I am building an application in Java (using NetBeans) that accepts user input through the console and prints out a statement using their name (given in user input). The following is the code:
package amazingpets;
import java.io.Console;
public class AmazingPets {
public static void main(String[] args) {
Console console = System.console();
String firstName = console.readLine("What is your name? ");
console.printf("My name is %s.\n",firstName);
}
}
However I keep getting the following error in the console:
Exception in thread "main" java.lang.NullPointerException
at amazingpets.AmazingPets.main(AmazingPets.java:14)
Java Result: 1
Can anyone please suggest a possible solution?
From the documentation of System#console, it returns:
The system console, if any, otherwise null.
So your code is equivalent to:
String firstName = null.readLine("What is your name? ");
I would suggest you to use Scanner scanner = new Scanner(System.in); instead.
System.console() returns a console if it exists. Java apps may be launched without a console.
Anywhy it seams this is a duplicate of this one (among others):
Why does System.console() return null for a command line app?
Hope it helps
Use Scanner instead of Console
As mentioned in this answer this answer
Isn't line 14 where you create firstName variable? In this case console may be null. Javadoc for Console says
` a unique instance of this class which can be obtained by invoking theSystem.console() method. If no console device is available then an invocation of that method will return null.`
When you run code in an IDE you will usually not have a console object. System.console() will thus return null and console.readLine("What is your name? "); will generate a NullPointerException. You can still read via System.in, so to read a line you can instead use:
Scanner sc = new Scanner(System.in);
String read = sc.nextLine();
Related
I am trying to take input from the keyboard but I am getting errors.
import java.util.*;
public class FirstClass{
public static void main(String[] args){
Scanner sc=new Scanner(System.in);
String name=sc.next();
System.out.println(name);
}
}
I copy pasted your code in a text editor inside a file called FirstClass.java
Compiled your code using javac FirstClass.java
Ran it using java FirstClass
This takes the cursor to the next line where an input is expected. I entered the string "hey", and your code printed out "hey".
In short, your code is working fine. You need to enter an input after you run your code.
PS: 'sc' is never closed is a warning and not an error. You should however consider using try-with-resources to prevent any resource leaks.
I have written a sample code:
import java.util.Scanner;
public class abcd {
public static void main(String[] args) {
System.out.print("please enter a: ");
Scanner a = new Scanner(System.in);
String b = a.next();
System.out.println(b);
}
}
I am able to compile and execute this code via Ubuntu terminal. In SciTe, it compiles fine, but when I run it, I am faced with this error:
please enter a: Exception in thread "main" java.util.NoSuchElementException
at java.util.Scanner.throwFor(Scanner.java:862)
at java.util.Scanner.next(Scanner.java:1371)
at abcd.main(abcd.java:8)
Any Suggestions?
EDIT: When I execute a file in terminal, I do: 'java abcd' Scite does: 'java -cp .abcd'. How are the two commands different and why isn't java -cp working?
It appears that there is a bug/improper implementation in the handling of standard input in SciTE on Linux/Unix.
The description of the bug and a workaround are in this PDF document: A Problem with SciTE Go Command on Linux
Note: this is not official documentation, but it seems to match your problem.
According to that document, when running a Java program through the "Go" command on SciTE, input is supposed to come from the output pane. However, on Linux this does not work properly, and it's as if you are reading from an empty stream.
When you are reading from an empty stream, Scanner sees the end-of-file marker when it attempts to read a value using next(), nextInt() etc. And it throws a NoSuchElementException as there is no input element in the stream.
Your options to work around this problem:
Try the method mentioned in the aforesaid document, to use "Go" in a Linux terminal instead of the output pane.
Run the program in a terminal and avoud the "Go" command altogether.
Use a different IDE which doesn't have this problem.
Try to use hasNext() before next();
import java.util.Scanner;
public class abcd {
public static void main(String[] args) {
System.out.print("please enter a: ");
Scanner a = new Scanner(System.in);
while(a.hasNext()) {
try {
String b = a.next();
System.out.println(b);
} catch (NoSuchElementException e) {}
}
}
}
I don't mean to offend, but using hasNext() as suggested in Alexander's answer won't solve this problem, it will only enable OP to handle it well. I don't think that is what he/she is looking for.
Now I am no expert by any means and for some reason your program code works on my machine... But anyways, a NoSuchElementException is thrown when your program is cycling over an iterable object and there is nothing more to cycle over, despite your program expecting something there. A quick look-up in the Java-docs of Scanner.next()
shows that this exception is thrown if there are no more tokens available for read.
Now, if I had to guess I would advise you to try using something other than Scanner.next() and see if that works.
The fact that it works on my machine but not on yours is somewhat surprising, so could you provide some information on how you try to run your program? Are you running it from the default command-line? Or within Scite? (If second is the case, I really won't be able to help you, I have never even touched Scite).
This question already has answers here:
Masking password input from the console : Java
(5 answers)
Closed 9 years ago.
So I have this application, where I read users input from command prompt, and when user is inputting his/her password, I would like to hide it such that everyone else cant see it like this:
Please enter your password below
this is password
I would like to show it like this:
Please enter your password below
****************
So is there any way of doing this in java console application?
Take a look at the Console class, it has readPassword()
Syntax:
public char[] readPassword(String fmt,Object... args)
Provides a formatted prompt, then reads a password or passphrase from the console with echoing disabled.
Parameters:
fmt - A format string as described in Format string syntax for the prompt text.
args - Arguments referenced by the format specifiers in the format string. If there are more arguments than format specifiers,
the extra arguments are ignored. The maximum number of arguments is
limited by the maximum dimension of a Java array as defined by the
Java Virtual Machine Specification.
Returns - A character array containing the password or passphrase read from the console, not including any line-termination
characters, or null if an end of stream has been reached.
Taken from the answer pointed in the comment:
import java.io.Console;
public class Main {
public void passwordExample() {
Console console = System.console();
if (console == null) {
System.out.println("Couldn't get Console instance");
System.exit(0);
}
console.printf("Testing password%n");
char passwordArray[] = console.readPassword("Enter your secret password: ");
console.printf("Password entered was: %s%n", new String(passwordArray));
}
public static void main(String[] args) {
new Main().passwordExample();
}
}
The thing to notice is that you are getting back a char[] and not a String. This is for security reasons and there is another great answer on this same topic on SO. You should destroy this char[] by overwriting it after your work is done. Strings can stay a longer time in memory till the GC collects them and this can be a security risk.
Run the example from the command line and not from an IDE. It may not work.
I'm a beginner. Just wondering why this code works perfectly fine in windows command prompt, but I get an:
Exception in thread "main" java.lang.NullPointerException
at Test1.main(Test1.java:13)
error in eclipse. This has happened a lot to me, and it's really stopping me from using eclipse.
Here's the code:
import java.io.Console;
public class Test1 {
public static void main(String[] args) {
Console myConsole = System.console();
for (int a = 0; a < 10; a++){
int a2 = a * a;
myConsole.printf("\n%d squared is: %d.",a,a2); //Problem with this line
}
System.exit(0);
}
}
The javadoc for System.console() states
Returns The system console, if any, otherwise null.
Eclipse must not associate a system console.
Use System.in instead, possibly with a java.util.Scanner for input. And System.out for output.
Simply put, System.console() is returning null in Eclipse, but not when run in a console. This is the documented behaviour:
Returns the unique Console object associated with the current Java virtual machine, if any.
Returns:
The system console, if any, otherwise null.
Why not just use System.out instead? After all, you don't need any of the functionality of Console.
System#console may return null in certain environments. Since youre simply outputting to the console, you don't need to use Console. Formatter can be used instead:
System.out.printf("\n%d squared is: %d.", a, a2);
I would like to make use of java.io.Console. I am trying to do so by invoking System.console(). This works..some of the time.
This is fine when I run my program like so:
java classn
However, I would like to read standard input from a file named input.in. When I try to do so via:
java classn < input.in
I receive a null pointer exception:
Exception in thread "main" java.lang.NullPointerException
at classn.main(classn.java:9)
Is there a fix so I can use Console along with input from a fix? I realise why it's returning null, I would just like to know if there's a way to hook the Console into what's being passed in via a file.
Well, you'd have to test whether System.console() returned null. If it did, you'd have to work without an interactive console - there's no getting around that. You can use System.in to get the information from the redirected file.
An alternative is to have a command-line option to read appropriate data from the given filename, but then interact with the console for the rest.
Often, the easyest way is to use the Scanner class, bounded to System. in:
Scanner sc = new Scanner (System.in);
Call your program
cat foo | java Sample
on linux/unix/bsd, or
type foo | java Sample
on Windows.