Chapter 16 Exercise 13, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

16.13 (Compare loans with various interest rates)
Rewrite Programming Exercise 5.21 to create a GUI,
as shown in Figure 16.41b. Your program should let
the user enter the loan amount and loan period in
the number of years from text fields, and it should
display the monthly and total payments for each interest
rate starting from 5 percent to 8 percent,
with increments of one-eighth, in a text area.

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class Exercise_13 extends Application {

    TextField tfLoanAmount = new TextField();
    TextField tfNumOfYears = new TextField();
    TextArea textArea = new TextArea();

    @Override
    public void start(Stage primaryStage) {

        EventHandler<ActionEvent> handler = event -> updateRates();
        tfNumOfYears.setOnAction(handler);
        tfLoanAmount.setOnAction(handler);

        HBox topPane = new HBox();
        topPane.setSpacing(10);
        topPane.setPadding(new Insets(5));
        Label lblLoanAmount = new Label("Loan Amount:", tfLoanAmount);
        lblLoanAmount.setContentDisplay(ContentDisplay.RIGHT);

        Label lblNumOfYears = new Label("Number Of Years:", tfNumOfYears);
        lblNumOfYears.setContentDisplay(ContentDisplay.RIGHT);

        Button btShowTable = new Button("Show Table");
        btShowTable.setOnAction(handler);
        topPane.getChildren().addAll(lblLoanAmount, lblNumOfYears, btShowTable);


        BorderPane borderPane = new BorderPane(textArea);
        borderPane.setTop(topPane);

        primaryStage.setScene(new Scene(borderPane));
        primaryStage.setTitle("Display Loan");
        primaryStage.show();
    }

    private void updateRates() {
        double annualInterestRate = 5.00;

        double loanAmount = Double.parseDouble(tfLoanAmount.getText());

        double numberOfYears = Double.parseDouble(tfNumOfYears.getText());

        String s = String.format("%-1s%20s%20s\n", "Interest Rate", "Monthly Payment", "Total Payment");
        // making loop to display different interest rates
        for ( ; annualInterestRate <= 8.00; annualInterestRate += 0.125) {

            // calculating monthly and total payment rates
            double monthlyInterestRate = annualInterestRate / 1200;
            double monthlyPayment = loanAmount * monthlyInterestRate / (1 - 1 / Math.pow(1 + monthlyInterestRate, numberOfYears * 12));

            double totalPayment = monthlyPayment * numberOfYears * 12;

            // making % string for formatting reasons...
            String str = "%";

            // Displaying formatted info
            s += String.format("%-1.3f%s%19.2f%30.2f \n", annualInterestRate, str, ((int) (monthlyPayment * 100) / 100.0), ((int) (totalPayment * 100) / 100.0));
        }

        textArea.setText(s);

    }

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