How to check if a string is numeric?

Question

I have a gpa program, and it works with the equalsIgnoreCase() method which compares two strings, the letter "a" to the user input, which checks if they put "a" or not. But now I want to add an exception with an error message that executes when a number is the input. I want the program to realize that the integer input is not the same as string and give an error message. Which methods can I use to compare a type String variable to input of type int, and throw exception?

Answer 1

Many options explored at http://www.coderanch.com/t/405258/java/java/String-IsNumeric

One more is

public boolean isNumeric(String s) {  
    return s != null && s.matches("[-+]?\\d*\\.?\\d+");  
}  

Might be overkill but Apache Commons NumberUtils seems to have some helpers as well.

Answer 2

If you are allowed to use third party libraries, suggest the following.

https://commons.apache.org/proper/commons-lang/javadocs/api-2.6/org/apache/commons/lang/math/NumberUtils.html

NumberUtils.isDigits(str:String):boolean
NumberUtils.isNumber(str:String):boolean

Answer 3

Use this

public static boolean isNum(String strNum) {
    boolean ret = true;
    try {

        Double.parseDouble(strNum);

    }catch (NumberFormatException e) {
        ret = false;
    }
    return ret;
}

Answer 4

You can also use ApacheCommons StringUtils.isNumeric - http://commons.apache.org/proper/commons-lang/javadocs/api-2.6/org/apache/commons/lang/StringUtils.html#isNumeric(java.lang.String)

Answer 5

Simple method:

public boolean isBlank(String value) {
    return (value == null || value.equals("") || value.equals("null") || value.trim().equals(""));
}


public boolean isOnlyNumber(String value) {
    boolean ret = false;
    if (!isBlank(value)) {
        ret = value.matches("^[0-9]+$");
    }
    return ret;
}

Answer 6

Use below method,

public static boolean isNumeric(String str)  
{  
  try  
  {  
    double d = Double.parseDouble(str);  
  }  
  catch(NumberFormatException nfe)  
  {  
    return false;  
  }  
  return true;  
}

If you want to use regular expression you can use as below,

public static boolean isNumeric(String str)
{
  return str.matches("-?\\d+(\\.\\d+)?");  //match a number with optional '-' and decimal.
}

Answer 7

public static boolean isNumeric(String string) {
    if (string == null || string.isEmpty()) {
        return false;
    }
    int i = 0;
    int stringLength = string.length();
    if (string.charAt(0) == '-') {
        if (stringLength > 1) {
            i++;
        } else {
            return false;
        }
    }
    if (!Character.isDigit(string.charAt(i))
            || !Character.isDigit(string.charAt(stringLength - 1))) {
        return false;
    }
    i++;
    stringLength--;
    if (i >= stringLength) {
        return true;
    }
    for (; i < stringLength; i++) {
        if (!Character.isDigit(string.charAt(i))
                && string.charAt(i) != '.') {
            return false;
        }
    }
    return true;
}

Answer 8

I wrote this little method lastly in my program so I can check if a string is numeric or at least every single char is a number.

private boolean isNumber(String text){
    if(text != null || !text.equals("")) {
        char[] characters = text.toCharArray();
        for (int i = 0; i < text.length(); i++) {
            if (characters[i] < 48 || characters[i] > 57)
                return false;
        }
    }
    return true;
}

Answer 9

You can use Character.isDigit(char ch) method or you can also use regular expression.

Below is the snippet:

public class CheckDigit {

private static Scanner input;

public static void main(String[] args) {

    System.out.print("Enter a String:");
    input = new Scanner(System.in);
    String str = input.nextLine();

    if (CheckString(str)) {
        System.out.println(str + " is numeric");
    } else {
        System.out.println(str +" is not numeric");
    }
}

public static boolean CheckString(String str) {
    for (char c : str.toCharArray()) {
        if (!Character.isDigit(c))
            return false;
    }
    return true;
}

}

Answer 10

Here's how to check if the input contains a digit:

if (input.matches(".*\\d.*")) {
    // there's a digit somewhere in the input string 
}

Answer 11

To check for all int chars, you can simply use a double negative. if (!searchString.matches("[^0-9]+$")) ...

[^0-9]+$ checks to see if there are any characters that are not integer, so the test fails if it's true. Just NOT that and you get true on success.