Android - Acción botón

package com.example.misegundoejemplo;

import android.support.v7.app.ActionBarActivity;

import android.support.v4.app.Fragment;

import android.os.Bundle;

import android.view.LayoutInflater;

import android.view.Menu;

import android.view.MenuItem;

import android.view.View;

import android.view.ViewGroup;

import android.widget.EditText;

import android.widget.ImageView;

import android.widget.TextView;

public class MainActivity extends ActionBarActivity {

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        if (savedInstanceState == null) {

            getSupportFragmentManager().beginTransaction()

                    .add(R.id.container, new PlaceholderFragment())

                    .commit();

        }

    }

    @Override

    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.

        getMenuInflater().inflate(R.menu.main, menu);

        return true;

    }

    @Override

    public boolean onOptionsItemSelected(MenuItem item) {

        // Handle action bar item clicks here. The action bar will

        // automatically handle clicks on the Home/Up button, so long

        // as you specify a parent activity in AndroidManifest.xml.

        int id = item.getItemId();

        if (id == R.id.action_settings) {

            return true;

        }

        return super.onOptionsItemSelected(item);

    }

    /**

     * A placeholder fragment containing a simple view.

     */

    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {

        }

        @Override

        public View onCreateView(LayoutInflater inflater, ViewGroup container,

                Bundle savedInstanceState) {

            View rootView = inflater.inflate(R.layout.fragment_main, container, false);

            return rootView;

        }

    }

    public void mostrar(View i){

    		TextView text1 = (TextView) findViewById(R.id.textView1);

    		EditText text2 = (EditText) findViewById(R.id.mensaje);

    		ImageView imag = (ImageView) findViewById(R.id.imageView1);

    		text1.setText(text2.getText());

    		if(imag.getVisibility()==4)

    			imag.setVisibility(0);

    		else

    			imag.setVisibility(4);

    		 i.setVisibility(View.VISIBLE);

    }

}