser(String username, String password)
    {
        String[] columns = { COL_1};
        SQLiteDatabase db = getReadableDatabase();
        String selection = COL_2 + "=?" + " and " + COL_3 + "=?";
        String[] selectionArgs = { username, password };
        Cursor cursor = db.query(TABLE_NAME,columns,selection,selectionArgs,null,null,null);
        int count = cursor.getCount();
        cursor.close();
        db.close();

        if(count>0)
            return true;
        else
            return false;
    }

    public int updatePassword(String username, String newPassword){

        SQLiteDatabase db = getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put("password",newPassword);
        String whereClause = "username = ?";
        String[] whereArgs = { username };
        int result = db.update(TABLE_NAME,contentValues,whereClause,whereArgs);

        db.close();
        return result;
    }
}

活动注册活动：

public class RegisterActivity extends AppCompatActivity {
    DatabaseHelper db;
    EditText mTextUsername;
    EditText mTextPassword;
    EditText mTextCnfPassword;
    ImageButton mButtonRegister;
    TextView mTextViewLogin;

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


        db = new DatabaseHelper(this);
        mTextUsername = (EditText)findViewById(R.id.edittext_username);
        mTextPassword = (EditText)findViewById(R.id.edittext_password);
        mTextCnfPassword = (EditText)findViewById(R.id.edittext_cnf_password);
        mButtonRegister = (ImageButton)findViewById(R.id.imagebutton_register);
        mTextViewLogin = (TextView)findViewById(R.id.textview_login);
        mTextViewLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent LoginIntent = new Intent(RegisterActivity.this,LoginPageActivity.class);
                startActivity(LoginIntent);
            }
        });

        mButtonRegister.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String user = mTextUsername.getText().toString().trim();
                String pwd = mTextPassword.getText().toString().trim();
                String cnf_pwd = mTextCnfPassword.getText().toString().trim();

                if(pwd.length() > 0 && user.length() > 0 && pwd.equals(cnf_pwd)) {
                    Long val = db.adduser(user,pwd);
                    if(val > 0){
                        Toast.makeText(RegisterActivity.this,"Successfully Registered.",Toast.LENGTH_SHORT).show();
                        Intent movetoLogin = new Intent(RegisterActivity.this,LoginPageActivity.class);
                        startActivity(movetoLogin);
                    }
                    else{
                        Toast.makeText(RegisterActivity.this,"Registration Error.",Toast.LENGTH_SHORT).show();
                    }

                }
                else{
                    Toast.makeText(RegisterActivity.this,"Check Your Fields Again.",Toast.LENGTH_SHORT).show();
                }

            }
        });
    }
}