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Scanner.next() and hasNext() creating infinite loop when reading from console [duplicate]

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: ");
}

Make Java Program Loop until x is entered?

I want to make my program loop until the user types in x instead of a number. I tried using a while statement but I do not know how to use it with multiple variables. Here is my code
public static void main(String[] args)
{
int denominatorOne = 1, numeratorOne = 1;
System.out.println("Welcome, type an \"x\" at any point to exit the program");
while (numeratorOne !=x)
{
Scanner in = new Scanner(System.in);
//Prompt the user for fraction one
System.out.print("Enter the first numerator (top number): ");
numeratorOne = in.nextInt();
System.out.print("Enter the first denominator (bottom number): ");
denominatorOne = in.nextInt();
}
}
The exact phrasing from my assignment is The program should run in loop and allow the user to exit with some special character input (e.g. x or X to exit)

First off, 'x' isn't a number and won't be accepted by nextInt or a comparison to 'x', you should trying checking to see if it has next int (in.hasNextInt()) and process depending. Besides the point, you can easily test two variables in a while loop. Assuming you set up the variables right to be chars:
do {
// scan code.
} while(!(numChar1.equals('x') && numChar2.equals('x')))

what you need to do is have a bool value that holds the loop and when have a if statement check for the keydown event in the loop
bool looping = true
while ( looping == true)
{
if (x button was pressed == true)
{looping = false
}
}

try changing it to
while(!numeratorOne.equals("x")){...}

You can just call the method over again in this case main();.
What I suggest however is to create a new method, in the method just checking the users input returning the input as a string. Then you can check the string in your main method, and if that's not the string you wanted then recall the method. Here's an example, please note I didn't use an IDE for this.
public String getMessage(){
Scanner input = System.in();
return input;
}
public void checkMessage(String wantedString){
if(!getMessage().equalsIgnoreCase(wantedString)){
System.out.println("Please retry");
checkMessage();
}
}
public static void main(String[] args){
checkMessage();
}

How to use indexOf? Please help a beginner out

So I want to know what indexOf() does. As I want to use it in my program it find out how many vowels are in a word that the user inputs.
public static boolean methodCheck(char a){
return "AEIOUaeiou".indexOf(a) != -1;
}
But that doesnt seem to work at all hahah. as I have no idea what indexOf() actually does. anyway here is my program so far(sorry if its bad I'm really new). I left 5 questions too that would help me a lot! please and thank you for your help :D
import java.util.Scanner;
public class vowelCounter {
private static String input = methodInput(); //1. is there any other way to make a global Scanner?
public static void main(String[] args){
System.out.println("Enter word");
System.out.println(input);
System.out.println("This word has" + methodCheck('a')); //2. what should i put in my parameters?
}
public static boolean methodCheck(char a){
return "AEIOUaeiou".indexOf(a) != -1; //3. what does this line do?
}
public static String methodInput(){
Scanner keyboard = new Scanner(System.in);
String input = keyboard.nextLine();
return input;
//4. the output is 'hastrue' why is that?
//5. how can i make this program better?
}
}

If you don't know what a method does, then the solution is to go look at what it does. For example, the java documentation will tell you that
public int indexOf(int ch)
Returns the index within this string of the first occurrence of the specified character
In either case, if no such character occurs in this string, then -1 is returned.
How you're using it is not necessarily wrong, considering how the method returns -1 if the character wasn't found. But if you want to check how many vowels there are in a word that the user enters, it wouldn't be right to check whether the word they entered is in the string of vowels.

All the standard Java libraries, classes and methods have Javadoc that describes what they do.
All you need to do is look up the Javadoc and they describe it.
In this case the Javadoc is at: http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#indexOf(int)
Your first step with any question like this should always be the documentation, then if that doesn't work try doing a web search looking for examples. For example 5 seconds on google putting in "java indexOf example" found me:
http://www.tutorialspoint.com/java/java_string_indexof.htm
Then if that doesn't work you can try asking the question here.

When you have the word boolean before the name of a method, that means that the method will return either the value true or the value false. And it's this true or false value that your program is printing out, on the same line as "This word has".
This particular method will return true if the character you pass to it is a vowel, or false otherwise. The method indexOf tells you which character of a String is the first one that is equal to the value that you pass in to the method. It returns 0 for the first character, 1 for the second character and so on. It returns -1 if none of the characters match. In this case, you're just checking whether the value returned by indexOf is or isn't -1 - in other words, whether the character is or isn't in the String "AEIOUaeiou".

indexOf(String str) Returns the index within this string of the first occurrence of the specified substring. If no such value of str exists, then -1 is returned.
For examples :
int num1 = "AEIOUaeiou".indexOf("a"); // it gives 5
int num2 = "AEIOUaeiou".indexOf("A"); // It gives 0
int num3 = "AEIOUaeiou".indexOf("z"); // It gives -1

1 Don't do that! Create a scanner in main, read input with it and then call your method(s).
2 How about countVowels(input)? You'd need to write an static int countVowels(String input) method.
3 Returns true since you pass in 'a'.
4 See number 3.
5 See number 2, and add a static boolean isVowel(char a).

Here is what the indexOf method does
string.indexOf(searchvalue,start)
Parameters
searchvalue : Required. The string to search for
start : Optional. Default 0. At which position to start the search
Return Value
Number : The position where the specified searchvalue occurs for the first time, or -1 if it never occurs
In simple terms, the index of method checks the first occurence of the value passed to it from the start position(if specified) and returns the position at which the value was first encountered in the string.
eg.
String s = "AEIOUaeiou";
s.indexOf("a"); //This would get a value of 5.
s.indexOf("v"); //This would get a value of -1, since it doesn't have the character v
To answer your questions,
You can directly declare the scanner as private and use it in the
entire program
`private static Scanner input = new Scanner(System.in);`
you can write a method that receives the String input by the user
and then checks if the String contains any of the vowels. You can
use indexOf or contains methods to check for the each vowel using
the indexOf method.
Already described above.
A better way to do it would be as follows.
public class vowelCounter{
public static void main (String[] args) {
Scanner keyboard = new Scanner (System.in); // No need to declare it as global. You use it only once.
System.out.println ("Enter word : "); //Prompt the user to enter a word
String input = keyboard.nextLine (); //Fetch the word that the user enters into a String
System.out.println ("This word has" + countVowel (input)); // Pass the string to the method to check if it has vowels.
}
private static int countVowel (String a) {
int count = 0;
String s = a.toLowerCase (); // convert the string to lower case so that you only have to check for the lower case characters
// Here you would need to check the number of times each vowel exists in the String and incremenet the count everytime.
return count;
}
}

Check for value in array

I need to check the array to see if the user input is already present, and display a message as to whether it is or isn't there. The first part is working, but I tried to create a method for the word check, and I'm not sure if I'm on the right path or not, cheers.
import java.util.Scanner;
public class InputLoop {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
String array[] = new String[10];
int num = array.length, i = 0;
System.out.println("Enter a word");
for (i = 0; i < num; i++) {
while (scan.hasNextInt()) // while non-integers are present...
{
scan.next(); // ...read and discard input, then prompt again
System.out.println("Bad input. Enter a word");
}
array[i] = scan.next();
WordCheck();
}
}
public void WordCheck(String[] i) {
Scanner scan = new Scanner(System.in);
System.out.println("Enter another word");
if (scan.next().equals(array[i])) {
System.out.println("The word has been found");
} else {
System.out.println("The word has not been found");
}
}
}

Right. You've clearly gone down a bad thought process, so let's just clear the slate and have a re-think.
Step one: You want to take some user input
Step two: Compare it with all previous user inputs to see if it's present.
If it is present, return a message indicating that value has been inputted.
otherwise ignore the input and continue execution
Repeat step one.
The solution
So, let's review what you've got, and how you need to change it.
public static void main(String[] args)
If I were you, I would avoid calling methods directly from here. If you do, every method will need to be static, which is a pointless adjustment in scope for the functionality of your class. Create a new instance of your class, inside the main method, and move this code to the class' constructor. This will remove the need to make every single method static.
Scanner scan = new Scanner(System.in);
String array[] = new String[10];
Okay, so you've created a scanner object that takes input from the System.in stream. That's a reasonable thing to do when taking input from the keyboard. You've also created an array to contain each item. If you only want the user to be able to type in 10 values, then this is fine. Personally, I would use an ArrayList, because it means you can take in as many user inputs as the user desires.
Secondly, you want a function to compare the input, with all other inputs. What you have at the moment clearly isn't working, so let's have another go at it.
You will need some input, userInput, and a collection to compare it against, allInputs.
allInputs needs to be accessible from any point in the program, so it's probably wise to make it into a field, rather than a local variable.
Then, because you're comparing userInput against all values, you're going to need a foreach loop:
for(String s : allInputs)
{
if(s.equals(userInput))
{
// Output message code.
}
}
Now the trick is fitting this inside a loop that works with this program. That is up to you.

One simple solution is to use a Set:
Set<String> words = new HashSet<String>();
Add words with the add() method and check if a word is already added with contains(word) method.
EDIT
If you must use Arrays you can keep the array sorted and do a binary search:
Arrays.sort(words);
boolean isAlreadyAdded = Arrays.binarySearch(words, newWord) >= 0;

You're going to have to loop through the entire array and check if scan.next() equals any of them - if so return true - as such:
String toCheck = scan.next();
for (String string : i) { //For each String (string) in i
if (toCheck.equals(i)) {
System.out.println("The word has been found");
return;
}
}
System.out.println("The word has not been found");
This supposes you call WordCheck(), passing the array to it - this method also has to be static for you to call it from the main() method.

You can use the arraylist.contains("name") method to check if there is a duplicate user entry.

How to get out of while loop in java with Scanner method "hasNext" as condition?

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: ");
}