Question

When a user enters a whole number, the program runs smoothly, but when the user enters a number that has a decimal at the end, the program crashes.

These are the errors that I get:

    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
    at java.lang.Integer.parseInt(Integer.java:458)
    at java.lang.Integer.parseInt(Integer.java:499)
    at BMI.main(BMI.java:11)

Here is my code:

import javax.swing.*;

public class BMI {
  public static void main(String args[]) {
    int height;  // declares the height variable
    int weight;  // declares the weight variable
    String getweight;
    getweight = JOptionPane.showInputDialog(null, "Please enter your weight in Kilograms");  // asks user for their weight
    String getheight;
    getheight = JOptionPane.showInputDialog(null, "Please enter your height in Centimeters");  // asks user for their height
    weight = Integer.parseInt(getweight);  // stores their weight
    height = Integer.parseInt(getheight);  // stores their height
    double bmi;  // declares the BMI variable
    bmi = weight / Math.pow(height / 100.0, 2.0);  // calculates the BMI
    double roundbmi;  // variable to round the BMI to make it more read-able
    roundbmi = Math.round(bmi);  // rounds the BMI
    JOptionPane.showMessageDialog(null, "Your BMI is: " + roundbmi);  // displays the calculated and rounded BMI
  }
}

Answer 1

An Integer only recognizes whole numbers. If you want to be able to capture floating point numbers, use either Float.parseFloat() or Double.parseDouble().

To make the answer more complete, let me give you a quick example of why "4.", "4.0", and "4" are represented in two different ways. The first two are considered floating point values (since Java will just assume you mean 4.0 regardless), and how they are represented in memory depends heavily on which datatype you use to represent them – either a float or a double.

A float represents 4.0 using the single-precision floating point standard, whereas a double would represent 4.0 using the double-precision floating point standard. An int represents the value 4 in base-2 instead (so it’d just be 2²).

Understanding how numbers are stored internally is critical and key to development, not just with Java. In general, it’s recommended to use Double since that gives the larger range of floating point numbers (and higher precision).