Main.java

package ch.zhaw.prog2.fxmlcalculator;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

import java.io.IOException;

/**
 * Main-Application. Opens the first window (MainWindow) and the common ValueHandler
 * @author
 * @version 1.0
 */
public class Main extends Application {

    private ValueHandler valueHandler;

	public static void main(String[] args) {
		launch(args);
	}

	@Override
	public void start(Stage primaryStage) {
		valueHandler = new ValueHandler();
		mainWindow(primaryStage);
	}

	private void mainWindow(Stage primaryStage) {
	    try {
            Parent root = FXMLLoader.load(getClass().getResource("MainWindow.fxml"));

            Scene scene = new Scene(root);
            primaryStage.setTitle("Return on investment calculator");
            primaryStage.setScene(scene);

            primaryStage.setMinWidth(250);
            primaryStage.setMinHeight(350);

            primaryStage.show();
        } catch (IOException e) {
	        throw new RuntimeException(e);
        }
	}
}

MainWindow.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.geometry.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.input.*?>
<?import javafx.scene.layout.*?>

<AnchorPane xmlns="http://javafx.com/javafx/11.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="ch.zhaw.prog2.fxmlcalculator.MainWindowController">
    <BorderPane AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
        <top>
            <MenuBar fx:id="menuBar" BorderPane.alignment="TOP_LEFT">
                <Menu text="Clear">
                    <CheckMenuItem fx:id="checkmenuitem_clear_initial_amount" text="Initial amount" />
                    <CheckMenuItem fx:id="checkmenuitem_return_in_percent" text="Return in %" />
                    <CheckMenuItem fx:id="checkmenuitem_annual_cost" mnemonicParsing="false" text="Annual Costs" />
                    <CheckMenuItem fx:id="checkmenuitem_number_years" mnemonicParsing="false" text="Number of years" />
                    <SeparatorMenuItem />
                    <MenuItem fx:id="menuitem_clear_values" text="Clear values" />
                    <SeparatorMenuItem />
                    <MenuItem fx:id="menuitem_clear_results" text="Clear results" />
                </Menu>
                <Menu text="?">
                    <MenuItem fx:id="menuitem_show_help" text="Show Help">
                        <accelerator>
                            <KeyCodeCombination alt="UP" code="F1" control="UP" meta="UP" shift="UP" shortcut="UP" />
                        </accelerator>
                    </MenuItem>
                </Menu>
            </MenuBar>
        </top>
        <center>
            <VBox spacing="10.0">
                <HBox alignment="CENTER_LEFT">
                    <Label minWidth="100.0" text="Initial amount" />
                    <TextField fx:id="textfield_initial_amount" />
                </HBox>
                <HBox alignment="CENTER_LEFT">
                    <Label contentDisplay="CENTER" minWidth="100.0" text="Return rate in %" />
                    <TextField fx:id="textfield_return_rate" />
                </HBox>
                <HBox alignment="CENTER_LEFT">
                    <Label minWidth="100.0" text="Annual cost" />
                    <TextField fx:id="textfield_annual_cost" />
                </HBox>
                <HBox alignment="CENTER_LEFT">
                    <Label minWidth="100.0" text="Number of years" />
                    <TextField fx:id="textfield_number_of_years" />
                </HBox>
                <VBox alignment="CENTER_LEFT" VBox.vgrow="ALWAYS">
                    <Label text="Results" />
                    <TextArea fx:id="textarea_results" editable="false" VBox.vgrow="ALWAYS" />
                </VBox>
                <BorderPane.margin>
                    <Insets bottom="10.0" left="10.0" right="10.0" top="10.0" />
                </BorderPane.margin>
            </VBox>
        </center>
        <bottom>
            <HBox alignment="CENTER" spacing="10">
                <Button fx:id="button_calculate" text="Calculate" />
                <Button fx:id="button_close" text="Close" />
                <Button fx:id="button_open_results" text="Results" />
                <BorderPane.margin>
                    <Insets bottom="10.0" left="10.0" right="10.0" />
                </BorderPane.margin>
            </HBox>
        </bottom>
    </BorderPane>
</AnchorPane>

MainWindowController.java

package ch.zhaw.prog2.fxmlcalculator;

import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.stage.Stage;

import java.io.IOException;


/**
 * Controller for the MainWindow. One controller per mask (or FXML file)
 * Contains everything the controller has to reach in the view (controls)
 * and all methods the view calls based on events.
 * @author
 * @version 1.0
 */
public class MainWindowController {
    @FXML private MenuBar menuBar;
    @FXML private CheckMenuItem checkmenuitem_clear_initial_amount;
    @FXML private CheckMenuItem checkmenuitem_return_in_percent;
    @FXML private CheckMenuItem checkmenuitem_annual_cost;
    @FXML private CheckMenuItem checkmenuitem_number_years;
    @FXML private MenuItem menuitem_clear_values;
    @FXML private MenuItem menuitem_clear_results;
    @FXML private MenuItem menuitem_show_help;

    @FXML private TextArea textarea_results;

    @FXML private TextField textfield_initial_amount;
    @FXML private TextField textfield_return_rate;
    @FXML private TextField textfield_annual_cost;
    @FXML private TextField textfield_number_of_years;

    @FXML private Button button_calculate;
    @FXML private Button button_close;
    @FXML private Button button_open_results;

    private ValueHandler valueHandler;

    /**
     * Constructor
     */
    public MainWindowController() {
        valueHandler = new ValueHandler();
    }

    // called by the FXML loader after the labels declared above are injected:
    public void initialize() {
        button_close.setOnAction((event) -> {
            Stage stage = (Stage) button_close.getScene().getWindow();
            stage.close();
        });

        button_calculate.setOnAction((event) -> {
            String initialAmount = textfield_initial_amount.getText();
            String returnRate = textfield_return_rate.getText();
            String annualCost = textfield_annual_cost.getText();
            String numberOfYears = textfield_number_of_years.getText();
            valueHandler.checkAndCalculateResult(initialAmount, returnRate, annualCost, numberOfYears);
            if (valueHandler.areValuesOk()) {
                textarea_results.setStyle("-fx-border-color: green; -fx-focus-color: green");
                textarea_results.setText(valueHandler.getResultBound());
            } else {
                textarea_results.setStyle("-fx-border-color: red; -fx-focus-color: red");
                textarea_results.setText(valueHandler.getResultBound());
            }
        });

        // menu bar connections
        menuitem_clear_values.setOnAction((event) -> {
            if (checkmenuitem_clear_initial_amount.isSelected()) {
                textfield_initial_amount.clear();
            }
            if (checkmenuitem_return_in_percent.isSelected()) {
                textfield_return_rate.clear();
            }
            if (checkmenuitem_annual_cost.isSelected()) {
                textfield_annual_cost.clear();
            }
            if (checkmenuitem_number_years.isSelected()) {
                textfield_number_of_years.clear();
            }
        });
        menuitem_clear_results.setOnAction(event -> textarea_results.clear());
        menuitem_show_help.setOnAction(event -> {
            textarea_results.setStyle("-fx-border-color: blue; -fx-focus-color:blue");
            textarea_results.setText(valueHandler.getHelpText());
        });

        valueHandler.resultBoundProperty().addListener((observable, oldValue, newValue) -> {

        });

        button_open_results.setOnAction(event -> {
            try {
                FXMLLoader loader = new FXMLLoader(getClass().getResource("ResultWindow.fxml"));
                Parent root = loader.load();

                Scene scene = new Scene(root);
                Stage stageOfNewWindow = new Stage();

                stageOfNewWindow.setTitle("Results");
                stageOfNewWindow.setScene(scene);

                stageOfNewWindow.setMinWidth(410);
                stageOfNewWindow.setMinHeight(280);

//                stageOfNewWindow.initOwner(textarea_results.getScene().getWindow());
//                stageOfNewWindow.initModality(Modality.WINDOW_MODAL);

                ((ResultWindowController) loader.getController()).setValueHandler(valueHandler);
                // we need to fire the calculate button to update the text on the new window
                button_calculate.fire();

                stageOfNewWindow.show();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        });
    } // initialize
}

ResultWindow.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import javafx.geometry.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.text.*?>
<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.TextArea?>
<?import javafx.scene.input.KeyCodeCombination?>
<?import javafx.scene.layout.BorderPane?>
<?import javafx.scene.layout.ColumnConstraints?>
<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.layout.RowConstraints?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.text.Font?>

<BorderPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="250.0" minWidth="400.0" prefHeight="250.0" prefWidth="400.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="ch.zhaw.prog2.fxmlcalculator.ResultWindowController">
   <center>
       <VBox minWidth="300.0" BorderPane.alignment="TOP_CENTER">
           <padding>
               <Insets left="10.0" right="10.0"/>
           </padding>
           <VBox spacing="5.0" VBox.vgrow="ALWAYS">
               <VBox.margin>
                   <Insets top="10.0"/>
               </VBox.margin>
               <Label text="Results">
                   <font>
                       <Font name="System Bold" size="12.0"/>
                   </font>
               </Label>
               <TextArea fx:id="textarea_results" editable="false" minHeight="100.0" VBox.vgrow="ALWAYS"/>
           </VBox>
       </VBox>
   </center>
   <bottom>
       <HBox alignment="BOTTOM_RIGHT" BorderPane.alignment="CENTER">
           <BorderPane.margin>
               <Insets bottom="5.0" left="5.0" right="5.0" top="5.0"/>
           </BorderPane.margin>
           <Button fx:id="closeForm" mnemonicParsing="false" onMouseClicked="#closeWindow" text="Close">
              <HBox.margin>
                  <Insets bottom="5.0" left="5.0" right="5.0" top="5.0"/>
              </HBox.margin>
           </Button>
       </HBox>
   </bottom>
</BorderPane>

ResultWindowController.java

package ch.zhaw.prog2.fxmlcalculator;


import javafx.fxml.FXML;
import javafx.scene.control.TextArea;
import javafx.stage.Stage;

/**
 * Controller for the MainWindow. One controller per mask (or FXML file)
 * Contains everything the controller has to reach in the view (controls)
 * and all methods the view calls based on events.
 * @author
 * @version 1.0
 */
public class ResultWindowController {
	@FXML private TextArea textarea_results;

	@FXML
	private void closeWindow() {
		Stage stage = (Stage) textarea_results.getScene().getWindow();
		stage.close();
	}

	public void setValueHandler(ValueHandler valueHandler) {
        valueHandler.resultBoundProperty().addListener((observable, oldValue, newValue) -> {
            if (valueHandler.areValuesOk()) {
                textarea_results.setStyle("-fx-border-color: green; -fx-focus-color: green");
                textarea_results.setText(valueHandler.getResultBound());
                textarea_results.layout();
            } else {
                textarea_results.setStyle("-fx-border-color: red; -fx-focus-color: red");
                textarea_results.setText(valueHandler.getResultBound());
            }
        });
    }

}

ValueHandler.java

package ch.zhaw.prog2.fxmlcalculator;

import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;

/**
 * Handles the values from the input form. Offers the {@link resultBound} StringProperty to listen from
 * a GUI (bind to a field in the GUI)
 * @author bles
 * @version 1.1
 * MODEL
 */
public class ValueHandler {

	private double initialAmount;
	private double returnInPercent;
	private double annualCost;
	private int numberOfYears;
	private boolean valuesOk = false;
	// Solution with bound properties
	private final StringProperty resultBound = new SimpleStringProperty();

	public ValueHandler() {
	}

	/**
	 * Check the input values are valid (can be improved)
	 * If not ok, fill error message to the results and return false
	 * If ok, set the date fields and return true
	 */
	private String checkValues(String initialAmount, String returnInPercent, String annualCost, String numberOfYears) {
		StringBuilder sb = new StringBuilder();
		valuesOk = true;
		if ("".equals(initialAmount) || Double.parseDouble(initialAmount)<=0) {
			sb.append("Please specify a positive  initial amount!\n");
			valuesOk = false;
		} else {
			this.initialAmount = Double.parseDouble(initialAmount);
		}
		if ("".equals(returnInPercent)) {
			sb.append("Please specify the annual return rate in %!\n");
			valuesOk = false;
		} else {
			this.returnInPercent = Double.parseDouble(returnInPercent)/100;
		}
		if ("".equals(annualCost) || Double.parseDouble(annualCost)<0) {
			sb.append("Please specify the annual cost!\n");
			valuesOk = false;
		} else {
			this.annualCost = Double.parseDouble(annualCost);
		}
		if ("".equals(numberOfYears) ||
				Double.parseDouble(numberOfYears) < 1 ||
				Double.parseDouble(numberOfYears) > 99 ||
				Math.round(Double.parseDouble(numberOfYears))!=Double.parseDouble(numberOfYears)) {

			sb.append("Please enter a time period in years!");
			valuesOk = false;
		} else {
			this.numberOfYears = Integer.parseInt(numberOfYears);
		}
		return sb.toString();
	}

	/**
	 * Calculates the result
	 * @return the result as a String
	 */
	private String calculateResult() {
		StringBuilder resultSB = new StringBuilder();
		double val = initialAmount;
		for(int i = 1; i <= numberOfYears; i++) {
			resultSB.append("After ");
			resultSB.append(i).append(" year(s): ");
			val = val * (1 + returnInPercent) - annualCost;
			resultSB.append(Math.round(val)).append("\n");
		}
		return resultSB.toString();
	}

	/**
	 * Result String can be "", if no calculation or check is done
	 * @return	String with the result of the value checking or the calculation
	 */
	public String getResultBound() {
		return resultBound.get();
	}

	/**
	 * Set the result to the string in the parameter
	 * @param infoText
	 */
	public void setResultBound(String infoText) {
		resultBound.set(infoText);
	}

	/**
	 * If the values checked by {@link checkValues} are ok, the return is true
	 * @return	true, if ok
	 */
	public boolean areValuesOk() {
		return valuesOk;
	}

	// Solution with bound properties
	/**
	 * The property to bind
	 * @return
	 */
	public StringProperty resultBoundProperty() {
		return resultBound;
	}
	/**
	 * Checks and calculates the result. All values as String (from the Text-Fields)
	 * If the check fails, an error message is set to the bound result property
	 * @param initialAmount
	 * @param returnInPercent
	 * @param annualCost
	 * @param numberOfYears
	 */
	public void checkAndCalculateResult(String initialAmount, String returnInPercent, String annualCost, String numberOfYears) {
		setResultBound(checkValues(initialAmount, returnInPercent, annualCost, numberOfYears));
		if(valuesOk) {
			setResultBound(calculateResult());
		}
	}

    public String getHelpText() {
	    return "Enter valid values to\n" +
            "- Initial amount (>0)\n" +
            "- Return in % (can be +/- or 0)\n" +
            "- Annual costs (>0)\n" +
            "- Number of years (>0)\n" +
            "\n" +
            "Calculate display the annual balance development!";
    }
}