I'm trying to progress displaying a file line by line with an Enter key, but the if statement that I try doesn't seem to work. If I disregard the if statement, it works, but it feels incomplete because then I'm asking for input and doing nothing with it.
This is what I have:
import java.util.Scanner;
import java.io.*;
public class LineByLine {
public static void main(String[] args) throws IOException {
Scanner in = new Scanner(System.in);
System.out.println("What is the filename?");
String input = in.nextLine();
BufferedReader buff = new BufferedReader(new FileReader(input));
String sen = buff.readLine();
System.out.println(sen);
Scanner enter = new Scanner(System.in);
while (sen != null){
String output = enter.next();
if (output.equals("")){
System.out.println(sen = buff.readLine());
}
}
}
}
I just don't know why my if statement doesn't work.
The core issue is that you misunderstand Scanner and its default configuration: Out of the box, scanner splits on any amount of whitespace. .next() asks for the next token; a token is the thing that appears in between the whitespace.
Thus, pressing enter 500 times produces zero tokens. After all, tokens are what's in between the separator, and the default separator is 'any amount of whitespace'. Pressing enter a bunch of time is still just you entering the same separator.
The underlying problem is that most people appear to assume that Scanner reads one line at a time. It doesn't do that. At all. But you want it to. So, tell it to! Easy peasy - make scanner do what you already thought it did:
Scanner in = new Scanner(System.in);
in.useDelimiter("\\R"); // a single enter press is now the separator.
You should also stop using nextLine on scanners. nextLine and any other next call do not mix. The easiest way to solve this problem is to only ever use nextLine and nothing else, or, never use nextLine. With the above setup, .next() gets you a token which is an entire line - thus, no need for nextLine, which is good news, as nextLine is broken (it does what the spec says it should, but what it does is counterintuitive. We can debate semantics on whether 'broken' is a fair description of it. Point is, it doesn't do what you think it does).
Also, while you're at it, don't make multiple scanners. And, to improve this code, resources must be properly closed. You're not doing that. Let's use try-with, that's what it is for.
public static void main(String[] args) throws IOException {
Scanner in = new Scanner(System.in);
in.useDelimiter("\\R");
System.out.println("What is the filename?");
String input = in.next();
try (BufferedReader buff = new BufferedReader(new FileReader(input))) {
String sen = buff.readLine();
System.out.println(sen);
while (sen != null){
enter.next(); // why does it matter _what_ they entered?
// as long as they pressed it, we're good, right? Just ignore what it returns.
System.out.println(sen = buff.readLine());
}
}
}
Related
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();
I have a client/server game where users take turns giving input to the game. I want anything a user types when it is not their turn to be ignored. I want to scan only the inputs the user gives after they are prompted. I am using java.util.Scanner for input, but I can't find a way to "flush" the scanner buffer without the program hanging. I also cannot find examples of other input reading methods doing what I want.
This is what I tried:
Scanner in = new Scanner(System.in);
//do game stuff
//I don't care about anything typed at this time
while(in.hasNextLine()){
in.nextLine();
}
System.out.println("Enter your move:");
String input = in.nextLine();
There is no way to disable a Scanner and re-enable it. Here are a few suggestions:
You can manually empty the Scanner before the player's turn like so (what (I think) you've been doing):
while(in.hasNext()) in.next();
Or, you can close the scanner at the end of the player's turn and instantiate a new one at the beginning of the next turn (you can't reopen a closed Scanner):
//Player's turn
//
//End of player's turn...
in.close();
//Beginning of player's turn
in = new Scanner(System.in);
This, however, will throw an IllegalStateException if you try to access the Scanner after it has been closed. To solve this, try (on Java 7 and later) the try-with-resources block:
//Overridden close method because you don't want to close System.in
try (Scanner in = new Scanner(new InputStreamReader(System.in)
{public void close() throws IOException {}})) {
System.out.println("Enter your move:");
String input = in.nextLine();
}
//End of player's turn
//Scanner is automatically closed and this code is out of the scanner's scope
I think this last option is your best bet. It restricts the scope of the Scanner to the relevant code, and it automates the instantiating/closing. Plus, it uses a (relatively) recent Java feature, so that might be the required answer to your problem.
I ran into an issue. Below is my code, which asks user for input and prints out what the user inputs one word at a time.
The problem is that the program never ends, and from my limited understanding, it seem to get stuck inside the while loop. Could anyone help me a little?
import java.util.Scanner;
public class Test{
public static void main(String args[]){
System.out.print("Enter your sentence: ");
Scanner sc = new Scanner (System.in);
while (sc.hasNext() == true ) {
String s1 = sc.next();
System.out.println(s1);
}
System.out.println("The loop has been ended"); // This somehow never get printed.
}
}
You keep on getting new a new string and continue the loop if it's not empty. Simply insert a control in the loop for an exit string.
while(!s1.equals("exit") && sc.hasNext()) {
// operate
}
If you want to declare the string inside the loop and not to do the operations in the loop body if the string is "exit":
while(sc.hasNext()) {
String s1 = sc.next();
if(s1.equals("exit")) {
break;
}
//operate
}
The Scanner will continue to read until it finds an "end of file" condition.
As you're reading from stdin, that'll either be when you send an EOF character (usually ^d on Unix), or at the end of the file if you use < style redirection.
When you use scanner, as mentioned by Alnitak, you only get 'false' for hasNext() when you have a EOF character, basically... You cannot easily send and EOF character using the keyboard, therefore in situations like this, it's common to have a special character or word which you can send to stop execution, for example:
String s1 = sc.next();
if (s1.equals("exit")) {
break;
}
Break will get you out of the loop.
Your condition is right (though you should drop the == true). What is happening is that the scanner will keep going until it reaches the end of the input. Try Ctrl+D, or pipe the input from a file (java myclass < input.txt).
it doesn't work because you have not programmed a fail-safe into the code. java sees that the scanner can still collect input while there is input to be collected and if possible, while that is true, it keeps doing so. having a scanner test to see if a certain word, like EXIT for example, is fine, but you could also have it loop a certain number of times, like ten or so. but the most efficient approach is to ask the user of your program how many strings they wish to enter, and while the number of strings they enter is less than the number they put in, the program shall execute. an added option could be if they type EXIT, when they see they need less spaces than they put in and don't want to fill the next cells up with nothing but whitespace. and you could have the program ask if they want to enter more input, in case they realize they need to enter more data into the computer.
the program would be quite simplistic to make, as well because there are a plethera of ways you could do it. feel free to ask me for these ways, i'm running out of room though. XD
If you don't want to use an EOF character for this, you can use StringTokenizer :
import java.util.*;
public class Test{
public static void main(){
Scanner sc = new Scanner (System.in);
System.out.print("Enter your sentence: ");
String s=sc.nextLine();
StringTokenizer st=new StringTokenizer(s," ");//" " is the delimiter here.
while (st.hasMoreTokens() ) {
String s1 = st.nextToken();
System.out.println(s1);
}
System.out.println("The loop has been ended");
}
}
I had the same problem and I solved it by reading the full line from the console with one scanner object, and then parsing the resulting string using a second scanner object.
Scanner console = new Scanner(System.in);
System.out.println("Enter input here:");
String inputLine = console.nextLine();
Scanner input = new Scanner(inputLine);
List<String> arg = new ArrayList<>();
while (input.hasNext()) {
arg.add(input.next().toLowerCase());
}
You can simply use one of the system dependent end-of-file indicators ( d for Unix/Linux/Ubuntu, z for windows) to make the while statement false. This should get you out of the loop nicely. :)
Modify the while loop as below. Declare s1 as String s1; one time outside the loop. To end the loop, simply use ctrl+z.
while (sc.hasNext())
{
s1 = sc.next();
System.out.println(s1);
System.out.print("Enter your sentence: ");
}
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();
}
}
I ran into an issue. Below is my code, which asks user for input and prints out what the user inputs one word at a time.
The problem is that the program never ends, and from my limited understanding, it seem to get stuck inside the while loop. Could anyone help me a little?
import java.util.Scanner;
public class Test{
public static void main(String args[]){
System.out.print("Enter your sentence: ");
Scanner sc = new Scanner (System.in);
while (sc.hasNext() == true ) {
String s1 = sc.next();
System.out.println(s1);
}
System.out.println("The loop has been ended"); // This somehow never get printed.
}
}
You keep on getting new a new string and continue the loop if it's not empty. Simply insert a control in the loop for an exit string.
while(!s1.equals("exit") && sc.hasNext()) {
// operate
}
If you want to declare the string inside the loop and not to do the operations in the loop body if the string is "exit":
while(sc.hasNext()) {
String s1 = sc.next();
if(s1.equals("exit")) {
break;
}
//operate
}
The Scanner will continue to read until it finds an "end of file" condition.
As you're reading from stdin, that'll either be when you send an EOF character (usually ^d on Unix), or at the end of the file if you use < style redirection.
When you use scanner, as mentioned by Alnitak, you only get 'false' for hasNext() when you have a EOF character, basically... You cannot easily send and EOF character using the keyboard, therefore in situations like this, it's common to have a special character or word which you can send to stop execution, for example:
String s1 = sc.next();
if (s1.equals("exit")) {
break;
}
Break will get you out of the loop.
Your condition is right (though you should drop the == true). What is happening is that the scanner will keep going until it reaches the end of the input. Try Ctrl+D, or pipe the input from a file (java myclass < input.txt).
it doesn't work because you have not programmed a fail-safe into the code. java sees that the scanner can still collect input while there is input to be collected and if possible, while that is true, it keeps doing so. having a scanner test to see if a certain word, like EXIT for example, is fine, but you could also have it loop a certain number of times, like ten or so. but the most efficient approach is to ask the user of your program how many strings they wish to enter, and while the number of strings they enter is less than the number they put in, the program shall execute. an added option could be if they type EXIT, when they see they need less spaces than they put in and don't want to fill the next cells up with nothing but whitespace. and you could have the program ask if they want to enter more input, in case they realize they need to enter more data into the computer.
the program would be quite simplistic to make, as well because there are a plethera of ways you could do it. feel free to ask me for these ways, i'm running out of room though. XD
If you don't want to use an EOF character for this, you can use StringTokenizer :
import java.util.*;
public class Test{
public static void main(){
Scanner sc = new Scanner (System.in);
System.out.print("Enter your sentence: ");
String s=sc.nextLine();
StringTokenizer st=new StringTokenizer(s," ");//" " is the delimiter here.
while (st.hasMoreTokens() ) {
String s1 = st.nextToken();
System.out.println(s1);
}
System.out.println("The loop has been ended");
}
}
I had the same problem and I solved it by reading the full line from the console with one scanner object, and then parsing the resulting string using a second scanner object.
Scanner console = new Scanner(System.in);
System.out.println("Enter input here:");
String inputLine = console.nextLine();
Scanner input = new Scanner(inputLine);
List<String> arg = new ArrayList<>();
while (input.hasNext()) {
arg.add(input.next().toLowerCase());
}
You can simply use one of the system dependent end-of-file indicators ( d for Unix/Linux/Ubuntu, z for windows) to make the while statement false. This should get you out of the loop nicely. :)
Modify the while loop as below. Declare s1 as String s1; one time outside the loop. To end the loop, simply use ctrl+z.
while (sc.hasNext())
{
s1 = sc.next();
System.out.println(s1);
System.out.print("Enter your sentence: ");
}