I have a class that creates multiple Integer objects and puts them into a LinkedList as shown below:
public class Shares<E> implements Queue<E> {
protected LinkedList<E> L;
public Shares() {
L = new LinkedList<E>();
}
public boolean add(E price) {
System.out.println("How many of these shares would you like?");
Scanner scanInt;
scanInt = new Scanner(System.in);
Integer noShares = scanInt.nextInt();
for (int i = 0; i < noShares; i++) {
L.addLast(price);
}
scanInt.close();
return true;
}
}
I have an application that scans for the input "add" from the console and if found, invokes the method add as shown below:
public class Application {
private static Scanner scan;
public static <E> void main(String[] args) {
Queue<Integer> S = new Shares<Integer>();
scan = new Scanner(System.in);
System.out.println("Please type add");
String sentence = scan.nextLine();
while (sentence.equals("quit") == false) {
if (sentence.equals("add")) {
System.out
.println("What price would you like to buy your shares at?");
S.add((Integer) scan.nextInt());
} else
System.exit(0);
sentence = scan.nextLine();
}
}
}
The application should allow the user to enter "add" as many times as they wish but the error "no line found" appears after the add method has been invoked.
I'm guessing this is because the Scanner in the method, has not been closed and then reopened when needed. Is this what is wrong with the program and if so, how would I go about fixing it?
Please note, this program is not finished, as I will be adding a selling method that sells these shares. That is why I am using a while loop.
Having multiple wrappers for any stream is a great way to really confuse yourself. I suggest you only ever wrap a stream once unless you really know what you are doing.
The simplest way to do this is to use a singleton in this case as it wraps another singleton (the best is to pass around the Scanner as an argument)
public class Application {
// use this Scanner in all you other code, don't create another one.
static final Scanner scan = new Scanner(System.in);
public static <E> void main(String[] args) {
Im guessing this is because the scanner in the method has not been closed
Once you close a stream it closes the underlying stream and you can't use it again. Only close System.in if you want to prevent it being used again.
how would I go about fixing it?
The best solution is to have all your Scanner use in one place, one method or one class. You have your main() do all the interaction with the user and pass the values to your data structure. Having objects which initialise themselves is a bad practice to get into and if you start doing this, it will plague you for the rest of your development days ;) (Seriously you will see this done again and again and its often a nightmare)
BTW Never exit a program without explanation. Calling System.exit(0); without even an error message is also a nightmare. I once worked on a project which has 260 calls to System.exit() often without an error message, you can imagine how much fun it is to diagnose a server just stopping for no apparent reason.
A first mistake is that this line of code
scanInt.close();
closes the System.in, not just the scanInt object. This means that after the first call to add, the scan object will only consume the input it already has and then you'll receive a NoSuchElementException: Remove this line.
Now, if you replace the last line you have with this
sentence = scan.nextLine();
System.out.println("sentence: \"" + sentence + "\"");
you will see that the last input you get before exiting is an empty String. So in the next loop you enter the else statement and your program stops execution. You can fix this problem by adding the following:
scan.nextLine(); // consume the first always empty String...
System.out.println("Please type add");
sentence = scan.nextLine(); // and then get the actual value
However, I will agree with Peter that you should not use multiple wrappers. Consider passing the Scanner object as an argument in the Shares class contractor.
Having multiple scanners (on same stream) is a very bad practice, because scanners consume the stream they share.
I've verified it while debugging the Scanner class source code, and there I’ve found:
a reference to the source input stream
a internal private buffer used to hold input.
So when a scanner instance consume its stream, basically it just read a bunch of bytes (1024) and the stream's position is moved ahead.
For example when the nextLine() method is invoket, behind the scenes the source.read() copy the result into the private buffer.
Obviously the state of other Scanner becomes corrupted (invalid).
Try to debug the Java source code yourself and/or look at the method Scanner.readInput().
Related
I am very new to Java but am working through the book Java: How to program (9th ed.) and have reached an example where for the life of me I cannot figure out what the problem is.
Here is a (slightly) augmented version of the source code example in the textbook:
import java.util.Scanner;
public class Addition {
public static void main(String[] args) {
// creates a scanner to obtain input from a command window
Scanner input = new Scanner(System.in);
int number1; // first number to add
int number2; // second number to add
int sum; // sum of 1 & 2
System.out.print("Enter First Integer: "); // prompt
number1 = input.nextInt(); // reads first number inputted by user
System.out.print("Enter Second Integer: "); // prompt 2
number2 = input.nextInt(); // reads second number from user
sum = number1 + number2; // addition takes place, then stores the total of the two numbers in sum
System.out.printf( "Sum is %d\n", sum ); // displays the sum on screen
} // end method main
} // end class Addition
I am getting the 'NoSuchElementException' error:
Exception in thread "main" java.util.NoSuchElementException
at java.util.Scanner.throwFor(Scanner.java:838)
at java.util.Scanner.next(Scanner.java:1461)
at java.util.Scanner.nextInt(Scanner.java:2091)
at java.util.Scanner.nextInt(Scanner.java:2050)
at Addition.main(Addition.java:16)
Enter First Integer:
I understand that this is probably due to something in the source code that is incompatible with the Scanner class from java.util, but I really can't get any further than this in terms of deducing what the problem is.
NoSuchElementException Thrown by the nextElement method of an Enumeration to indicate that there are no more elements in the enumeration.
http://docs.oracle.com/javase/7/docs/api/java/util/NoSuchElementException.html
How about this :
if(input.hasNextInt() )
number1 = input.nextInt(); // if there is another number
else
number1 = 0; // nothing added in the input
You should use hasNextInt() before assigning value to variable.
NoSuchElementException will be thrown if no more tokens are available. This is caused by invoking nextInt() without checking if there's any integer available. To prevent it from happening, you may consider using hasNextInt() to check if any more tokens are available.
I faced this Error with nextDouble(), when I input numbers such as 5.3, 23.8 ... I think that was from my PC depending on computer settings that use Arabic (23,33 instead 23.33), I fixed it with add:
Scanner scanner = new Scanner(System.in).useLocale(Locale.US);
You must add input.close() at the end...
This error is mostly occur in case of 0nline IDE's on which you are testing your code. It is not configured properly, as if you run the same code on any other IDE/Notepad it works properly because the online IDE is not designed such a way that it will adjust the input code of your format, So you have to take input as the Online IDE supports.
If I may, I solved this issue today by realizing that I had multiple functions that used an instance of a Scanner, each. So basically, try refactoring so that you have only one instance opened and then closed in the end - this should work.
For anyone using gradle's application plugin, you must wire it to the standard console in build.gradle(.kts) otherwise it will keep throwing the NoSuchElementException error if you try to use scanner.
For groovy:
run {
standardInput = System.in}
For gradle kotlin dsl:
tasks.withType<JavaExec>() {
standardInput = System.`in`}
Integer#nextInt throws NoSuchElementException - if input is exhausted
You should check if there is a next line with Integer#hasNextLine
if(sc.hasNextLine()){
number1=sc.nextInt();
}
I added a single static scanner (sc) at the top of my class and closed it (sc.close()) when coming out of the whole class wherever I used return statements. Again that's one instance of scanner as suggested by another answer, which should be static.
package com.example.com;
import java.util.Scanner;
public class someClass {
static Scanner sc = new Scanner(System.in);
//Whole world of methods using same sc.
//sc.close()); return;
}
Other than that you can add #SuppressWarnings("resource") on the top of the troubling method to make the warning go away. But be careful about resource leaks.
This question already has an answer here:
How to use java.util.Scanner to correctly read user input from System.in and act on it?
(1 answer)
Closed 5 years ago.
public static char[] puzzleInput() {
printEnterPuzzleMessage();
Scanner puzzleS = new Scanner(System.in);
if(puzzleS.hasNext()) {
char[] puzzle = puzzleS.next().toCharArray();
while(!isLegalPuzzleStructure(puzzle)) {
printIllegalPuzzleMessage();
puzzleInput();
}
return puzzle;
}
puzzleS.close();
return null;
}
public static void main(String[] args) throws Exception{ //Q - 8
Scanner fileName = new Scanner(System.in);
if(!fileName.hasNext()) {
System.out.println("No argument has been received");
System.exit(0);
}
String filePath = fileName.nextLine();
fileName.close();
Scanner vocabulary = new Scanner(new File(filePath));
String[] vocabularyArr = scanVocabulary(vocabulary);
vocabulary.close();
printReadVocabulary(filePath, vocabularyArr.length);
printSettingsMessage();
printEnterPuzzleMessage();
char[] puzzle = puzzleInput();
Hi, a beginner in Java is here.
In the function puzzleInput, I open a Scanner to get an input from the user. For some reason, the program won't give me a chance to put in input, and therefor the argument (puzzle) gets a null as default, and later when puzzle is needed not as a null - throws a NullPointerException.
There are many other functions in the code, but most of them are just a print commands, and the ones who are not were being checked by me, and are OK.
The problem is just the scanner won't give me a chance to put in an input.
Some points I'd like to clarify further:
1. The first Scanner (fileName) is not being skipped by the program, and I'm able to give it an argument.
2. I made sure I closed all the other scanners i've opened before.
Can someone explain me what I'm doing wrong?
program won't give me a chance to put in input
Your problem is that you are closing your Scanner in main:
Scanner fileName = new Scanner(System.in);
...
fileName.close();
This in turn closes the System.in input-stream which then cannot be reused in your puzzleInput() method because it is already closed. The right thing to do here is to pass in the Scanner variable into your puzzleInput() method and continue to reuse it there and not try to open up a new Scanner.
public static char[] puzzleInput(Scanner scanner) {
printEnterPuzzleMessage();
if(scanner.hasNext()) {
...
// don't close it here
return null;
}
...
Scanner scanner = new Scanner(System.in);
...
puzzleInput(scanner);
Couple of other comments:
Calling a Scanner fileName is not a good pattern. Choosing good names for your variables will help make the code self-documenting. scanner would be a better name of course.
When dealing with any input/output, it is a good practice to wrap any opening method in a try/finally block so it gets close properly. See also the try-with-resources functionality added in Java 7.
If you want a chance to do something with the input with a prompt, why not assign it to a String variable? This allows you to manipulate the input however you want later on too.
String input = scannerName.nextLine();
This is a diary program which allows you to write something in your diary (obviously). After typing enter and pressing enter, the page closes and its gonna be safed in a list. My problem is that it only runs once when I have Pages(); in the main method, so I tried this loop. It doesnt work for me and i dont know why. Need some help
import java.util.ArrayList;
import java.util.Scanner;
public class NotizbuchKlasse{
public static void Pages() {
System.out.println("day 1 : Write something in your diary.");
System.out.println("Write enter if you are done writing.");
ArrayList<String> List = new ArrayList<String>();
String ListInList;
Scanner write = new Scanner(System.in);
do {
ListInList = write.next();
List.add(ListInList);
} while (! ListInList.equals("enter"));
List.remove(List.size()-1);
write.close();
System.out.println("This is now your page. Your page is gonna be created after writing something new.");
System.out.println(List);
}
public static void main(String[]Args){
boolean run = true;
do{
Pages();
} while(run);
}
}
Error:
This is now your page. Your page is gonna be created after writing something
new.
Exception in thread "main" [hello]
day 1 : Write something in your diary.
Write enter if you are done writing.
java.util.NoSuchElementException
at java.util.Scanner.throwFor(Unknown Source)
at java.util.Scanner.next(Unknown Source)
at NotizbuchKlasse.Pages(NotizbuchKlasse.java:12)
at NotizbuchKlasse.main(NotizbuchKlasse.java:24)
You need to check whether there is something to read before you read it. You're not currently, and that's why you're getting a NoSuchElementException.
You do this via Scanner's has* methods.
For example:
ArrayList<String> List = new ArrayList<String>();
Scanner write = new Scanner(System.in);
while (write.hasNextLine()) {
String ListInList = write.nextLine();
if (ListInList.equals("enter")) break;
List.add(ListInList);
}
// No need to remove the last item from the list.
But also, I notice that you have a loop in your main method, where you call Pages() in that loop. If you close write, you also close System.in; once a stream is closed, you can't re-open it. So if you try to read things from System.in the next time you call Pages(), the stream is already closed, so there's nothing to read.
Simply don't call write.close(). You shouldn't close streams that you didn't open in general; and you didn't open System.in (the JVM did when it started up), so don't close it.
You want to be using a while loop like this:
while (write.hasNextLine()) {
ListInList = write.nextLine();
if (doneWriting(ListInList)) { // Check for use of enter.
break; // Exit the while loop when enter is found.
}
List.add(ListInList); // No enter found. Add input to diary entry.
}
where doneWriting() is a method (that you write!) which checks to see if the user has typed enter.
Here is the documentation for the next() method of Scanner. If you read it, you will see that it throws the exception you are getting when it runs out of tokens.
If you want a little bit more a casual explanation here is a question that was asked previously about next() versus nextLine().
I'm practicing HashSet from Java book by Cay S. Horstmann and Gary Cornell and I think there's a mistake in example code on page 687. We have a Scanner importing words to HashSet and it looks like this (I removed some unneeded code to make a problem more visible):
Set<String> words = new HashSet<String>();
Scanner in = new Scanner(System.in);
while (in.hasNext()) {
String word = in.next();
words.add(word);
}
The problem is there's no way stop this loop. Or maybe there's something I'm missing?
To stop the loop I've added another static helper method:
public static boolean isStop(Scanner in) {
if (in.next().equals("stop")) {
return true;
}
return false;
}
And now the main code looks like this:
Set<String> words = new HashSet<String>();
Scanner in = new Scanner(System.in);
while (!isStop(in)) {
String word = in.next();
words.add(word);
}
Is there any other way to stop scanner loop? I can't believe that book's author has made a mistake ?
The loop stops as soon as this condition is false:
in.hasNext()
I.e., there are no more words.
Inside the loop is a command to read the next word:
in.next()
So words will continue to be read until the Scanner has no more words to read. This loop will terminate at the end of whatever the Scanner is reading.
Since you are scanning System.in, the loop won't stop as it will keep on adding "words" to your Set but there's no visible error in the program.
Your idea of selecting a keyword to stop the loop once a user input matches that keyword sounds good.
You don't really need a static method using your Scanner as argument for that.
Just add the following after String word = in.next();:
if (word.equalsIgnoreCase("stop")) {
System.out.printf("Quitting with set: %s%n", words);
in.close();
return; // assuming method is void
}
The scanner will continue while there are still words in the input, as others explained. Note that when we're talking about System.in, it usually waits until the user enters more text, and so will not terminate until the user closes the stream (supplies the appropriate end-of-file for the operating system). In Unix/Linux, for the loop to terminate, the user will need to use control-D.
The loop won't stop as it will keep on adding "words" to your Set but there's no visible error in the program.
Your idea of selecting a keyword to stop the loop once a user input matches that keyword sounds good.
You don't really need a static method using your Scanner as argument for that.
Just add change your code as follows:
Set<String> words = new HashSet<String>();
Scanner in = new Scanner(System.in);
System.out.print("Type a word...");
while (in.hasNext()) {
String word = in.next();
if (word.equalsIgnoreCase("stop")) {
System.out.printf("Quitting with set: %s%n", words);
return; // assuming method is void
}
else {
words.add(word);
System.out.print("Type a word (or \"stop\" to quit)...");
}
}
IF there is code to be executed after the loop then yes, I'd say the authors made a mistake. Write to them about it! If this is an example of adding items to a Set then the example is fine. It all depends on what the authors' intent of the example was.
hasNext() is a blocking method meaning it will always wait for more input. There are related questions about this. Your way of "fixing this" is what the general consensus has done.
I have a program that needs to read lines of input. It needs to be many lines at once. For example:
As I enter my time machine or
maybe not,
I wonder whether free will exists?
I wonder whether free will exists
maybe not
as I enter my time machine or.
That all gets entered at one time by the user. I was trying to use .hasNextLine() method from Scanner class, but it is not returning false.... it waits for input again. Ive been looking around for a solution and it appears that .hasNextLine() waits for input, but i do not know what alternative to use. Any suggestions? The actual code looks like:
while(input.hasNextLine());
{
line += input.nextLine();
}
Thanks for your help
Perhaps you should use some sort of "stop" sequence meaning when the user enters a particular character sequence, it will break out the loop. It might look something like:
public static void main(String args[]){
final String stopSequence = "/stop";
final Scanner reader = new Scanner(System.in);
String input = reader.nextLine();
while(!input.equalsIgnoreCase(stopSequence)){
//process input
input = reader.nextLine();
}
}