build-a-selfdestructing-message-android-app: Error Messages and Dialogs

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.usenameField);
    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.sign_up_error_message)
                    .setTitle(R.string.sign_up_error_title)
                    .setPositiveButton(android.R.string.ok, null);
                AlertDialog dialog = builder.create();
                dialog.show();
            }
            else {
                // create the new user!
            }
        }
    });
}




@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;
}

<EditText
    android:id="@+id/usernameField"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true"
    android:ems="10"
    android:hint="@string/username_hint" >

    <requestFocus />
</EditText>

<EditText
    android:id="@+id/passwordField"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/usernameField"
    android:layout_below="@+id/usernameField"
    android:ems="10"
    android:inputType="textPassword"
    android:hint="@string/password_hint" />

<EditText
    android:id="@+id/emailField"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/passwordField"
    android:layout_below="@+id/passwordField"
    android:ems="10"
    android:inputType="textEmailAddress"
    android:hint="@string/email_hint" />

<Button
    android:id="@+id/signupButton"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/emailField"
    android:layout_below="@+id/emailField"
    android:text="@string/sign_up_button_label" />