Check whether a double is a number in java

/**
 * @author lautturi.com
 * Java example: determine whether a double is a number in java
 */

import java.util.*;
import java.util.regex.Pattern;

public class Lautturi {
	public static void main(String[] args) {

		double delta = -1;
		double val = Math.sqrt(delta);
		
		if(Double.isNaN(val)) {
			System.out.println("It's not a number"); 
		}
		
	}
}

output:

It's not a number