---- activity main

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:background="#fff"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin">

    <android.support.design.widget.TextInputLayout
        android:id="@+id/text_input_username"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <EditText
            android:id="@+id/et_username"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="UserName"/>

    </android.support.design.widget.TextInputLayout>

    <android.support.design.widget.TextInputLayout
        android:layout_below="@id/text_input_username"
        android:id="@+id/text_input_password"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <EditText
            android:id="@+id/et_password"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:inputType="textPassword"
            android:hint="Password"/>

    </android.support.design.widget.TextInputLayout>

    <Button
        android:text="Login"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/text_input_password"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="60dp"
        android:id="@+id/action_login" />

</RelativeLayout>


---activity home screen

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_home_screen"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.lpirro.applogin.HomeScreenActivity">

    <TextView
        android:text="TextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true"
        android:id="@+id/textview_username" />
</RelativeLayout>


----- homescreenjava
package com.example.lpirro.applogin;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
import android.widget.Toast;

public class HomeScreenActivity extends AppCompatActivity {

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

        // Prelevo i dati passati dall'Activity precedente attraverso gli Extras dell'Intent
        Intent intent = getIntent();
        String username = intent.getStringExtra("username");

        TextView textViewUsername = (TextView)findViewById(R.id.textview_username);

        // Setto il testo della TextView con l'username
        textViewUsername.setText(username);
    }
}

---- activity main

package com.example.lpirro.applogin;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private final String USERNAME = "lpirro";
    private final String PASSWORD = "valentinorossi";
    private EditText editTextUsername;
    private EditText editTextPassword;
    private Button buttonLogin;


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

        // Istanze degli oggetti del layout
        editTextUsername = (EditText)findViewById(R.id.et_username);
        editTextPassword = (EditText)findViewById(R.id.et_password);
        buttonLogin = (Button)findViewById(R.id.action_login);

        // Registro l'acoltatore
        buttonLogin.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        // Prelevo username e password dalle EditText
        String username = editTextUsername.getText().toString();
        String password = editTextPassword.getText().toString();

        // Controllo se i dati inseriti nelle EditText corrispondono e in caso positivo avvio una
        // nuova intent che mostra l'username dell'utente loggato. In caso negativo mostra un Toast con "Login non riuscito"
        if(username.equals(USERNAME) && password.equals(PASSWORD)){
            Intent intent = new Intent(MainActivity.this, HomeScreenActivity.class);
            intent.putExtra("username", username);
            startActivity(intent);
        }
        else
            Toast.makeText(this, "Login non riuscito", Toast.LENGTH_SHORT).show();
    }
}