with this short code you can delete empty space at start and end of the string. If the string is "" return the message "error" else you ave a string
EditText user = findViewById(R.id.user);
userString = user.getText().toString().trim();
if (userString.matches("")) {
Toast.makeText(this, "Error", Toast.LENGTH_SHORT).show();
return;
}else{
Toast.makeText(this, "Ok", Toast.LENGTH_SHORT).show();
}