Validate user input (positive and numerical)

[neveser]

I can't figure out a way to validate user input. The input needs to be both positive and numerical.
So far the code won't accept any letters, but it still accepts negative numbers.

//Get input (minutes and payment) from user
        try{
        minutesIn = Double.parseDouble(minutesField.getText());
        paymentIn = Double.parseDouble(paymentField.getText());
        }
        catch(Exception e)
        {
        return;  
        }

I am a total noob here and the books I have don't seem to cover this. Actually, both books don't seem to cover much of anything I need.

[Tolls]

So check the values.
Create a validate<whatever those values represent>() method that checks them and returns false if they aren't valid, or even throws an exception.