Showing error message if input is not number

When the user enters invalid input, sc.nextDouble() will throw InputMismatchException. That will crash your current program, the code printing the "Invalid Input" message will never be reached.

You can wrap your code in a try block, and catch this exception:

try {
    System.out.print("Enter value of a = ");
    a = sc.nextDouble();
    // ...
} catch (InputMismatchException e) {
    System.out.print("Invalid Input. \nGood Bye.");
    // return or exit with failure
}

The "Good Bye" message suggests you want to exit on invalid input.

If you don't actually want to exit, then you can wrap the user input part, and the try-catch block within a loop, with limited or unlimited retries.

To guide against wrong input from the user, you should put both the input statements and all arithmetic operations into a try .... catch block.

Try the code below :

DecimalFormat df = new DecimalFormat("0.00#");
Scanner sc = new Scanner(System.in);

Double a,b,c;
System.out.println("Welcome to the Grand Quadratic Calculator ! ");
System.out.print("Enter value of a = ");

try { //this line will disallowing the program from terminating when the users inputs an invalid value

a = sc.nextDouble();
System.out.print("Enter value of b = ");
b = sc.nextDouble();
System.out.print("Enter value of c = ");
c = sc.nextDouble();
double xone = (-b + Math.sqrt(b * b - 4 * a * c)) / 2 * a;
double xtwo = (-b - Math.sqrt(b * b - 4 * a * c)) / 2 * a;

    if(b * b - 4 * a * c >= 0)
    {
        System.out.println("x1 = " + df.format(xone));
        System.out.print("x2 = " + df.format(xtwo));
    }
}
catch(InputMismatchException err){
    System.out.println("Invalid Input. \nGood Bye.");
}

The else statement have been removed and the catch block will handle all that.

I hope that worked.