I'm learning android and I'm currently doing a calculator.
I've already done the xml part, where I put the buttons 
And I'm trying to finish the Java file, which is the code that makes the calculator work

BUT in the Java file I've got this error:
Error:(22, 64) error: cannot find symbol class OnClickListener

and I don't know what to do :c I'm still learning
I've seen that I should put 

```
    public class MainActivity extends ActionBarActivity implements View.OnClickListener {

```

But I don't understand what's that and I don't know if that will bug the calculator.

That's the Java file (MainActivity)

```
package com.example.glow.pruebas;

import android.content.DialogInterface;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity implements OnClickListener {

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

        Button n0 = (Button) findViewById(R.id.B0);
        n0.setOnClickListener(this);
        Button n1 = (Button) findViewById(R.id.B1);
        n1.setOnClickListener(this);
        Button n2 = (Button) findViewById(R.id.B2);
        n2.setOnClickListener(this);
        Button n3 = (Button) findViewById(R.id.B3);
        n3.setOnClickListener(this);
        Button n4 = (Button) findViewById(R.id.B4);
        n4.setOnClickListener(this);
        Button n5 = (Button) findViewById(R.id.B5);
        n5.setOnClickListener(this);
        Button n6 = (Button) findViewById(R.id.B6);
        n6.setOnClickListener(this);
        Button n7 = (Button) findViewById(R.id.B7);
        n7.setOnClickListener(this);
        Button n8 = (Button) findViewById(R.id.B8);
        n8.setOnClickListener(this);
        Button n9 = (Button) findViewById(R.id.B9);
        n9.setOnClickListener(this);

        Button coma = (Button) findViewById(R.id.Bcoma);
        coma.setOnClickListener(this);
        Button igual = (Button) findViewById(R.id.Bigual);
        igual.setOnClickListener(this);

        Button suma = (Button) findViewById(R.id.B6sumar);
        suma.setOnClickListener(this);
        Button resta = (Button) findViewById(R.id.B5restar);
        resta.setOnClickListener(this);
        Button mul = (Button) findViewById(R.id.Bmult);
        mul.setOnClickListener(this);
        Button division = (Button) findViewById(R.id.Bdividir);
        division.setOnClickListener(this);
        Button raiz = (Button) findViewById(R.id.raiz);
        raiz.setOnClickListener(this);
        Button elevado = (Button) findViewById(R.id.BElevado);
        elevado.setOnClickListener(this);

        Button DEL = (Button) findViewById(R.id.BDEL);
        DEL.setOnClickListener(this);
        Button AC = (Button) findViewById(R.id.BAC);
        AC.setOnClickListener(this);

        Button sin = (Button) findViewById(R.id.Bsin);
        sin.setOnClickListener(this);
        Button cos = (Button) findViewById(R.id.Bcos);
        cos.setOnClickListener(this);
        Button tan = (Button) findViewById(R.id.Btan);
        tan.setOnClickListener(this);

        Button secreto = (Button) findViewById(R.id.Bsecreto);
        secreto.setOnClickListener(this);

    }
    @Override
    public void onClick(View v) {
        TextView pantalla = (TextView) findViewById(R.id.texto);
        int seleccion = v.getId();

        try {
            switch (seleccion) {
                case R.id.B0:
                    pantalla.setText("0");
                    break;
                case R.id.B1:
                    pantalla.setText("1");
                    break;
                case R.id.B2:
                    pantalla.setText("2");
                    break;
                case R.id.B3:
                    pantalla.setText("3");
                    break;
                case R.id.B4:
                    pantalla.setText("4");
                    break;
                case R.id.B5:
                    pantalla.setText("5");
                    break;
                case R.id.B6:
                    pantalla.setText("6");
                    break;
                case R.id.B7:
                    pantalla.setText("7");
                    break;
                case R.id.B8:
                    pantalla.setText("8");
                    break;
                case R.id.B9:
                    pantalla.setText("9");
                    break;

                case R.id.Bcoma:
                    pantalla.setText(",");
                    break;

                case R.id.Bmult:
                    break;
                case R.id.B5restar:
                    break;
                case R.id.B6sumar:
                    break;
                case R.id.Bdividir:
                    break;
                case R.id.BAC:
                    break;

                case R.id.Bsin:
                    break;
                case R.id.Bcos:
                    break;
                case R.id.Btan:
                    break;
            }

        }catch(Exception e){
            pantalla.setText("error");
        };

    }
}

```