import java.sql.Connection;
import java.sql.DriverManager;

public class MainActivity extends AppCompatActivity {
    public void onMyButtonClick(View view) {
        connectToDB();
    }

    public Connection connectToDB(){
        Connection con=null;
        try{
            Class.forName("oracle.jdbc.driver.OracleDriver");
            con= DriverManager.getConnection(
                    "jdbc:oracle:thin:@server.host:1521:SID","user","passwd");
            Toast.makeText(getApplicationContext(), "OK", Toast.LENGTH_LONG).show();
        }catch(Exception e){
            Toast.makeText(getApplicationContext(), "Error - " + e.getMessage(), Toast.LENGTH_LONG).show(); }
        return con;
    }

android.os.NetworkOnMainThreadException

package com.example.isidorov.myapp4;

import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;



public class MainActivity extends AppCompatActivity {

    TextView tvInfo;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        tvInfo = (TextView) findViewById(R.id.textView);
    }

    public void btnOracleOnClick(View view) {
        new MyTask().execute();
    }

    class MyTask extends AsyncTask<Void, Void, Void> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            tvInfo.setText("Begin");
        }

        @Override
        protected Void doInBackground(Void... params) {
            try {
                oraclass.connectToDB();
            } catch (Exception e) {
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            super.onPostExecute(result);
            tvInfo.setText("This error - " + oraclass.err);
        }
    }

}