When user presses "enter" key or space enter then error message pops up

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I would like to print an error message when the user presses enter or space enter instead of a string. I have tried isEquals("") and isEmpty() but haven't found anything that works yet.

Here's my code:

import java.util.Scanner;

public class check{
    public static void main(String args[]){
      System.out.println("Enter a number: ");
      Scanner keyboard = new Scanner(System.in);
      String input = keyboard.next();

      if(input.equals("")){
        System.out.println("Empty");
      } else {
        System.out.println("number inputed");
    }
  }

}

One way to do this, change keyboard.next() to keyboard.nextLine(), use trim() to remove unnecessary spaces, check with isEmpty().

String input = keyboard.nextLine().trim();
if (input.isEmpty()) {
    // error message
} else {
    // good to go
}

When User Presses Key Trigger, Hi guys, I can change a variable to 1 when a user presses the number 1 on the keyboard but I also need to use negative numbers, so is it  When a user presses a key, Windows Forms determines which event to raise based on whether the keyboard message specifies a character key or a physical key. For more information about character and physical keys, see How Keyboard Input Works .

import java.util.Scanner;

public class check{
    public static void main(String args[]){
      System.out.println("Enter a number: ");
      Scanner keyboard = new Scanner(System.in);
      String input = keyboard.nextLine();


      if(input.trim().equals("")){
        System.out.println("Empty");
      } else {
        System.out.println("number inputed");
    }
  }
}

Set TextEntry to the typed value when user presses the enter key , Hi there I am working with a simulation and i have text entry fields where once the user has typed in the information, the user must press the  Below step by step execution will happen when we pressed a key on the keyboard:-. 1) The keyboard sends a scan code of the key to the keyboard controller (Scan code for key pressed and key released is different) 2) The keyboard controller interprets the scan code and stores it in a buffer.

Strangely, I don't get an error when running your code. However, I noticed that your code simply doesn't react to an empty input (just pressing enter). If you want to check for that, you can use keyboard.nextLine().

Judging by the rest of your code, it seems like you want the user to input only a number. An easy way to check if the user entered an integer if you're using Scanner is keyboard.hasNextInt().

Meaning you can do something like this:

if(keyboard.hasNextInt()) {
  int yourNumber = keyboard.nextInt();
  System.out.println("Your number is: " + your Number);
}
else {
  System.out.println("Please enter a valid integer");
}

How to: Detect When the Enter Key Pressed, This example consists of a Extensible Application Markup Language (XAML) file and a code-behind file. Example. When the user presses the  Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Learn more How to detect when the user presses Enter in an input field

To check whether the string input is empty, you can use the String.isEmpty() method. Look below:

String input = keyboard.nextLine();
if(!input.isEmpty()) {
  //the input is not empty! 
} 
else {
  //the input is empty! 
} 

Note, however, that since you want to receive numbers as inputs you should not retrieve them as strings. Below is an example where the program retrieves a double from the user. Scanner provides many methods to validate the user's input. In this case, I'm using hasNextDouble() to check whether the input is a number.

Scanner scanner = new Scanner(System.in); 
System.out.println("Enter a number:");

while(!scanner.hasNextDouble()) {
  System.out.println("That's not a number!");
  scanner.next();
} 

double numberInput = scanner.nextDouble();
System.out.println("The entered number was " + numberInput);

WEB ENGINEERING, change <input> <textarea> <select> click onclick When user clicks the mouse etc. keydown onkeydown When user presses the key down Form elements  Key events indicate when the user is typing at the keyboard. Specifically, key events are fired by the component with the keyboard focus when the user presses or releases keyboard keys. For detailed information about focus, see How to Use the Focus Subsystem.

I made a sample program similar to yours and used nextLine() instead of next(). When user enters space and clicks enter he will print "space" else "a number".

Beginning Java Programming: The Object-Oriented Approach, Fires KeyEvent when user types on the keyboard and the component the listener is keyPressed and keyReleased: Code that reacts when user presses and  Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Learn more How do I tell when the enter key is pressed in a TextBox?

How to Write a Key Listener (The Java™ Tutorials > Creating a GUI , The second kind is either a key-pressed or key-released event. In general, you react to only key-typed events unless you need to know when the user presses keys  When the enter key is pressed, I want to call a Javascript function (User defined), but when I press it, the form is Stack Overflow Products

keyPress event in C# , KeyDown event in C# , KeyUp event in C#, C# Key Press - Most Windows Forms applications process keyboard input KeyDown Event : This event raised as soon as the user presses a key on the  KeyDown Event : This event raised as soon as the user presses a key on the keyboard, it repeats while the user keeps the key depressed. KeyPress Event : This event is raised for character keys while the key is pressed and then released. This event is not raised by noncharacter keys, unlike KeyDown and KeyUp, which are also raised for

Handling Key Presses Made on a Physical Keyboard, Detect when the user presses and releases keys on a physical keyboard. Framework. UIKit. On This Page. Overview; See Also