Getting SignUpActivity to work with Fragments...

package com.corestorm.ribbit;

import android.app.Activity;
import android.app.AlertDialog;
import android.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.Button;
import android.widget.EditText;

public class SignUpActivity extends Activity {

    protected EditText mUsername;
    protected EditText mPassword;
    protected EditText mEmail;
    protected Button mSignUpButton;

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

        mUsername = (EditText) findViewById(R.id.usernameField);
        mPassword = (EditText) findViewById(R.id.passwordField);
        mEmail = (EditText) findViewById(R.id.emailField);
        mSignUpButton = (Button) findViewById(R.id.signupButton);
        mSignUpButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                String username = mUsername.getText().toString();
                String password = mPassword.getText().toString();
                String email = mEmail.getText().toString();

                username = username.trim();
                password = password.trim();
                email = email.trim();

                if (username.isEmpty() || password.isEmpty() || email.isEmpty()) {
                    AlertDialog.Builder builder = new AlertDialog.Builder(
                            SignUpActivity.this);
                    builder.setMessage(R.string.signup_error_message)
                            .setTitle(R.string.signup_error_title)
                            .setPositiveButton(android.R.string.ok, null);
                    AlertDialog dialog = builder.create();
                    dialog.show();
                } else {
                    // create new user!
                }
            }
        });

        if (savedInstanceState == null) {
            getFragmentManager().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.sign_up, 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_sign_up,
                    container, false);
            return rootView;
        }
    }

}