Use If to check integer

Question

How can I use If to check the input must be integer and can't type any english words.

int numWeight = sc.nextInt();
if (numWeight == (int)numWeight) {
    ((Salad)menu[itemNum - 1]).setWeight(numWeight);

    System.out.println(menu[itemNum - 1].showOrderDetails());
    System.out.println("-------------------------------------------");
    System.out.println("Total No. of items ordered :");

    orderedItem[TtlOrderNum] = menu[itemNum - 1];
    TtlOrderNum += 1;

    Continue();
}
else {
    System.out.println("input must a be integer");
}

When I use this code to run it and type integer, it shows:

Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Unknown Source)
    at java.util.Scanner.next(Unknown Source)
    at java.util.Scanner.nextInt(Unknown Source)
    at java.util.Scanner.nextInt(Unknown Source)
    at SaladAndDrinkOrderSystem.placeOrder(SaladAndDrinkOrderSystem.java:69)
    at TestSaladAndDrinkOrderSystem.main(TestSaladAndDrinkOrderSystem.java:23)

What do you think if(numWeight == (int)numWeight) will do ? — shmosel
– shmosel, Commented Apr 20, 2018 at 20:34
By entering a String-representation of an int. The if-clause is not throwing this Exception. — Turing85
– Turing85, Commented Apr 20, 2018 at 20:34
Possible duplicate of Checking for user input to be only integers in Java — user7392562
– user7392562, Commented Apr 20, 2018 at 20:34

smac89 · Accepted Answer · 2018-04-20 21:04:21Z

1

You need to do something like:

int numweight;
try {
    numweight = sc.nextInt();
    // Rest of your code here...
} catch (InputMismatchException e) {
    System.err.println("Please enter an integer");
}

In order to not have java throw an input mismatch exception.

edited Apr 20, 2018 at 21:04

smac89

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answered Apr 20, 2018 at 20:41

Raymo111

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4 Comments

shmosel Over a year ago

This will loop infinitely on bad input.

smac89 Over a year ago

@shmosel I don't see a loop. How does it "loop" infinitely?

shmosel Over a year ago

@smac89 I assumed the idea was to loop until valid input. But you're right, it's not mentioned explicitly.

Raymo111 Over a year ago

@smac89 Thanks for the edit! I am used to Eclipse's Content Assist, so I rarely type the whole method.

Augusto Accorsi · Accepted Answer · 2018-04-20 22:00:55Z

Note that if you pass a non Integer value to your numWeight you will raise an input missmatch exception cannot convert from xxxx to int, so in my opinion you could use a try-catch assuming you will receive an int and if you do not receive it the catch prints to you that it must be an Integer, something like this:

    int numWeight;
    try {
        numWeight = sc.nextInt();       
        ((Salad)menu[itemNum - 1]).setWeight(numWeight);

        System.out.println(menu[itemNum - 1].showOrderDetails());
        System.out.println("-------------------------------------------");
        System.out.println("Total No. of items ordered :");

        orderedItem[TtlOrderNum] = menu[itemNum - 1];
        TtlOrderNum += 1;

        Continue();
    }
    catch(InputMismatchException e) {
         System.err.println("Input must be an Integer");
    }

this way if the sc.nextInt() do not be an Integer you will print the error saying that it sould be.

But, if you want to keep with your original idea of check is the value is an Integer, you could use instaceof operator, something like this

    Integer numWeight = sc.nextInt();
    if (numWeight instanceof Integer) {
        ((Salad)menu[itemNum - 1]).setWeight(numWeight);

        System.out.println(menu[itemNum - 1].showOrderDetails());
        System.out.println("-------------------------------------------");
        System.out.println("Total No. of items ordered :");

        orderedItem[TtlOrderNum] = menu[itemNum - 1];
        TtlOrderNum += 1;

        Continue();
    }
    else {
        System.out.println("input must a be integer");
    }

Remember that if you choose for the second approach you will raise an exception if the st.nextInt() not be an int.

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