How to add int through JTextfield input

Question

my code works fine except the fact I cannot obtain an accurate totalBill! Below is my code the area of issue is within the button_1 action. I am sure it is something simple with my math but the outcome for example if I input: 100 (FOR AMOUNT SPENT) 15 (FOR TOTAL TAX AMOUNT) and 10 (FOR TOTAL TIP AMOUNT) comes out to 123, just 2 short of being correct. For the life of me I have retried the fractional math and I am just stumped I am sure it is something relatively easy I just don't see it. Any help is much appreciated.

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class unit6_Q2 extends JFrame {

    JPanel main_1 = new JPanel();
    JLabel main_2 = new JLabel();
    JTextField question_1 = new JTextField("Enter amount here",30);
    JTextField question_2 = new JTextField("Enter tax percent here", 30);
    JTextField question_3 = new JTextField("Enter tip amount here", 30);
    JButton button_1 = new JButton("Calculate");

    public unit6_Q2() {
        setTitle("Tutorial");
        setVisible(true);
        setSize(400,200);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        main_1.add(question_1);
        main_1.add(question_2);
        main_1.add(question_3);

        //if I want the use of enter on each TextField
        /*
        question_1.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e) {
                String input_1 = question_1.getText();              
            }
        });

        question_2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                String input_2 = question_2.getText();
            }
        });
        question_3.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                String input_2 = question_3.getText();
            }
        });  */

        main_1.add(button_1);
        button_1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                String input_1 = question_1.getText();  
                String input_2 = question_2.getText();
                String input_3 = question_3.getText();
                int amountSpent = Integer.parseInt(input_1);
                int amountTaxed = Integer.parseInt(input_2);
                int amountTipped = Integer.parseInt(input_3);

                int totalTax = (int)(amountSpent * (amountTaxed*(1/100.0f)));
                int totalTipped =(int)(amountSpent * (amountTipped*(1/100.0f)));
                int totalBill = totalTipped + totalTax + amountSpent;
                JOptionPane.showMessageDialog(null, totalBill);
            }
        });
        add(main_1);
    }
    public static void main(String[] args) {

        unit6_Q2 display = new unit6_Q2();
    }
}

Answer 1

It is because you are truncating fractial data.

            int totalTax = (int)(amountSpent * (amountTaxed*(1/100.0f)));
            int totalTipped =(int)(amountSpent * (amountTipped*(1/100.0f)));

Use Math.round() insteed of (int) cast or simply stick to the floating points.

The way you are doing is if lets say totalTax would be 1.9, it will become 1 due to int cast.

Answer 2

Hint, in order to solve such problems, it often helps to "reduce" them to their core. In your case: computations. There is absolutely no need to add UI complexity. You see ...

public class Test {
  public static void main(String[] args) {
     int amountSpent = Integer.parseInt("100");
     int amountTaxed = Integer.parseInt("15");
     int amountTipped = Integer.parseInt("10");

     int totalTax = (int)( amountSpent * amountTaxed/100.0f );
     System.out.println(totalTax);
     int totalTipped =(int)(amountSpent * amountTipped/100.0f);
     System.out.println(totalTipped);
     int totalBill = totalTipped + totalTax + amountSpent;
     System.out.println(totalBill);
    }
 }

Just works (as it avoids the "intermediate" conversion from float to int; which gives you those rounding errors).

So, my answer is more of a strategy how to resolve such problems:

1. Get rid of anything that doesn't contribute to the problem. In your case: UI elements and such things
2. Then study the exact rules when doing maths, like here