How to use an if statement to check if user input is a certain variable type?

Question

I'm creating a math program where a user inputs an answer. I want to display a message that says "All solutions must be entered as decimal numbers". How would I make sure that the user inputs a double and if not display that message. I've tried this so far:

if(userLetter.equalsIgnoreCase("a")) {
    System.out.print("What is the solution to the problem:" + " " + ran1Shift + " " + "+" + " " + ran2Shift + " = ");
    double userNum = input.nextDouble();
        if(userNum == additionAnswer){
            System.out.println("That's correct!");
        }
        else{
            System.out.println("The correct solution is: " + additionAnswer);
        }
    }

So basically I have it set now to display whether the answer is exactly true or false but how could I make another part which catches if the user enters a non-double? Thank you for your help :)

Elliott Frisch · Accepted Answer · 2016-01-30 21:22:06Z

2

Assuming input is a Scanner, then you can call Scanner.hasNextDouble() which returns true if the next token in this scanner's input can be interpreted as a double value using the nextDouble() method. Something like,

if (input.hasNextDouble()) {
    double userNum = input.nextDouble();
    if (userNum == additionAnswer) {
        System.out.println("That's correct!");
    } else {
        System.out.println("The correct solution is: " + additionAnswer);
    }
} else {
    System.out.println("All solutions must be entered as decimal numbers");
}

answered Jan 30, 2016 at 21:22

Elliott Frisch

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2 Comments

kg5425 Over a year ago

Thank you! I figured that could work I just wasn't sure how to implement it.

Elliott Frisch Over a year ago

If your question is answered, please accept an answer.

Makoto · Accepted Answer · 2016-01-30 21:23:28Z

0

nextDouble is guaranteeing that it will only scan a token that looks like a double from standard input. Since you want to catch the invalid input, you'll have to do more work.

The below is re-entrant; it will allow the user to keep adding input until valid input is brought in. I leave the prompting of the user as an exercise for the reader.

boolean valid = false;
do {
    try {
        double userNum = Double.parseDouble(input.nextLine());
        // other code to use `userNum` here
        valid = true;
    } catch(NumberFormatException e) {
        System.out.println("Not a valid double - please try again.");
    }
} while(!valid);

answered Jan 30, 2016 at 21:23

Makoto

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Comments

Man H · Accepted Answer · 2016-01-30 21:30:43Z

0

You can use a loop to allow user to re-enter a number in case he make a mistake.

System.out.println("Enter a double number:");
while (!input.hasNextDouble())
{
    System.out.println("Invalid input\n Enter a double number:");
    input.next();
}
double userInput = input.nextDouble();    
System.out.println("You entered: " + userInput);

answered Jan 30, 2016 at 21:30

Man H

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