[SOLVED] android - onclick could not find method

I am having an issue where onclick won't find the method it's supposed to execute. Android tries to call the onClick method public void fahrplan(View v,Connection conn) but is unable to find it. Details of my code are below, and the error is after that. Please help me determine why android is unable to find the onClick method. Thanks.

Right now there's only one activity, so there's no other code running.

MainActivity.java

    package eu.rathenau.fahrgemeinschaft;
    
    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.TextView;
    
    import java.sql.Connection;
    import java.sql.DriverManager;
    import java.sql.ResultSet;
    import java.sql.Statement;
    
    public class MainActivity extends AppCompatActivity {
    
    
    
        String x;
        TextView testTextView;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
             testTextView =(TextView)findViewById(R.id.textView);
            connect();
        }
    
        public  Connection connect() {
            try {
                Class.forName("com.mysql.jdbc.Driver");
                Connection conn = DriverManager.getConnection("jdbc:mysql://"ip-address"/MarcelGerber?user="username"&password="password");
                return conn;
            } catch (Exception e) {
                return null;
            }
        }
    
        public  String getAllKürzel(Connection conn) {
            if (conn == null) {
                return null;
            }
            Statement st = null;
            ResultSet rs = null;
            try {
                st = conn.createStatement();
                rs = st.executeQuery("SELECT Kürzel FROM Benutzer");
                int length = 0;
                if (rs.last()) {
                    length = rs.getRow();
                    rs.beforeFirst();
                }
                String[] kürzel = new String[length];
                while(rs.next()) {
                    kürzel[rs.getRow() - 1] = rs.getString("Kürzel");
                }
    
                x=kürzel[1];
                return x;
            } catch (Exception e) {
                return null;
            } finally {
                if (rs != null) {
                    try {
                        rs.close();
                    } catch (Exception e) {
                    }
                }
                if (st != null) {
                    try {
                        st.close();
                    } catch (Exception e) {
                    }
                }
            }
    
    
    
        }
    
        public void fahrplan (View v,Connection conn){
    
            testTextView.setText(getAllKürzel(conn));
        }
    
    
    }

activity_main.xml

    <?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:paddingBottom="@dimen/activity_vertical_margin"
        android:paddingLeft="@dimen/activity_horizontal_margin"
        android:paddingRight="@dimen/activity_horizontal_margin"
        android:paddingTop="@dimen/activity_vertical_margin"
        tools:context="eu.rathenau.fahrgemeinschaft.MainActivity">
    
    
        <Button
            android:layout_width="match_parent"
            android:layout_height="70dp"
            android:text="@string/fahrplan"
            android:id="@+id/buttonFahrplan"
            android:layout_alignParentTop="true"
            android:layout_centerHorizontal="true"
            android:onClick="fahrplan"/>
    
        <TextView
            android:layout_width="match_parent"
            android:layout_height="70dp"
            android:text="hallo"
            android:id="@+id/textView"
            android:layout_below="@+id/buttonFahrplan"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="66dp" />
    </RelativeLayout>

AndroidManifest.xml

    <?xml version="1.0" encoding="utf-8"?>
    <manifest xmlns:android="http://schemas.android.com/apk/res/android"
        package="eu.rathenau.fahrgemeinschaft">
    
        <application
            android:allowBackup="true"
            android:icon="@mipmap/ic_launcher"
            android:label="@string/app_name"
            android:supportsRtl="true"
            android:theme="@style/AppTheme">
            <activity android:name=".MainActivity"
                android:screenOrientation="portrait">
                <intent-filter>
                    <action android:name="android.intent.action.MAIN" />
    
                    <category android:name="android.intent.category.LAUNCHER" />
                </intent-filter>
            </activity>
        </application>
    
    </manifest>

Log

    04-22 11:20:39.830 28066-28066/eu.rathenau.fahrgemeinschaft E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                  Process: eu.rathenau.fahrgemeinschaft, PID: 28066
                                                                                  java.lang.IllegalStateException: Could not find method fahrplan(View) in a parent or ancestor Context for android:onClick attribute defined on view class android.support.v7.widget.AppCompatButton with id 'buttonFahrplan'
                                                                                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.resolveMethod(AppCompatViewInflater.java:321)
                                                                                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:280)
                                                                                      at android.view.View.performClick(View.java:4856)
                                                                                      at android.view.View$PerformClick.run(View.java:19956)
                                                                                      at android.os.Handler.handleCallback(Handler.java:739)
                                                                                      at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                      at android.os.Looper.loop(Looper.java:211)
                                                                                      at android.app.ActivityThread.main(ActivityThread.java:5373)
                                                                                      at java.lang.reflect.Method.invoke(Native Method)
                                                                                      at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1020)
                                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:815)

It would be really nice if you could help me.