Currently, our code has no error-checking. It assumes that the user will enter valid numbers. You can enter an integer into a double, or a float into a double; it just adds the decimal and a zero when you do that. But, the problem is, you can't put in a byte or a Boolean into an integer field. The code that you're looking at right now shows what will happen if you enter something other than a number. As you can see, it results in a build failed notification, among all the other things.
The next code block shows how we can make sure the numbers entered are valid. We'll make use of a 'while' loop that will continuously process until a valid number is entered. The Scanner class also has another method, hasNextDouble( ) and corresponding methods for integer and float, that lets us know if the value is actually a double.
Our while loop will run if the input value is not a double. The exclamation point (!) in the while statement tells Java to process if the readme value is not a double. It's an inversion of the value. When the value is not valid, we display an error. This will continue forever until the user enters a valid number.
Take special note of the next( ) function after the warning. This gives the user another attempt at entering a valid number. If they don't, the loop simply repeats, and they try again until they get it right. Check out the following code to get a good idea of how all this plays out.
public class Main {
public static void main(String[] args) {
Scanner readme = new Scanner(System.in);
System.out.println("Enter Two Numbers (Press Enter after each):");
while(!readme.hasNextDouble()) {
System.out.println("Try entering a number");
readme.next();
}
double n1, n2, n3;
n1 = readme.nextDouble();
n2 = readme.nextDouble();
n3 = n1 + n2;
System.out.println("Total = " + n3);
}
}
}
And if we enter bogus data, it will keep telling us to enter a valid number, which you can see on the next set of code here.
Eventually, the data entry is complete and the numbers are added together.
Now we can look at the final output, which has error checking and adds the two numbers, which you can see below. There are comments/notes within the code. As programmers we should always be sure to read those notes, as they often contain very helpful information!:
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
//declare and initialize numeric variables
//for use in the program
double n1 = 0, n2 = 0, n3 = 0;
Scanner readme = new Scanner(System.in);
System.out.println("Enter The First Number (Press Enter After It):");
//initial the state this variable showing that the value has not been successfully read
boolean valueSuccessfullyRead = false;
// while the value has not been successfully read
while (!valueSuccessfullyRead) {
// check if the next input value is a double
if (readme.hasNextDouble()) {
// if the next input value is a double, read it and record that it has been successfully read
n1 = readme.nextDouble();
valueSuccessfullyRead = true;
} else {
// if the next input value is not a double, read but do not store it
eadme.next();
}
// if a double value has not been successfully read, print this prompt before the loop repeats if (!valueSuccessfullyRead) {
System.out.println("Try entering a number");
}
}
System.out.println("First Number Read. Enter The Second Number (Press Enter After It):");
//initial the state this variable showing that the value has not been successfully read
valueSuccessfullyRead = false;
// while the value has not been successfully read
while (!valueSuccessfullyRead) {
// check if the next input value is a double
if (readme.hasNextDouble()) {
// if the next input value is a double, read, and record that it has been successfully read
n2 = readme.nextDouble();
valueSuccessfullyRead = true;
} else {
// if the next input value is not a double, read but do not store it readme.next();
}
// if a double value has not been successfully read, print this prompt before the loop repeats
if (!valueSuccessfullyRead) {
System.out.println("Try entering a number");
}
}
n3 = n1 + n2;
System.out.println("Total = " + n3);
/*
NOTE: The while loops above are virtually identical and repeated blocks of code.
They could easily become a single JAVA method which returns an input double value. That would be a more 'proper' and 'professional' implementation of this programming logic. You will learn more about JAVA methods in a later lesson.
*/
}
