Reimplament this app to use property listeners to perform calculations whenever the user updates the bill amount or changes the custom tip percentage, rather than the "calculate" button (remove button). Also use a property binding to update the label that displays the tip percentage:

import javafx.application.Application; import javafx.fxml.FXMLLoader; import javafx.scene.Parent; import javafx.scene.Scene; import javafx.stage.Stage; public class TipCalculator extends Application { @Override public void start(Stage stage) throws Exception { Parent root = FXMLLoader.load(getClass().getResource("TipCalculator.fxml")); Scene scene = new Scene(root); // attach scene graph to scene  stage.setTitle("Tip Calculator"); // displayed in window's title bar  stage.setScene(scene); // attach scene to stage  stage.show(); // display the stage  } public static void main(String[] args) { // create a TipCalculator object and call its start method  launch(args); } } 

import java.math.BigDecimal; import java.math.RoundingMode; import java.text.NumberFormat; import javafx.beans.value.ChangeListener; import javafx.beans.value.ObservableValue; import javafx.event.ActionEvent; import javafx.fxml.FXML; import javafx.scene.control.Label; import javafx.scene.control.Slider; import javafx.scene.control.TextField; public class TipCalculatorController { // formatters for currency and percentages  private static final NumberFormat currency = NumberFormat.getCurrencyInstance(); private static final NumberFormat percent = NumberFormat.getPercentInstance(); private BigDecimal tipPercentage = new BigDecimal(0.15); // 15% default   // GUI controls defined in FXML and used by the controller's code  @FXML private TextField amountTextField; @FXML private Label tipPercentageLabel; @FXML private Slider tipPercentageSlider; @FXML private TextField tipTextField; @FXML private TextField totalTextField; // calculates and displays the tip and total amounts  @FXML private void calculateButtonPressed(ActionEvent event) { try { BigDecimal amount = new BigDecimal(amountTextField.getText()); BigDecimal tip = amount.multiply(tipPercentage); BigDecimal total = amount.add(tip); tipTextField.setText(currency.format(tip)); totalTextField.setText(currency.format(total)); } catch (NumberFormatException ex) { amountTextField.setText("Enter amount"); amountTextField.selectAll(); amountTextField.requestFocus(); } } // called by FXMLLoader to initialize the controller  public void initialize() { // 0-4 rounds down, 5-9 rounds up  currency.setRoundingMode(RoundingMode.HALF_UP); // listener for changes to tipPercentageSlider's value  tipPercentageSlider.valueProperty().addListener( new ChangeListener() { @Override public void changed(ObservableValueextends Number> ov, Number oldValue, Number newValue) { tipPercentage = BigDecimal.valueOf(newValue.intValue() / 100.0); tipPercentageLabel.setText(percent.format(tipPercentage)); } } ); } } 

xml version="1.0" encoding="UTF-8"?>  import javafx.geometry.Insets?> import javafx.scene.control.Button?> import javafx.scene.control.Label?> import javafx.scene.control.Slider?> import javafx.scene.control.TextField?> import javafx.scene.layout.ColumnConstraints?> import javafx.scene.layout.GridPane?> import javafx.scene.layout.RowConstraints?>  <GridPane hgap="8.0" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" xmlns="http://javafx.com/javafx/8.0.60" xmlns:fx="http://javafx.com/fxml/1" fx:controller="TipCalculatorController"> <columnConstraints> <ColumnConstraints halignment="RIGHT" hgrow="SOMETIMES" minWidth="10.0" /> <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" /> columnConstraints> <rowConstraints> <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" /> <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" /> <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" /> <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" /> <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" /> rowConstraints> <children> <Label text="Amount" /> <Label fx:id="tipPercentageLabel" text="15%" GridPane.rowIndex="1" /> <Label text="Tip" GridPane.rowIndex="2" /> <Label text="Total" GridPane.rowIndex="3" /> <TextField fx:id="amountTextField" GridPane.columnIndex="1" /> <TextField fx:id="tipTextField" editable="false" focusTraversable="false" GridPane.columnIndex="1" GridPane.rowIndex="2" /> <TextField fx:id="totalTextField" editable="false" focusTraversable="false" GridPane.columnIndex="1" GridPane.rowIndex="3" /> <Slider fx:id="tipPercentageSlider" blockIncrement="5.0" max="30.0" value="15.0" GridPane.columnIndex="1" GridPane.rowIndex="1" /> <Button maxWidth="1.7976931348623157E308" mnemonicParsing="false" onAction="#calculateButtonPressed" text="Calculate" GridPane.columnIndex="1" GridPane.rowIndex="4" /> children> <padding> <Insets bottom="14.0" left="14.0" right="14.0" top="14.0" /> padding> GridPane>