How to add a OnClicklistener to this kind of code

Question

Our app should connect to a SQL Database. It is in our Network. The App should edit data in the database. We have built the connection and want to set a onclicklistener to a Button, witch causes the connection code to connect.

this is the code we have got:

public class Werte_aendern extends AppCompatActivity {

TextView tvIP;

String Textauslesen = tvIP.getText().toString();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    tvIP = (TextView) findViewById(R.id.tvIP);
    setContentView(R.layout.activity_werte_aendern);
}




     Connection con = null;
    //private static String dbHost = "192.168.40.148"; // Hostname
     String dbPort = "3306";      // Port -- Standard: 3306
     String dbName = "wasserwerte";   // Datenbankname
     String dbUser = "App";     // Datenbankuser
     String dbPass = "fruitcake";      // Datenbankpasswort

    private Werte_aendern(){
        try {
            Class.forName("com.mysql.jdbc.Driver"); // Datenbanktreiber für JDBC Schnittstellen laden.

            // Verbindung zur JDBC-Datenbank herstellen.
            con = DriverManager.getConnection("jdbc:mysql://"+Textauslesen+":"+ dbPort+"/"+dbName+"?"+"user="+dbUser+"&"+"password="+dbPass);
          //  Statement createStatement();
          //  SQLiteDatabase wasserwerte =

        } catch (ClassNotFoundException e) {
            Toast.makeText(getApplicationContext(), "Treiber nicht gefunden", Toast.LENGTH_SHORT).show();
        } catch (SQLException e) {
            Toast.makeText(getApplicationContext(), "Verbindung nicht möglich", Toast.LENGTH_SHORT).show();
            Toast.makeText(getApplicationContext(), "SQLException: " + e.getMessage(), Toast.LENGTH_SHORT).show();
            Toast.makeText(getApplicationContext(), "SQLState: " + e.getSQLState(), Toast.LENGTH_SHORT).show();
            Toast.makeText(getApplicationContext(), "VendorError: " + e.getErrorCode(), Toast.LENGTH_SHORT).show();
        }
    }

}

We are noobs, but we have to do this for a schoolproject.
Can you help us please?

Answer 1

I've updated your code. An Activity should always be named [Whatever]Activity. Donnot if "WerteAendern" is a correct name since I don't speak German (I assume it's German).

  public class WerteAendernActivity extends AppCompatActivity {

    TextView tvIP;

    // You should get the text from the View AFTER inflating the layout and find it with
    // findViewById. Otherwise it's gonna crash.
    String textauslesen;
    private Button connectBtn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // First you inflate the layout
        setContentView(R.layout.activity_werte_aendern);
        // Then you get the views
        tvIP = (TextView) findViewById(R.id.tvIP);
        // Probably should be somewhere else as there is no interesting text to retrieve from the 
        // view at the moment
        textauslesen = tvIP.getText().toString();  

        connectBtn = (Button) findViewById(R.id.connection_button); // ! You need to add a Button in your layout
        connectBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                // Add some test, ... if you're already connected
                connectToDataBase();
            }
        });
    }

    Connection con = null;
    //private static String dbHost = "192.168.40.148"; // Hostname
    String dbPort = "3306";      // Port -- Standard: 3306
    String dbName = "wasserwerte";   // Datenbankname
    String dbUser = "App";     // Datenbankuser
    String dbPass = "fruitcake";      // Datenbankpasswort


    // Method to connect to the database.
    // !!! You're not supposed to override the constructor of an Activity!
    private void connectToDataBase() {
        try {
            Class.forName("com.mysql.jdbc.Driver"); // Datenbanktreiber für JDBC Schnittstellen laden.

            // Verbindung zur JDBC-Datenbank herstellen.
            con = DriverManager.getConnection("jdbc:mysql://" + textauslesen + ":" + dbPort + "/" + dbName + "?" + "user=" + dbUser + "&" + "password=" + dbPass);
            //  Statement createStatement();
            //  SQLiteDatabase wasserwerte =

        } catch (ClassNotFoundException e) {
            Toast.makeText(getApplicationContext(), "Treiber nicht gefunden", Toast.LENGTH_SHORT).show();
            e.printStackTrace();
        } catch (SQLException e) {
            Toast.makeText(getApplicationContext(), "Error! See Exception logs", Toast.LENGTH_SHORT).show();
            // The logs will be displayed in the Logcat window in Android Studio 
            e.printStackTrace();
        }
    }
}