MainActivity.java

package com.app.mungoi.minhaprimeiraapp;

import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.FrameLayout;


public class MainActivity2 extends ActionBarActivity {

    /** Framelayout que ira conter o tablet caso seja um smartphone*//
    private FrameLayoutLayout mContainer;

    private Fragment mFragment;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_activity);

        mContainer = (FrameLayout) findViewById(R.id.container);

        if(mContainer==null)
        {
            //O xml carregado nao contem nenhum framelayout logo estamos a correr a aplicacao de um tablet
            //serao apresentados dois paineis

        }
        else
        {
            //estamos no layout do celular devemos colocar o primeiro fragment no container
            mFragment = new FragmentA();

            //Pegar um Fragment Transaction responsavel por adicionar, modificar e remover fragments

            FragmentTransaction mFragmentTrasaction = getSupportFragmentManager().beginTransaction();

            /**Adiciona um fragment passando o id do container(FrameLayout), o fragment, e uma tag opcional para poder identificar o fragment posteriormente
             *
             */
            mFragmentTrasaction.add(R.id.container,mFragment);
            
            //E sempre importante fazer o commit para que as mudancas seja efectuadas caso nao todo codigo acima nao tera nenhum efeito
            mFragmentTrasaction.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.menu_main_activity2, menu);
        return true;
    }

}