I'm learning Java and working on some projects for fun. One issue that I have run in to is that when I use a Scanner object Eclipse warns me that:
Resource Leak: 'scan' is never closed.
So, I added a scan.close(); at the end of my code and that takes care of the warning.
The problem comes in because I have other classes in the same package that also use scanner objects and and Eclipse tells me to close scanner in those classes respectively. However, when I do that it seems like it closes ALL of the scanner objects and I get errors during run time.
Here is an example of what causes the error:
import java.util.Scanner;
public class test2 {
public static void main(String [] args) {
Scanner scan = new Scanner(System.in);
int test = 0;
do {
//Do stuff
test = scan.nextInt();
System.out.println(test);
scanTest scanTest = new scanTest();
scanTest.test();
} while (test != 0);
scan.close();
}
}
import java.util.Scanner;
public class scanTest {
public void test() {
Scanner scanner = new Scanner(System.in);
int blah = scanner.nextInt();
System.out.println(blah);
scanner.close();
}
}
After scanner is closed in the scanTest class and the do loop in test2 is entered again an error occurs at the line test = scan.nextInt();
I tried moving the creation of the scanner object into the do loop just to make a new object every time as well but the error still occurs.
Not sure why this is happening or how I can make sure all my I/O objects are closed out without running into problems.
One post I came across mentioned that when System.in is closed I cannot be re-opened. If this is the case would I just need to make sure a scanner object with System.in is closed at the very end of the program and #suppress all of the other scanner warnings in other classes? Or would that still leave all those scanner objects open (bad)?
First, this is no memory leak.
Second, when you close a stream wrapper, the default implementation is for it to close the stream that it wraps. This means that the first time you close your Scanner (as it is written), yes, you close System.in.
In general, one would like to avoid closing System.in if they were meaning to read from System.in again. The best way to go about this depends on your program.
One might copy the information from System.in into a buffer of some sort and then scan the buffer. One might not close the Scanner, reusing it in other locations. One might even de-reference the Scanner for garbage collection and create multiple new Scanners on System.in.
These solutions are not all equivalent, some are considered much better than others; but, it all depends on the calling program. Experiment with a few, and if you run into a problem, open a new StackOverflow question where you show the relevant portions of your code, a description of the problem, the example input, and the wrong output (along with the desired output).
Good luck.
Yes, when you close a scanner you will be closing the underlying stream (in this case System.in). To avoid this, either create a global variable of scanner which can be used by all classes or have a central point for shutting down the scanner (just before the program exits would be ideal)
Don't name all your scanners the same. If you have multiple in one thing like this:
import java.util.Random;
import java.util.Scanner;
public class DayThree {
public static void main(String[] args) {
**Scanner textScanner = new Scanner(System.in);**
// boolean operands
// String(or objects) .equals() "this".equals("that") false
// primitive data types == 'a'=='a' -> true 5==6 false
// != 'a'!='a' -> false 5!=6 true
// ! !(true) -> false !(false) true
// > 5 > 4 -> true 'a' > 'b' false
// < 5 < 4 -> false
// <=
// >=
// && -> and 5 < 6 && 7 > 10 -> false
// if either side of and is false the outcome is false
// || -> or 5 < 6 || 7 > 10 -> true
// if either side of or is true the outcome is true
//System.out.println(!(5 < 10) && (7>3) || (true && false || true));
/* <-- this is a multi line comment
System.out.println("What is the most amazing show on tv this week? ");
String show = textScanner.nextLine().toLowerCase(); //this is case sensitive
show = show.toLowerCase(); // changes the strng to a lowercase version
show = show.toUpperCase();
if(show.equalsIgnoreCase("game of thrones")){ // .equalsIgnoreCase( ignores caps/lower)
System.out.println("Yes it is!");
}
else{
System.out.println("You are wrong.");
System.out.println(show + " is clearly inferior to Game of Thrones.");
}
System.out.println("Who is your favorite character in " + show + ".");
String character = textScanner.nextLine().toLowerCase();
if(character.contains("dragon")){
System.out.println("CGI magic is so cool!");
}
else if(character.contains("lanister")){
System.out.println("Wrong house.");
}
else{
System.out.println(character + "is pretty cool I guess...");
}
*/
// asdf alternate multi line comment use ctrl + / on highlighted text.
// doing this a second time undoes the comment
// sdaf
// asdf
// asdf
// asdf
// 1. ask about favorite something (pet)
// 2. save that into a string all lowercase
// 3. have a series of if else (x3) and else statements about the something
//NOTE: DO NOT END CONDITIONALS WITH ; example: if(boolean); IS WRONG.
**Scanner numScanner = new Scanner(System.in);** // the variable tells you what to use it for
Random rand = new Random(); // this makes a new random object
System.out.println("Pick a number.");
int num = numScanner.nextInt();
int sNum = rand.nextInt(9) + 1; // gives me a random num between 1-10
// nextInt(bound)gives you a num from 0-bound
//adding one gives you a num from 1 - bound + 1
if(num > sNum){
System.out.println("Too high");
System.out.println("The number was " + sNum);
}
else if(num < sNum){
System.out.println("Too low");
System.out.println("The number was " + sNum);
}
else{
System.out.println("Wow are you psychic? ");
}
textScanner.close();
numScanner.close();
}//main method
}
Put the *scanner name goes here*.close(); for each one of your scanners. If they all have the same name then change the ones that do something different from and other scanner.
Related
int selection;
while (true) {
selection = printing();
if (selection == 4) {
id = starting();
if (id < 1 || id > 10) {
if (id == -20150901) {
System.out.println("Exit code entered");
break;
}
id = incorrectId(id);
}
}
}
public static int printing(){
Scanner sc = new Scanner(System.in);
System.out.print("Main menu\n1: check balance\n2: withdraw\n3: deposit\n4: exit\nEnter a choice: ");
System.out.print("Enter a choice: ");
int selection = sc.nextInt();
return selection;
}
This is part of my java code.
The NoElementException occured in the third line.
If the whole code is needed, I will copy and paste here and explain what it is about.
How can I solve this exception?
I want to get keyboard input every time the loop starts.
Thank you.
Your code creates a new Scanner wrapping System.in each time you call printing(). This is incorrect. Instead, your application should create exactly one Scanner wrapping System.in, save it somewhere, and reuse it for each printing() call and all other places where you are reading from System.in.
The problem with creating multiple Scanner objects is that that the hashNext* and next* methods can cause characters to be "read ahead" characters into the Scanner's buffer. So the first printing() could "consume" all characters.
Now this may not be the actual cause of your problem. Other possible causes could be:
standard input could be empty, or
you may be (directly or indirectly) closing the System.in in some other part of the code.
However, the problem I identified is a bug that would cause application failures in other contexts ... and it is advisable to understand and fix it.
I'm currently working on an assignment for school and I am almost done but I just have one large problem I need to fix before I can add the final bit.
I need to create a program that prompts you to enter either 1 or 2, Afterwards it asks you to enter three words/names and saves them into an array.
Then, depending on whether you picked 1 or 2, it prints them in alphabetical order or flips around the lowercase and uppercase letters. I didn't add that part yet because I'm trying to fix a problem related to the very first input.
When you input a number other than 1 or 2, I am instructed to display an error message and ask for input again. I am pretty sure what I need to do is get the entire program to go back to the beginning because copy/pasting the entire program again would be bad, lol
A big problem is probably that I'm using if/else statements with for loops inside when I might need to put the entire thing inside a loop? But I'm not sure what condition I would use to start the loop if I put the entire code in it. I must be missing something here.
With what I have now, it gets stuck saying invalid input even if you put in a 1 or 2.
import java.util.Scanner;
public class IsabellaPiantoniLab5 {
public static void main (String[]args) {
//Ask for input
Scanner input = new Scanner(System.in);
System.out.print("Please choose either a number 1 or number 2.");
int numChoice = input.nextInt();
//if choice is 1 or 2
if (numChoice == 1 || numChoice == 2) {
System.out.println("Please enter three names: ");
String nameInput[] = new String[4];
//input loop
for (int i= 0; i < nameInput.length; i++) {
nameInput[i] = input.nextLine();
}
System.out.println("Values are:");
//display values if 1
if (numChoice == 1) {
for (int i=1; i<4; i++) {
System.out.println(nameInput[i]);
}
}
//display values if 2
else if (numChoice == 2) {
for (int i=1; i<4; i++) {
System.out.println(nameInput[i]);
}
}
}
//retry if invalid------i restart from the beginning if this happens
else if (numChoice != 1 || numChoice != 2) {
System.out.println("Invalid value. Please try again.");
//continue;
}
}
}
System.exit(0);
This will terminate the app, thus you can start it again using command line ( START [your app path])
Or
RunTime.getRuntime().exec(“Your app”);
System.exit(0);
Edit I misunderstood the question, I thought you wanted to restart the whole app
After discussing the approach with #csm_dev
It is way either to ask for the user input one more time by emptying the field and showing a message “please enter a valid input” with a clarification message
Trying to print a file based off the user's input as mentioned in the title. Basically, my program has been altered from one that I previously created which reads data from a file, so I know that the file has been imported correctly (not the problem).
The problem I have is that I'm trying to make the program print the entirety of the .txt file if the user chooses a specific number, in this case '1'. My current code so far is:
import java.io.FileReader;
import java.util.Scanner;
public class InputOutput {
public static void main(String[] args) throws Exception {
// these will never change (be re-assigned)
final Scanner S = new Scanner(System.in);
final Scanner INPUT = new Scanner(new FileReader("C:\\Users\\JakeWork\\workspace\\Coursework\\input.txt"));
System.out.print("-- MENU -- \n");
System.out.print("1: Blahblahblah \n");
System.out.print("2: Blahblahblah \n");
System.out.print("Q: Blahblahblah \n");
System.out.print("Pick an option: ");
if (S.nextInt() == 1) {
String num = INPUT.nextLine();
System.out.println(num);
}
I feel as if my if statement is totally off and I'm heading in the entire wrong direction, could anyone point me in the right and give me a helping hand?
You're close, but not quite there.
You a reading the user input correctly, but now you need the file contents in a loop.
if(S.nextInt() == 1) {
while (INPUT.hasNextLine()) {
System.out.println(INPUT.nextLine());
}
}
This will keep looking as long as the file contents hasNextLine
You can safely remove the String option = S.next();
Also, just a small bit of naming convention nitpicking, don't use all upper case letters for variable names unless they are meant to be static. Also, the first letter of a variable is generally lower case.
if (S.nextInt() == 1) {
// check if there is input ,if true print it
while((INPUT.hasNextLine())
System.out.println(INPUT.nextLine());
}
Also, for menu scenarios like this, consider using a switch statement, then place a call to the menu-printing (that you move to a separate method) in the default case, so that if you enter something wrong, you can reprint the menu choices. Also, the switch statement is more readable (arguably) than a bunch of if's, like this:
int option = S.nextInt();
switch(option) {
case 1 :
while(INPUT.hasNextLine()) {
System.out.println(INPUT.nextLine());
}
break;
case 2 :
//Do stuff
break;
default :
//Default case, reprint menu?
}
}
This question already has answers here:
Why Wont My Java Scanner Take In input?
(3 answers)
Closed 7 years ago.
When I run my code, it works right up until it asks the question "which operation do you want to use from ( sum , subst , multi , div )". No matter what the user picks, there is no response from my program!
Why is this happening?
import java.util.Scanner;
import java.io.*;
public class three3 {
public static void main (String[] args) {
int x;
int y;
int opera;
String oper;
Scanner in = new Scanner (System.in);
System.out.println(" write the first number ");
x = in.nextInt();
System.out.println(" write the second number ");
y = in.nextInt();
System.out.println(" which operation do you want to use from ( sum , subst , multi , div )");
oper = in.nextLine();
if (oper == "sum") {
opera=x+y;
System.out.println(" the sum of two numbers is " + opera );
}
if (oper == "subst") {
opera = x - y;
System.out.println(" the subtraction of two numbers is " + opera );
}
if (oper == "multi") {
opera = x * y;
System.out.println(" the multi of two numbers is " + opera );
}
if (oper == "div") {
opera = x / y;
System.out.println(" the division of two numbers is " + opera );
}
}
}
Because none of those if-clauses is executed.
You're comparing Strings with == which is wrong. Use oper.equals("sum") instead. See this question for reference. The conclusion for you is to always use equals for Strings.
You need to call in.nextLine() right after the last call to in.nextInt() The reason is that just asking for the next integer doesn't consume the entire line from the input, and so you need skip ahead to the next new-line character in the input by calling in.nextLine().
int y = in.nextInt();
in.nextLine();
This pretty much has to be done each time you need to get a new line after calling a method that doesn't consume the entire line, such as when you call nextBoolean() etc.
In addition, you don't check for String equality with the == operator, use the .equals() String method instead.
The problem is that in.nextLine() consumes the \n inserted implicitly when you clicked enter after the int was entered. That means that the program doesn't expect any other input from the user. To fix this you could consume a new line with in.nextLine() before putting it int your actual variable, something like this:
System.out.println(" write the second number ");
y=in.nextInt();
System.out.println(" which operation do you want to use from ( sum , subst , multi , div )");
in.nextLine(); //New line consuming the \n
oper=in.nextLine();
if(oper.equals("sum")){//replace == by .equals
opera=x+y;
}
Apart from that, and as runDOSrun said, you should replace the comparisons of strings from a==b to a.equals(b)
Adding on to other people's points, you should also consider using else if{} and else{} statements so you can catch invalid input.
I made a program that asks for 3 integers to output type of triangle. Everything runs and compiled successfully, however, it seems the part where it asks the user to see if they want to loop it again, the online compiler outputs the error:
Exception in thread "main" java.util.NoSuchElementException
at java.util.Scanner.throwFor(Scanner.java:838)
at java.util.Scanner.next(Scanner.java:1347)
at Assignment5.main(Assignment5.java:56)
import java.util.Scanner;
public class Assignment5 {
public static void main (String[]args)
{
for (int a = 0; a < Integer.MAX_VALUE; a++)
{
Scanner userInput = new Scanner(System.in);
Scanner answer = new Scanner(System.in);
int x,y,z;
System.out.println("Enter the sides of the triangle: ");
x = userInput.nextInt();
y = userInput.nextInt();
z = userInput.nextInt();
Tri isos = new Tri(x,y,z);
Tri equal = new Tri(x,y,z);
Tri scalene = new Tri(x,y,z);
// check the equilateral triangle
System.out.println(equal.toString() + " triangle:");
if (equal.is_isosceles())
System.out.println("\tIt is isosceles");
else
System.out.println("\tIt is not isosceles");
if (equal.is_equilateral())
System.out.println("\tIt is equilateral");
else
System.out.println("\tIt is not a equilateral");
if (equal.is_scalene())
System.out.println("\tIt is scalene");
else
System.out.println("\tIt is not scalene");
System.out.println("Would you like to enter values again? (y/n)" );
String input = answer.next(); //Exception is thrown from here
if (input.equals("y"))
{
System.out.println("ok");
}
else if(!input.equals("y"))
{
System.out.println("Ok, bye.");
break;
}
}
}
}
NoSuchElementException:
Thrown by the nextElement method of an Enumeration to indicate that
there are no more elements in the enumeration.
You're getting this exception because Scanner#next doesn't read the new line character, which is the character when you press enter (\n), so in the next for iteration, you're trying to read it, which causes the exception.
One possible solution is to add answer.nextLine() right after answer.next() in order to swallow this extra \n.
Example of your code:
Iteration (a) | input for scanner | Data for scanner
--------------+-----------------------+-------------------
0 | "Hello" (And enter) | Hello
1 | \n | PROBLEM!
to me it seems that answer.next() does not actually have any value assigned to it
usually int name = answer.next() name is assigned what ever answer is. What i mean is that name cant be assigned a value because answer.next() doesn't have one.
At least this is my understanding. The alternative is the get rid of answer.next and use the other scanner.
actually an edit to this.
a scanner reads from files or the console. You have one scanner already (userInput) the second scanner isn't actually doing anything as well as it being an actual scanner, it doesn't have anything to read.
get rid of answer as a scanner, replace is with an int, String, double and have
int answer = userInput.nextInt();
or
double answer = userInput.nextDouble();
or
String answer = userInput.nextLine();
As you said the code runs for you but doesn't when compiled and executed on an online compiler. The answer scanner is exhausted because it doesn't have any elements.
It's embarrassing but i once got the same error when compiling my code on an online compiler, it turned out i wasn't supplying input beforehand to the input section and was expecting the online compiler to ask for the input.
Since you are using two scanners to get input from console, try using the scanner userInput to take the input from a file instead. (It may vary for different online compilers, but there will be an option to provide input from file)