import javafx.application.*;
import javafx.scene.*;
import javafx.scene.control.*;
import javafx.scene.layout.FlowPane;
import javafx.stage.*;
import javafx.event.*;
import javafx.geometry.Pos;
public class Main extends Application{
Label lbUsername;
Label lbPassword;
Label lbInfoPanel;
Button btnLogIn;
TextField flUsername;
PasswordField flPassword;
String Username;
String Password;
public static void main(String[] args) {
    launch(args);
}
public void start(Stage myStage){
    myStage.setTitle("Login");
    FlowPane rootNode = new FlowPane(10,10);
    rootNode.setAlignment(Pos.CENTER);
    Scene myScene1 = new Scene(rootNode, 300, 300);
    myStage.setScene(myScene1);
    lbUsername = new Label("Enter a Username");
    lbPassword = new Label("Enter a password");
    btnLogIn = new Button("Log in");
    flUsername = new TextField();
    flPassword = new PasswordField();
    lbInfoPanel = new Label();
    Username = new String();
    Password = new String();
    btnLogIn.setOnAction(new EventHandler<ActionEvent>() {

        @Override
        public void handle(ActionEvent event) {
            Username = flUsername.getText();
            Password = flPassword.getText();
            if ((Username == "") || (Username == null) || (Password == "") || (Password == null))
                lbInfoPanel.setText("You forget to enter username of password");

        }
    });
    rootNode.getChildren().addAll(lbUsername, flUsername, lbPassword, flPassword,
            btnLogIn,lbInfoPanel);
    myStage.show();
}

if (Username.isEmpty() || Password.isEmpty()){
            lbInfoPanel.setText("You forget to enter username of password");
    }

//вот пример:
String name1 = "User";
String name2 = "Login";
String name3 = "User";
String name4 = "login";

//Присвоется значение True так как строки идеентичны
boolean check1 = name1.equals(name3);

//Присвоется значение False так как строки Login и login не идеентичны, ведь не забываем то что Java чувствителен к регистру.
boolean check2 = name2.equals(name4);

//присвоется значение False так как строка не пустая и содержит какой-то текст
boolean check3 = name3.isEmpty();

boolean b = strOne.equals(strTwo);

if (Username == null || Password == null) {
    lbInfoPanel.setText("Something going wrong");
} else if (Username.isEmpty() || Password.isEmpty()) {
    lbInfoPanel.setText("You forget to enter username of password");
}