how to throw exception from catch block in java code example

Example 1: in java how to throw exception from function

public void doChangePin(int oldPin, int pin) throws Exception {	//need to add throws followed by exception name
		if (oldPin == pinCode) {
			pinCode = pin;
		} else {
			throw new Exception("some message");	//throwing the exception by creating its new object
		}
	}

Example 2: java throw an exception

public static void main(String[] args) {
	Scanner kb = new Scanner(System.in);
    System.out.println("Enter a number");
    try {
    	double nb1 = kb.nextDouble();
    	if(nb1<0)
        	throw new ArithmeticException();
        else System.out.println( "result : " + Math.sqrt(nb1) );
    } catch (ArithmeticException e) {
        System.out.println("You tried an impossible sqrt");
    }
}