JavaFX  in Java 

class Calculator
{ public double calculate
  ( final char operator,
    final double number1,
    final double number2 )
  { final double result; switch( operator )
    { case '+': result = number1 + number2; break;
      case '-': result = number1 - number2; break;
      case '*': result = number1 * number2; break;
      case '/': result = number1 / number2; break;
      default:  result = 0;                 break; }
    return result; }}
public final class Main extends javafx.application.Application
{ private final javafx.scene.control.TextField n
   = new javafx.scene.control.TextField();
  private final javafx.scene.control.TextField m
  = new javafx.scene.control.TextField();
  private final javafx.scene.control.TextField r
  = new javafx.scene.control.TextField();
  final Calculator calculator = new Calculator();
  final @java.lang.Override public void start
  ( final javafx.stage.Stage primaryStage )
  {
    final javafx.scene.control.MenuBar menuBar
    = new javafx.scene.control.MenuBar();
    final javafx.scene.control.Menu actionMenu
    = new javafx.scene.control.Menu( "Aktion" );
    final javafx.scene.control.Menu exitMenu
    = new javafx.scene.control.Menu( "Beenden" );
    menuBar.getMenus().addAll( actionMenu, exitMenu );
    final javafx.scene.control.MenuItem addMenuItem
    = new javafx.scene.control.MenuItem( "Addieren" );
    final javafx.scene.control.MenuItem subtractMenuItem
    = new javafx.scene.control.MenuItem( "Subtrahieren" );
    final javafx.scene.control.MenuItem multiplyMenuItem
    = new javafx.scene.control.MenuItem( "Multiplizieren" );
    final javafx.scene.control.MenuItem menuItemDivide
    = new javafx.scene.control.MenuItem( "Teilen" );
    actionMenu.getItems().addAll
    ( addMenuItem, subtractMenuItem,
      multiplyMenuItem, menuItemDivide );
    final javafx.scene.control.MenuItem closeMenuItem
    = new javafx.scene.control.MenuItem( "Schliessen" );
    exitMenu.getItems().add( closeMenuItem );
    final javafx.scene.layout.HBox hBox1
    = new javafx.scene.layout.HBox( 10 );
    n.setPrefColumnCount( 3 );
    m.setPrefColumnCount( 3 );
    r.setPrefColumnCount( 3 );
    hBox1.getChildren().addAll
    ( new javafx.scene.control.Label( "Zahl 1:" ),   n,
      new javafx.scene.control.Label( "Zahl 2:" ),   m,
      new javafx.scene.control.Label( "Ergebnis:" ), r );
    hBox1.setAlignment( javafx.geometry.Pos.CENTER );
    final javafx.scene.layout.HBox hBox2
    = new javafx.scene.layout.HBox( 10 );
    final javafx.scene.control.Button addButton
    = new javafx.scene.control.Button( "Addieren" );
    final javafx.scene.control.Button subtractButton
    = new javafx.scene.control.Button( "Subtrahieren" );
    final javafx.scene.control.Button multiplyButton
    = new javafx.scene.control.Button( "Multiplizieren" );
    final javafx.scene.control.Button divideButton
    = new javafx.scene.control.Button( "Dividieren" );
    hBox2.getChildren().addAll
    ( addButton, subtractButton, multiplyButton, divideButton );
    hBox2.setAlignment( javafx.geometry.Pos.CENTER );
    final javafx.scene.layout.VBox vBox
    = new javafx.scene.layout.VBox( 12 );
    vBox.getChildren().addAll( menuBar, hBox1, hBox2 );
    final javafx.scene.Scene scene
    = new javafx.scene.Scene( vBox, 300, 100 );
    primaryStage.setTitle( "Rechner" );
    primaryStage.setScene( scene );
    primaryStage.show();
    addMenuItem.setOnAction(      e -> perform( '+' ));
    subtractMenuItem.setOnAction( e -> perform( '-' ));
    multiplyMenuItem.setOnAction( e -> perform( '*' ));
    menuItemDivide.setOnAction(   e -> perform( '/' ));
    closeMenuItem.setOnAction(    e -> java.lang.System.exit( 0 ));
    addButton.setOnAction(        e -> perform( '+' ));
    subtractButton.setOnAction(   e -> perform( '-' ));
    multiplyButton.setOnAction(   e -> perform( '*' ));
    divideButton.setOnAction(     e -> perform( '/' )); }
  private void perform( final char operator )
  { r.setText
    ( calculator.calculate
      ( operator,
        java.lang.Double.parseDouble( n.getText()),
        java.lang.Double.parseDouble( m.getText() )) + "" ); }; }