isNumber method accepts letter D

Question

My method checks a user input if it is an integer. If it is, it prints a certain something in my code, if it is not an integer, it prints Invalid input. It works good except for when i input any integer ending with the letter "d", it accepts it as an integer. How do I fix this? Here's my code so far.

static boolean isNumber(String text) {
    try {
        Double.parseDouble(text);
        return true;
    } catch (NumberFormatException e) {
        System.out.println("Input is non-numerical or incorrect.");
        return false;
    }
}

do {
     System.out.print("Input loan principal amount : ");
     input = s.nextLine();
} while (!isNumber(input));
     double loan = Double.parseDouble(input);

why you parse double when you need an integer? do `Integer.parseInt()` instead — Stranger in the Q, Feb 22 '20 at 08:55
Does this answer your question? [How to check if a String is numeric in Java](https://stackoverflow.com/questions/1102891/how-to-check-if-a-string-is-numeric-in-java) — BitNinja, Feb 22 '20 at 08:55

score 2 · Accepted Answer · answered Feb 22 '20 at 09:04

According to Java Language Specification, floating point types can have suffixes (shortent version):

FloatingPointLiteral:
    DecimalFloatingPointLiteral
    HexadecimalFloatingPointLiteral

DecimalFloatingPointLiteral:
    Digits . Digitsopt ExponentPartopt FloatTypeSuffixopt
    . Digits ExponentPartopt FloatTypeSuffixopt
    Digits ExponentPart FloatTypeSuffixopt
    Digits ExponentPartopt FloatTypeSuffix

...

FloatTypeSuffix: one of
    f F d D

If you already using java.util.Scanner, you can use nextDouble() or nextBigDecimal() (which is better if your program is going to deal with money values like in your case).

It would be better to point to the documentation of [`Double.parseDouble`](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Double.html#parseDouble(java.lang.String))/[`Double.valueOf`](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Double.html#valueOf(java.lang.String)) as that is the formal definition of its behaviour. — Mark Rotteveel, Feb 22 '20 at 11:30

score 0 · Answer 2 · answered Feb 22 '20 at 09:00

This question is a typical homework question for beginners.

Your teacher does not want you to find shortcuts and use existing functions that will give you the answer in one line, but to create a loop and check every character in the string one by one.

Like this:

for (char c : text.getChars()) {
    //code that checks if c is a digit character and exits if it is not
}

return true;

Remember that trying to use existing functions like parseDouble or parseInt will fail for very long numbers that should still be valid!

isNumber method accepts letter D

2 Answers2