dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])

    compile 'com.android.support:design:22.2.0'
    compile 'com.android.support:appcompat-v7:22.2.0'
  }

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:background="#e3e3e3"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="@dimen/activity_horizontal_margin"
    tools:context=".LoginActivity"
    android:orientation="vertical">

    <RelativeLayout
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_weight="0.5"
      android:orientation="vertical">

      <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:gravity="center"
        android:text="Welcome"
        android:textSize="30sp"
        android:textColor="#333333"/>

    </RelativeLayout>


    <LinearLayout
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_weight="0.5"
      android:orientation="vertical">

      <EditText
        android:id="@+id/username"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="textEmailAddress"/>

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

      <Button
        android:id="@+id/btn"
        android:layout_marginTop="4dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Login"/>

    </LinearLayout>

  </LinearLayout>

<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
 </style>

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

    <EditText
      android:id="@+id/username"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:inputType="textEmailAddress"
      android:hint="Username"/>

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

  <android.support.design.widget.TextInputLayout
    android:id="@+id/passwordWrapper"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_below="@id/usernameWrapper"
    android:layout_marginTop="4dp">

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

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

final TextInputLayout usernameWrapper = (TextInputLayout) findViewById(R.id.usernameWrapper);
final TextInputLayout passwordWrapper = (TextInputLayout) findViewById(R.id.passwordWrapper);

usernameWrapper.setHint("Username");
passwordWrapper.setHint("Password");

  btn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
      // STUB
    }
  });

  private void hideKeyboard() {
    View view = getCurrentFocus();
    if (view != null) {
      ((InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE)).
        hideSoftInputFromWindow(view.getWindowToken(), InputMethodManager.HIDE_NOT_ALWAYS);
    }
  }

  private static final String EMAIL_PATTERN = "^[a-zA-Z0-9#_~!$&'()*+,;=:.\"(),:;<>@\\[\\]\\\\]+@[a-zA-Z0-9-]+(\\.[a-zA-Z0-9-]+)*$";
  private Pattern pattern = Pattern.compile(EMAIL_PATTERN);
  private Matcher matcher;

  public boolean validateEmail(String email) {
    matcher = pattern.matcher(email);
    return matcher.matches();
  }

public boolean validatePassword(String password) {
    return password.length() > 5;
  }

public void onClick(View v) {
    hideKeyboard();

    String username = usernameWrapper.getEditText().getText().toString();
    String password = passwordWrapper.getEditText().getText().toString();

    // TODO: Checks

    // TODO: Login
  }

public void onClick(View v) {
    hideKeyboard();

    String username = usernameWrapper.getEditText().getText().toString();
    String password = usernameWrapper.getEditText().getText().toString();
    if (!validateEmail(username)) {
      usernameWrapper.setError("Not a valid email address!");
    } else if (!validatePassword(password)) {
      passwordWrapper.setError("Not a valid password!");
    } else {
      usernameWrapper.setErrorEnabled(false);
      passwordWrapper.setErrorEnabled(false);
      doLogin();
    }
  }

public void doLogin() {
    Toast.makeText(getApplicationContext(), "OK! I'm performing login.", Toast.LENGTH_SHORT).show();
    // TODO: login procedure; not within the scope of this tutorial.
  }

 <style name="AppTheme" parent="Theme.A
  <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
    <item name="colorAccent">#3498db</item>
  </style>