Java Checking Whether a String Is a Valid Number

Java Checking Whether a String

Is a Valid Number

Problem

You need to check whether a given string contains a valid number, and, if so, con-
vert it to binary (internal) form.

Solution

Use the appropriate wrapper class’s conversion routine and catch the
NumberFormatException . This code converts a string to a double :

// StringToDouble.java
public static void main(String argv[]) {
String aNumber = argv[0];
// not argv[1]
double result;
try {
result = Double.parseDouble(aNumber);
} catch(NumberFormatException exc) {
System.out.println("Invalid number " + aNumber);
return;
}
System.out.println("Number is " + result);
}

Explained Of course, that lets you validate only numbers in the format that the designers of the wrapper classes expected. If you need to accept a different definition of numbers, you could use regular expressions to make the determination. There may also be times when you want to tell if a given number is an integer num- ber or a floating-point number. One way is to check for the characters . , d , e , or f in the input; if one of these characters is present, convert the number as a double . Otherwise, convert it as an int :

// Part of GetNumber.java
private static Number NAN = new Double(Double.NaN);
/* Process one String, returning it as a Number subclass
*/
public Number process(String s) {
if (s.matches(".*[.dDeEfF]")) {
try {
double dValue = Double.parseDouble(s);
System.out.println("It's a double: " + dValue);
return new Double(dValue);
} catch (NumberFormatException e) {
System.out.println("Invalid a double: " + s);
return NAN;
}
} else // did not contain . d e or f, so try as int.
try {
int iValue = Integer.parseInt(s);
System.out.println("It's an int: " + iValue);
return new Integer(iValue);
} catch (NumberFormatException e2) {
System.out.println("Not a number:" + s);
return NAN;
}
}

See Also 

A more involved form of parsing is offered by the DecimalFormat class. JDK 1.5 also features the Scanner class.