Java异常处理问题

Question

My final project for Object Oriented Programming is a program employing objects, classes, and user inputs to calculate whether the user can afford a car based on their income and a user-defined percentage of their monthly income that the total cost of ownership of the car is not to exceed. The program functions as desired, but i need to implement exception handling to catch incorrect inputs (ie string characters in a double field). My tutor and I cannot figure out how to properly catch the exceptions and prompt the user to reenter an appropriate value.

Here's the code I need to handle exceptions for:

System.out.println("What is the price of the car?");
    car.setPrice(sc.nextDouble());
    System.out.println("What is your down payment?");
    car.setDownPayment(sc.nextDouble());
    car.setOutTheDoor();
    System.out.println("Please enter your interest rate (APR) as a decimal value. For example, an interest rate of 3% would be entered as '.03.'");
    car.setRate(sc.nextDouble());
    System.out.println("What is the maturity of your loan? The maturity of your loan is the length of time in months over which the loan will be paid.");
    car.setMaturity(sc.nextInt());
    car.setPayment();

    System.out.println("The car you've chosen is a" + car.getModelName());
    System.out.println("The MSRP is: $" + car.getStickerPrice());
    System.out.println("The down payment is: $" + car.getDownPayment());
    System.out.println("The OTD price is: $" + car.getOutTheDoor());
    System.out.println("The interest rate is: " + car.getRate());
    System.out.println("The maturity is: " + car.getMaturity());
    System.out.println("Your monthly payment is: $" + car.getPayment());

    System.out.println("Let's see if you can afford a " +car.getModelName() + "!");
    while(personalFinance.getSalary() <= 0)
    {
        System.out.println("What is your annual salary after taxes and deductions?");
        personalFinance.setSalary(sc.nextDouble());
        if(personalFinance.getSalary() <= 0)
        {
            System.out.println("You need to make money to afford a car! Please reenter a value that is greater than 0.");
        }       
    }


    System.out.println("Approximately how many miles do you drive a year?");
    car.setMilesDriven(sc.nextInt());
    System.out.println("What is the average fuel efficiency of the " + car.getModelName() + "?");
    car.setFuelEfficiency(sc.nextInt());
    System.out.println("What is the average cost of fuel in your area?");
    car.setPricePerGallon(sc.nextDouble());
    System.out.println("What is your annual insurance rate?");
    car.setInsurance(sc.nextDouble());
    System.out.println("What is the percentage of your monthly income that you do not want your monthly cost of ownership to exceed? Enter the percent as decimal value. For example, if you do not want your monthly payment to exceed 20% of your monthly income, then enter .20.");
    personalFinance.setMonthlySalary(sc.nextDouble());

Answer 1

If you are using Java 1.8, you can create a generic helper method for the handling of the InputMismatchException that occurs when the user enters some string that cannot be converted to the desired type.

public static <T> T nextValid(final String prompt, final Scanner sc, final Function<Scanner, T> nextFn) {
    do {
        System.out.println(prompt);
        try {
            return nextFn.apply(sc);
        } catch (InputMismatchException e) {
            System.out.println("Invalid input, please try again.");
            sc.next(); // skip invalid token.
        }
    } while (true);
}

And you use it like this:

int x = nextValid("Give me an integer:", sc, Scanner::nextInt);
double y = nextValid("Now a double:", sc, Scanner::nextDouble);

Then it will keep asking until a valid input from the user.