Android : How do I rewrite this so that I only use one activity?

I have two different activities, and each one is a different page of the app. But I want to put everything into one activity. I also noticed that a lot of the code in these two activities are similar or exactly the same.

But the problem is, when I put in the new code from DisplayMessageActivity, I just get led back to the first page when I click "NEXT". (In simpler words, when I click "NEXT", I just go back to the page that I made in MainActivity) Any help is appreciated!

Here is my MainActivity:

package com.example.myfirstapp;

import android.app.Activity;
import android.app.Fragment;
import android.graphics.Typeface;
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.content.Intent;
import android.widget.EditText;
import android.widget.TextView;
//import android.widget.TextView;


public class MainActivity extends Activity {
    public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";

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


        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.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;
        }
    }

    /** Called when the user clicks the Send button */
    public void sendMessage(View view) {
        Intent intent = new Intent(this, DisplayMessageActivity.class);
        EditText editText = (EditText) findViewById(R.id.edit_message);
        String message = editText.getText().toString();
        intent.putExtra(EXTRA_MESSAGE, message);
        startActivity(intent);
    }
}

And here is my DisplayMessageActivity:

package com.example.myfirstapp;

import android.app.Activity;
import android.app.ActionBar;
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.content.Intent;
import android.widget.TextView;
import android.graphics.Typeface;
import android.os.Build;

import com.example.myfirstapp.R;

public class DisplayMessageActivity extends Activity {

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

        // Get the message from the intent
        Intent intent = getIntent();
        String message = "Hi " + intent.getStringExtra(MainActivity.EXTRA_MESSAGE) + "!";

        // Create the text view
        //   TextView textView = new TextView(this);
        //   textView.setTextSize(25);
        //   textView.setText(message);
        TextView textView1 = (TextView) findViewById(R.id.text1);
        TextView textView2 = (TextView) findViewById(R.id.text2);


        textView1.setText(message);
        textView1.setTextSize(35);
        textView2.setTextSize(30);


        // Font path
        String fontPath = "fonts/Sketchy.ttf";

        // Loading Font Face
        Typeface tf = Typeface.createFromAsset(getAssets(), fontPath);

        // Applying font
        textView1.setTypeface(tf);
        textView2.setTypeface(tf);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.display_message, 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_display_message, container, false);
            return rootView;
        }
    }

    public void sendMessage(View view) {
        Intent intent = new Intent(this, FlashlightActivity.class);
        startActivity(intent);
    }

}