Java scanner как очистить

Очистка буфера Scanner

Снова я с нубскими вопросами.
Но поиск по гуглу мне ничего путевого не дал.

1 2 3 4 5 6 7 8 9 10 11 12 13 14

while (true){ System.out.println("Input point of menu\n 0 - Save\n 1 - Load\n 2 - Print\n 4 - exit"); try {int i=scan.nextInt(); if (i>=0&i3){ mainMenu[i].runMenu(); // Ну собственно говоря неважно, что там. :)))) } else if (i==4) { break;} } catch (java.util.InputMismatchException e){ System.out.println (e); System.out.println ("input number. "); String s=scan.nextLine(); // Вот это конечно помогает, но это же криво! } }

Так вот.
Если убрать строчку 12, то на сколько я понимаю, при внесении, например, символа, scan не очищается при переходе в catch.
И при переходе на следующую итерацию, scan.nextInt() опять получает символ и опять все по кругу.

как сделать так, чтобы без вот этого String s=scan.nextLine(); все очищалось?

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Clear Scanner in Java

1. Java Clear Scanner Using nextLine()
2. Create a New Scanner Object to Clear Scanner in Java

The Scanner class in Java is often used to take input or output. We create an object of the Scanner class to use its functions.

We cannot use the close() method of Scanner because once a Scanner is closed by this method, we cannot take input as the input stream has been closed.

There are other ways to clear the Scanner in Java, and below are the examples that explain these methods.

Java Clear Scanner Using nextLine()

To clear the Scanner and to use it again without destroying it, we can use the nextLine() method of the Scanner class, which scans the current line and then sets the Scanner to the next line to perform any other operations on the new line.

In the example below, inside the while loop, we ask the user for an input and check if it is a valid binary using hasNextInt(radix) . If it is a valid binary, then it will be stored in the binary variable, and if it is not a binary value, then a message will be printed to ask the user to input a valid binary value.

In this situation, if we don’t clear the Scanner , the loop will never end. If the value is not binary, then the Scanner should go to a new line to take a new input. This is is why nextLine() is used to skip the current line and go to a new line.

import java.util.Scanner;  public class ClearScanner   public static void main(String[] args)    Scanner input = new Scanner(System.in);  int binary = 0;  int radix = 2;   while (binary == 0)    System.out.print("Please input a valid binary: ");   if (input.hasNextInt(radix))   binary = input.nextInt(radix);  > else   System.out.println("Not a Valid Binary");  >  input.nextLine();  >   System.out.print("Converted Binary: " + binary);  > > 

Create a New Scanner Object to Clear Scanner in Java

Another method to clear the Scanner is to create a new Scanner object when the user types a value other than a binary value. This method works because when a new object is created, the existing Scanner object is cleared, and a new input stream is started.

import java.util.Scanner;  public class ClearScanner   public static void main(String[] args)    Scanner input = new Scanner(System.in);  int binary = 0;  int radix = 2;   while (binary == 0)    System.out.print("Please input a valid binary: ");   if (input.hasNextInt(radix))   binary = input.nextInt(radix);  > else   System.out.println("Not a Valid Binary");  >   input = new Scanner(System.in);  >   System.out.print("Converted Binary: " + binary);   > > 

Please input a valid binary: 23 Not a Valid Binary Please input a valid binary: 11101 Converted Binary: 29 

Rupam Saini is an android developer, who also works sometimes as a web developer., He likes to read books and write about various things.

Related Article — Java Scanner

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Java Scanner reset()

Scanner reset in Java, as the name suggests, resets the scanner object. In many programming interview problems, we need to be comfortable with different ways to handle the input. Knowing about the functionalities that these methods provide can always come in handy.

This article discusses the Scanner use cases in detail. We’ll also discuss:

• What Is Scanner in Java?
• How to Use the Scanner in Java?

• Java Scanner reset() Method
• Java Clear Scanner Using the nextLine() Method
• How to Clear Scanner in Java?
• Scanner API (Constructors and Methods)
• Interview Questions Related to Java Scanner
• FAQ

What Is Scanner in Java?

Scanner is a class present in the java.util package. It breaks the input into tokens using a delimiter pattern, which by default is whitespace. These tokens can later be converted into values of different types using the various next methods.

Input: Ronaldo has an extensive collection of 16 royal cars.

Here if we use space as a delimiter, below are the tokens that we get:

[“Ronaldo”, “has”, “an”, “extensive”, “collection”, “of”, ”16”, “royal”, “cars”]

Now, we can convert these tokens into Integers or String based on the user requirements. So in our example, the token “Ronaldo” will be String, whereas the token “16” will be an Integer.

Scanner class breaks this input into tokens and provides methods like next(), nextLine(), nextInt(), nextDouble(), etc to convert the tokens to String, Integer, Double etc respectively. So parsing the input in a given way becomes easy. Also, it enables us to provide our own delimiter to parse the input string.

How to Use the Scanner in Java?

There are various ways to read input using the Scanner class. Let’s understand them one by one.

Use Hard-coded Input Strings

public static void main(String[] args)

String input = «Ronaldo has an extensive collection of 16 royal cars»;

Scanner scanner = new Scanner(input);

String token = scanner.next();

System.out.println(«\»» + token + «\» is a number»);

System.out.println(«\»» + token + «\» is a string»);

Explanation:

Here, we have hardcoded the input in a String and passed it to a Scanner object. Finally, we read each of the tokens and check if it’s a string or a number. The above program also shows that the default delimiter for Scanner is whitespace.

Use Hard-coded Input Strings Split by Custom Delimiters

public static void main(String[] args)

String input = «Comma,separated,input,string»;

Scanner scanner = new Scanner(input);

String token = scanner.next();

Token — Comma

Token — separated

Token — input

Token — string

Explanation:

Here, we have hardcoded the input in a String and passed it to a Scanner object. Also, using the useDelimiter() function, we specify a comma as a delimiter. Then we read each of the tokens using the next() method. Also, as per the output, we can see that the input string splits based on commas.

Reading the Input From Console

public static void main(String[] args)

Scanner scanner = new Scanner(System.in);

String firstToken = scanner.next();

String newLine = scanner.nextLine();

System.out.println(«Next String — » + firstToken);

System.out.println(«Read Entire Line — » + newLine);

Console Input:

Console Output :

Next String — Hi

Read Entire Line — This is a new Line

Explanation:

Here, we read the input string from the console and then parse them. As evident from the output, the next() method in the Scanner class returns the next token, whereas you can use the nextLine() method to read the current line, excluding any line separator at the end.

Read Input From File

You can also use Scanner along with FileReader to read input from a file.

Scanner scanner = new Scanner(new FileReader(«fullQualifiedFilePath»));

public static void main(String[] args) throws FileNotFoundException

Scanner scanner = new Scanner(new FileReader(«fullyQualifiedFilePath»));

String firstToken = scanner.next();

String newLine = scanner.nextLine();

System.out.println(«Next String — » + firstToken);

System.out.println(«Read Entire Line — » + newLine);

Here, we save our input in a file and pass the fullyQualifiedPath name as a parameter to the Scanner object.

Java Scanner reset() Method

We use the reset method of java.util.Scanner to reset the Scanner object. Also, once we reset the scanner, it discards all the state information which has been changed by the invocation of useDelimiter(), useRadix(), and useLocale() methods.

public static void main(String[] args)

String input = «This is a random input»;

Scanner scanner = new Scanner(input);

//changing the delimiter, radix and locale.

System.out.println(«Changed Delimiter = «+scanner.delimiter());

System.out.println(«Changed Radix = «+scanner.radix());

System.out.println(«Changed Locale = «+scanner.locale());

// resetting the delimiter, radix and locale to defaults.

// default values of scanner delimiter, radix, and locale.

System.out.println(«Updated Delimiter = «+scanner.delimiter());

System.out.println(«Updated Radix = «+scanner.radix());

System.out.println(«Updated Locale = «+scanner.locale());

Changed Delimiter = ,

Changed Radix = 30

Changed Locale = fr_FR

Updated Delimiter = \p+

Updated Radix = 10

Updated Locale = en_US

Explanation:

As evident from the console output, although we had set the delimiter as comma, radix as 30, and locale as FRANCE, once the reset method of Scanner is invoked, it resets the value to default.

Java Clear Scanner Using the nextLine() Method

Using nextLine, we can skip the current line and move to the next line.

import java.util.Scanner;

public static void main(String[] args)

Scanner scanner = new Scanner(System.in);

int hexaDecimalNumber = 0;

int radix = 16;

while (hexaDecimalNumber == 0)

System.out.println(«Please enter a valid number»);

// check if token is hexadecimal.

if (scanner.hasNextInt(radix))

hexaDecimalNumber = scanner.nextInt(radix);

System.out.println(«Invalid Hexadecimal Number»);

// skip current line and goto newLine to read input again.

scanner.nextLine();

System.out.println(«Hexadecimal Number = » + hexaDecimalNumber);

scanner.close();

Please enter a valid number

Invalid Hexadecimal Number

Please enter a valid number

Explanation:

The nextLine() method present in the Scanner class scans the current line. It then sets the Scanner to the next line to perform other operations on the new line, which helps us re-use the scanner object without destroying it or re-instantiating it.

How to Clear Scanner in Java?

To clear the Scanner object, we can re-instantiate the object again. We have a similar use-case as above; the only difference is instead of going to a new line, now we will re-initialize it.

import java.util.Scanner;

public static void main(String[] args)

Scanner scanner = new Scanner(System.in);

int hexaDecimalNumber = 0;

int radix = 16;

while (hexaDecimalNumber == 0)

System.out.println(«Please enter a valid number»);

// check if token is hexadecimal.

if (scanner.hasNextInt(radix))

hexaDecimalNumber = scanner.nextInt(radix);

System.out.println(«Invalid Hexadecimal Number»);

// re-initialize the scanner object.

scanner = new Scanner(System.in);

System.out.println(«Hexadecimal Number = » + hexaDecimalNumber);

scanner.close();

Please enter a valid number

Invalid Hexadecimal Number

Please enter a valid number

Scanner API (Constructors and Methods)

Scans the next token as an Integer.

Scans the next token as an Float.

nextBigInteger()

Scans the next token as an BigInteger.

Scans the next token as a Double.

Scans past the current line and returns skipped input.

Scans and returns the next token.

Resets the Scanner delimiter, radix, locale to default.

Coding Interview Questions Related to Java Scanner

• Read an input string separated by numbers and chars. You have two strings that contain decimal values, but the strings are too large to be converted to double. Add the strings and return the result.

Java Scanner reset FAQS

Question 1: Is the performance of the Scanner class better as compared to the BufferedReader class?

The Scanner class is significantly slower in performance as compared to BufferedReader class.

Question 2: Can the Scanner class be used along with FileReader class in Java?

Yes, FileReader class can be passed as a parameter to the Scanner constructor.

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Article contributed by Problem Setters Official

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