Using an unassigned variable in an if-statement (Java)

This is a pretty common problem I run into when I'm programming, and I'm a beginner so it stumps me for whatever reason. Here's my code:

boolean valid = false;  
        do {  
            double newPrice;  
            System.out.print("Enter new price: $");  
            newPrice = scan.nextDouble();  

            if(newPrice > 0){  
                b[bookChosen].price() = new newPrice;  
                valid = true;  
            }  
        }while(!valid);  

The newPrice double in the if-statement cannot be found, and I know why, I just can't think of a way to be able to see if the user's input is a viable number.
Thanks in advance.