Java. How to make an error message for when userInput is a string instead of an int?

    Scanner scanner = new Scanner(System.in);
    String input = scanner.nextLine();//get the next input line
    scanner.close();
    Integer value = null;
    try
    {
        value = Integer.valueOf(input); //if value cannot be parsed, a NumberFormatException will be thrown
    }
    catch (final NumberFormatException e)
    {
        System.out.println("Please enter an integer");
    }


    if(value != null)//check if value has been parsed or not
    {
        //do something with the integer

    }

Consider the following code that both ensures that an integer is provided, and prompts the user to enter the correct type of input. You could extend the class to handle other restrictions (min/max values, etc.)

TestInput Class

package com.example.input;
import java.util.Scanner;
public class TestInput {
    public static void main(String[] args) {
        int n;
        Scanner myScanner = new Scanner(System.in);

        n = Interact.getInt(myScanner,"Enter value for ?", "An integer is required");
        System.out.println("Resulting input = " + n);
    }
}

Interact Class

package com.example.input;
import java.util.Scanner;
public class Interact {

    public static int getInt(Scanner scanner, String promptMessage,
            String errorMessage) {
        int result = 0;

        System.out.println(promptMessage);
        boolean needInput = true;
        while (needInput) {
            String line = scanner.nextLine();
            try {
                result = Integer.parseInt(line);
                needInput = false;
            } catch (Exception e) {
                System.out.println(errorMessage);
            }
        }
        return result;
    }
}

First of all, you need to get the String the user typed for the '?' in the question. If you do it simply with System.in and System.out, you would do it like this:

BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
String line = in.readLine();

If you thought of an easy version with GUI, you could use JOptionPane like this:

String line = JOptionPane.showInputDialog("4 + ? = 12");

Else, if you have a real GUI, you could read the user's input from a JTextField or similar:

String line = textField.getText();

(In this case you could also use a JFormattedTextField that filters the input for an Integer. This will make the check whether the input is an Integer or not unnecessary.)

Now, you need to parse the String to an int:

int value;
try
{
    value = Integer.parseInt(line);
}
catch (NumberFormatException nfe)
{
    // the user didn't enter an Integer
}
doSomethingWith(value);

The content of the catch-block differs from the variant you took above. If you took the first one with System.in and System.out, you could write something like this:

System.out.println("Your input is not an Integer! Please try again");

If you took the version with JOptionPane, you could write something like this:

JOptionPane.showMessageDialog(null, "Your input is not an Integer! Please try again");

Else, you need to have a JLabel for that witch has initially no content. Now, set it for 3 seconds to the text:

errorLabel.setText("Your input is not an Integer! Please try again");
new Thread(() ->
{
    try { Thread.sleep(3000); } // sleep 3 sec
    catch (InterruptedException ie) {} // unable to sleep 3 sec
    errorLabel.setText("");
}).start();

Note that the last version is also compatible with the second one.

Now, you should either generate a new question, or repeat the read/parse procedure until the user entered an Integer.