Develop Reference

Trying to write a very simple program involving a password (java)

So I tried out having a password check in my program, the goal is, if the user types the correct password then the program will reply: "You Pass", if not the "You're wrong". The problem is Eclipse tells me that "Fish (that's the password) cannot be resolved to a variable"
import java.util.Scanner;
public class Password {
public static void main(String args[]) {
Scanner scan = new Scanner(System.in);
String password;
password = Fish;
System.out.println("What is the password? ");
scan.nextLine();
if (scan.equals(password)) {
System.out.println("You pass!");
}
else {
System.out.println("You're wrong!");
}
}
}
I tried resolving the issue in Eclipse's way, but with their method I get the wrong password when I type it after running the program:
import java.util.Scanner;
public class Password {
private static final String Fish = null;
public static void main(String args[]) {
Scanner scan = new Scanner(System.in);
String password;
password = Fish;
System.out.println("What is the password? ");
scan.nextLine();
if (scan.equals(password)) {
System.out.println("You pass!");
}
else {
System.out.println("You're wrong!");
}
}
}
I tried looking this up online, I'm actually reading Java Programming for Dummies, and still no luck, hopefully you can help me, I'd appreciate it!
Thanks!

You have wriiten some wrong syntaxes for it.Try this code.I have added necessary comments to let you make undersatand that what I have done in the following code.
public static void main(String args[]) {
Scanner scan = new Scanner(System.in);
String password;
password = "Fish"; // assigning value to String password
System.out.println("enter value");
String temp=scan.nextLine(); //scanner takes nextline entered and assign it to temp
// if (scan.equals(password)) is wrong ,you want to compare value taken by scanner , what you have wriiten is incorrect. scan is object of Scanner.Not value taken by it
if(temp.equals(password)) //if entered value eqauls your assigned password value
{
System.out.println("You pass!"); // else print you pass
}
else
{
System.out.println("You're wrong!"); //else print "you are wrong"
}

First of all,you're never reading your input in a variable!and then you're trying to check equality between the "scan" object which is of type Scanner.Instead read input "scan.nextLine();" in a variable say "var" and then call var.equals(password).Please also check that your String "Fish" is null initially which will give you a NullPointerException!

if (isValid(password))
{
System.out.println("You pass!");
}
else
{
System.out.println("You're wrong!");
}

Welcome to the Java World!
In your program you are making a small mistake by calling the equals on a Scanner object.
Also remember to close the close-able scanner resource once you are done with it.
Please refer the code below.
import java.util.Scanner;
public class Password {
private static final String Fish = "Fish";
public static void main(String args[]) {
Scanner scan = new Scanner(System.in);
System.out.println("What is the password? ");
if (scan.nextLine().equals(Fish)) {
System.out.println("You pass!");
}
else {
System.out.println("You're wrong!");
}
}
}

How to make a ONE static scanner global variable without closing scan constantly and use it within methods and the main method?

I want to create a static scanner but i will like to put the try catch block around it so it can automatically close avoiding resources
leaks and or this exception:
Exception in thread "main" java.util.NoSuchElementException: No line found
at java.util.Scanner.nextLine(Scanner.java:1585)
at softwareEngineer.UserApp1.main(UserApp1.java:82)
Essentially I only want to create one static scanner declaration and use it throughout the main program and includes the static methods, at this point my code will require to create separate scanner for each method and you are force "scan.close()". the code below will recieve a exception handling error due to multiple scanner that was open and did not closein the program.
I updated the code now i get null pointer exception
import java.util.Scanner;
public class UserApp1 {
static User currentCustomer = null; //single object
static Scanner scan;
//-------------------------------------------------------
// Create a list, then repeatedly print the menu and do what the
// user asks until they quit
//-------------------------------------------------------
public static void main(String[] args) {
scan = new Scanner(System.in);)//scanner to avoid resource leak
printMenu(); //print menu system from another function
String choice = scan.nextLine(); //reads an input
final String EXIT_now = "0";
final String BACK = "back";
while (!(choice.equalsIgnoreCase(EXIT_now))){
switch(choice) {
case 1: break;
case 2:
currentCustomer = loginInput();<---- errors happens here
if(currentCustomer != null){
System.out.println("You have successfully login");
}
break;
default:
System.out.println("Sorry, invalid choice");
break;
} //ends switch
printMenu(); //print menu system from another function
choice = scan.nextLine(); //reads an input
}//ends while
System.out.println("\t\t GoodBye!\n Thank you for trying our program.");
System.exit(0);
}//ends main
//----------------------------
// Print the user's choices
//----------------------------
public static void printMenu() {
System.out.println("\t\t The User Login System ");
System.out.println("\t\t ======================");
System.out.println("The Menu Options:");
System.out.println("1: Register an Account");
System.out.println("2: Login to your Account");
System.out.println("3: Reset Password");
System.out.println("0: Quit/Exit ");
System.out.println("Please enter your selection > ");
} //ends printMenu
public static User loginInput(){
System.out.print( "\nFollow the Prompts to Log-In to your Account \n ");
System.out.print( "\nPlease enter your userid : \n ");
String userid = scan.nextLine();// <---- errors happens here
System.out.print( "\nPlease enter your password: \n ");
String pass = scan.nextLine();
currentCustomer = AccountList.loginUser(userid, pass);
if (currentCustomer != null)
{
return currentCustomer;
}
return null;
}//ends loginInput
}//ends class*

You're using a try-with-resources, which will automatically close it when you finish the try block. Try setting it to a variable like so:
public class MyClass {
private static Scanner scan;
public static void main(String[] args) {
scan = new Scanner(System.in);
}
}
Avoid making multiple scanners with the System.in input as well, as they will consume the stream and then you have an entirely different problem.

Avoid using a static global Scanner at all, by passing the Scanner instance you want to work with to the relevant methods. Consider this simplified example:
public static void main(String[] args) {
try(Scanner in = new Scanner(System.in)) {
String choice = in.nextLine().trim();
if(choice.equals("1")) {
doOp1(in);
} else if(choice.equals("2")) {
doOp2(in);
} else {
System.err.println("Invalid choice. Goodbye.");
}
}
}
// Method takes an open, functioning Scanner as an argument, therefore
// it doesn't need to close it, or worry about where it came from, it
// simply uses it, does what it needs to do, and returns, trusting
// the caller to properly close the Scanner, since it opened it.
private void doOp1(Scanner in) {
System.out.print("What is your name? ");
String name = in.nextLine().trim();
System.out.print("What is your favorite color? ");
String color = in.nextLine().trim();
}
private void doOpt2(Scanner in) {
...
}
You want to compartmentalize your resources to ensure they are limited in scope and easy to close. Putting them in global state of any kind makes that very difficult. Instead, separate the opening and closing of the resource from the code using it. This sort of compartmentalization makes for much more maintainable, readable, and testable code.
For instance, by passing an already open Scanner to your core business logic functions, you can mock a real user's behavior and create a test to ensure your code remains stable, by constructing a Scanner that reads from a hard coded String, and passing that into your method, without needing to run the whole class and type in the behavior your testing manually again and again.

Use of Scanner class in recursive functions

I am trying to use a recursive method/function, which uses the Scanner class. Closing the scanner, causes an exception to be thrown the next time the same method is called within the recursion. A workaround to this is not to close the scanner at all, but this is not a right approach. I suspect the same scanner object is used between recursive calls, so that's why closing it creates havoc. If my assumption is correct then closing the scanner in the last method call would be a valid workaround (i.e. no actual resource leak). Is there anything I may be missing before jumping into Scanner and related implementation code?
EDIT
The answers provided were really useful and enlightening. In summary, the problem is the constant re-opening and closing of the scanner, and not recursion per se. The reason I would avoid passing the scanner object as parameter is that this example simulates a larger project, calling multiple recursive functions and I would have to pass the scanner object in all of them.
On the practical side, and from the answers provided, I think just closing the scanner in the last recursive call would work without having any resource leaks. Any related opinions would be welcome, esp. if you see something wrong with my approach.
Here is an example of my initial experiment:
package scanner;
import java.util.Scanner;
public class Main {
public static void acceptValidInput() {
System.out.print("Enter a number greater than 10: ");
Scanner sc = new Scanner(System.in);
int i = sc.nextInt();
// Adding this will make an exception to be thrown:
sc.close();
if (i <= 10) {
acceptValidInput();
}
}
public static void main(String[] args) {
acceptValidInput();
System.out.println("Your input is valid");
}
}

Once you start to consume an input stream using a Scanner, you should not try to read from it in any other way anymore. In other words, after you have constructed a Scanner to read from System.in, you need to use it for all further reading from System.in. This is because Scanner buffers input, so you have no idea how much input it has already consumed but not emitted yet.
Therefore, I recommend that you construct one Scanner, then use it for all the reading:
public class Main {
public static void acceptValidInput(Scanner sc) {
System.out.print("Enter a number greater than 10: ");
int i = sc.nextInt();
if (i <= 10) {
acceptValidInput(sc);
}
}
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
acceptValidInput(sc);
System.out.println("Your input is valid");
sc.close();
}
}

It works:
package scanner;
import java.util.Scanner;
public class Main {
public static void acceptValidInput(Scanner sc) {
int i = sc.nextInt();
if (i <= 10) {
System.out.print("Enter a number greater than 10: ");
acceptValidInput(sc);
}
}
public static void main(String[] args) {
System.out.print("Enter a number greater than 10: ");
Scanner sc = new Scanner(System.in);
acceptValidInput(sc);
sc.close();
System.out.println("Your input is valid");
}
}
The result is:
Enter a number greater than 10: 4
Enter a number greater than 10: 5
Enter a number greater than 10: 11
Your input is valid
Process finished with exit code 0

Closing the scanner closes also the underlying input stream. In this case it is the System.in stream - you shouldn't do this. Either do not close it or create a single scanner for all method calls.

public class abc{
public void acceptValidInput() {
System.out.print("Enter a number greater than 10: ");
Scanner sc = new Scanner(System.in);
int i = sc.nextInt();
// Adding this will make an exception to be thrown:
if (i <= 10) {
acceptValidInput();
}
}
public static void main(String[] args) {
while(true){
abc a=new abc();
a.acceptValidInput();
System.out.println("Your input is valid");
}
}}
try this.

Looping in Java(Beginners)

I have to use a loop in my code so that when someone enters yes, they can re-enter their names as many times as they want, but I have no idea how to do this. Any help is appreciated, here is my code:
public static void main(String [] args)
{
// Create a Scanner object to read input.
Scanner keyboard = new Scanner(System.in);
//Get the user's name.
System.out.print("What is your name?");
String name = keyboard.nextLine();
System.out.println("Hello there," + name);
System.out.println("Would you like to enter another name? Please enter Yes Or No.");
String reply = keyboard.nextLine();
if (reply == "yes")
{
}
}
}

This reply == "yes" is not how you compare Strings in Java. This compares there memory locations, not there contents (and it's unlikely there memory locations are going to be equal).
Instead you need to use reply.equals("yes") or if you don't care about doing a case comparison, you can use reply.equalsIgnoreCase("yes") instead
do {
// The remainder of your code...
} while (reply.equalsIgnoreCase("yes"));
Updated
You may also wish to have a read through The while and do-while statements and The for Statement, which covers the basics of looping in Java

Use a do-while loop:
public static void main(String[] args) {
// Create a Scanner object to read input.
Scanner keyboard = new Scanner(System.in);
do {
//Get the user's name.
System.out.print("What is your name?");
String name = keyboard.nextLine();
System.out.println("Hello there," + name);
System.out.println("Would you like to enter another name? Please enter Yes Or No.");
} while (keyboard.nextLine().equalsIgnoreCase("yes"));
System.out.println("Bye!");
keyboard.close();
}
}

use this
import java.util.*;
public class prob13 {
public static void main(String [] args)
{
// Create a Scanner object to read input.
Scanner keyboard = new Scanner(System.in);
//Get the user's name.
while(true){
System.out.print("What is your name?");
String name = keyboard.nextLine();
System.out.println("Hello there," + name);
System.out.println("Would you like to enter another name? Please enter Yes Or No.");
String reply = keyboard.nextLine();
if(reply.equals("no"))
break;
}
}
}
The reason for this is to loop through as long as the answer is not no.
or you could use this if you want the answer to always be yes
import java.util.*;
public class prob13 {
public static void main(String [] args)
{
// Create a Scanner object to read input.
Scanner keyboard = new Scanner(System.in);
String reply="yes";
//Get the user's name.
while(reply.equals("yes")){
System.out.print("What is your name?");
String name = keyboard.nextLine();
System.out.println("Hello there," + name);
System.out.println("Would you like to enter another name? Please enter Yes Or No.");
reply = keyboard.nextLine();
}
}
}

I think this will work (untested):
public static Scanner keyboard = new Scanner(System.in); // global
public static void main(String [] args)
{
getName();
}
public static void getName()
{
System.out.print("What is your name?");
String name = keyboard.nextLine();
System.out.println("Hello there," + name);
rerun();
}
public static void rerun()
{
System.out.println("Would you like to enter another name? Please enter \"yes\" or \"no\".");
String reply = keyboard.nextLine();
if (reply.equals("yes")) getName();
else System.exit();
}
}
First we call the getName() method and run through that once. Then we make a call to the rerun() method. This method will test if we want to re-run the program. If the user types in "yes", then we repeat the whole process. If we type in anything besides "yes", the program quits.
Besides the fact that your code is unfinished, the only real problem with your code is that you try to compare strings with the == operator. See MadProgrammer's answer as to why that is wrong.

The simplest (and probably clearest) way is to wrap what you want to repeat in a do-while statement:
public static void main(String [] args)
{
// Create a Scanner object to read input.
Scanner keyboard = new Scanner(System.in);
String reply;
do {
//Get the user's name.
System.out.print("What is your name?");
String name = keyboard.nextLine();
System.out.println("Hello there," + name);
System.out.println("Would you like to enter another name? Please enter Yes Or No.");
reply = keyboard.nextLine();
} while ("yes".equals(reply));
}
}
The reply variable must be declared before the block, because it is accessed in the loop condition (a variable is only visible in the block it is declared in, so if reply were declared in the loop, it would not be available to the loop condition).
I changed the loop condition because the == operator compares Strings by reference, i.e. it will check whether both sides point to the same String object. The equals method, in contrast, checks that the content of the Strings is equal (i.e. they contain the same characters in the same order).

initialization problems?

I am trying to make a program that lets the user enter an unknown value of names and then output the longest name entered. This is my code so far. When i compile I have several errors and they are all the same "cannot find symbol". Do i need to initialize those variables if so where?
import java.util.Scanner;
public class Name
{
public static void main(String[] args)
{
Scanner kb = new Scanner(System.in);
longestName(kb);
}
public static void longestName(Scanner sc)
{
String name=kb.nextLine();
biggestName=name;
System.out.println("Type -1 if you want to quit");
int number=kb.nextInt();
While (number !=-1);
{
String name1=kb.nextLine();
if (name1.length() > biggestName)
{
biggestName=name1;
}
System.out.println("Do you want to continue? Type -1 to quit.");
int number1=kb.nextInt();
}
System.out.println("Longest name is "+biggestName);
}
}
Thanks for the help guys fixed the errors, and some other changes and the program gives the correct output.
import java.util.Scanner;
public class Name
{
public static void main(String[] args)
{
Scanner kb = new Scanner(System.in);
longestName(kb);
}
public static void longestName(Scanner kb)
{
String biggestName;
System.out.println("Enter the first name");
String name=kb.nextLine();
biggestName=name;
System.out.println("Type -1 if you want to quit");
int number=kb.nextInt();
while (number !=-1)
{
System.out.println("Enter another name");
Scanner kb1 = new Scanner(System.in);
String name1=kb1.nextLine();
int length1=biggestName.length();
int length2=name1.length();
if (length2 > length1)
{
biggestName=name1;
}
System.out.println("Do you want to continue? Type -1 to quit.");
number=kb.nextInt();
}
System.out.println("Longest name is "+biggestName);
}
}

There are quite a few errors in your code. Without explaining every error in detail, here is an example of a modified version which works:
import java.util.Scanner;
public class Name
{
public static void main(String[] args)
{
Scanner kb = new Scanner(System.in);
longestName(kb);
}
public static void longestName(Scanner sc)
{
System.out.println("Enter name, or type '-1' if you want to quit");
String name=sc.nextLine();
String biggestName="";
while (!name.equals("-1"))
{
if (name.length() > biggestName.length())
{
biggestName=name;
}
name=sc.nextLine();
}
System.out.println("Longest name is "+biggestName);
}
}

You passed in your Scanner to longestName, but in longestName, you named the parameter sc. Use sc instead of kb in longestName.
Use lowercase while instead of While; remove the semicolon following the while; a semicolon there means that that is the body, instead of the { } block below it.
I assume that at the bottom of the while loop, that you want to assign the next integer to number, not a new variable number1 that immediately goes out of scope.
You didn't declare what biggestName is (or name).

Two errors here :
While (number !=-1);
While should be while, and the ; makes an infinite loop.
And another problem is that you don't change number in the loop anyway.

1-
public static void longestName(Scanner sc)
Either change the name of the scanner to kb, or change every kb within the method to sc.
2- See Scanner issue when using nextLine after nextXXX
3- Use while instead of While, and remove the ;.

I can see the below problems in the code:
longestName() method should be using the reference name sc instead of kb (since kb is having scope only in main method)
The variable biggestName is not declared. It should be either declared as a class variable or a variable in longestName() method and should be of type String
It is not While, it is while with 'w' in smaller case
There should not be a semicolon after the while statement
At the end of the while loop, the number to be compared for the while loop is to be calculated and is currently assigned to wrong variable. kb.nextInt() should be assigned to variable number and not to number1 since the variable number1 is never read/used.
The > operator can not be applied for String types. In the line if (name1.length() > biggestName), we are comparing int with String and will result in compilation error. The line should be modified as if (name1.length() > biggestName.length())
Method nextInt() will cause InputMismatchException to be thrown if you are providing an input which is not a number.
Now I feel I should have written a corrected code like Joe Elleson did. But hope this answer helps.