Java Scanner Class

java.util.Scanner is a new feature in Java 5. We can obtain user input through the Scanner class.

The following is the basic syntax for creating a Scanner object:

Scanner s = new Scanner(System.in);

Next, we demonstrate a simple data input, and obtain the input string through the next() and nextLine() methods of the Scanner class. Before reading, we generally need to use hasNext and hasNextLine to determine whether there is still input data:

Using the next method:

ScannerDemo.java file code:

import java.util.Scanner; public class ScannerDemo { public static void main(String[] args) { Scanner scan = new Scanner(System.in); //Receiving data from the keyboard //Receiving a string using the next method System.out.println("Receiving using the next method:"); //Determine whether there is still input if (scan.hasNext()) { String str1 = scan.next(); System.out.println("The input data is:" + str1); } scan.close(); } }

The output after executing the above program is:

$ javac ScannerDemo.java
$ java ScannerDemo
next方式接收：
example com
输入的数据为：example

It can be seen that the com string was not output. Next we look at nextLine.

Using the nextLine method:

ScannerDemo.java file code:

import java.util.Scanner; public class ScannerDemo { public static void main(String[] args) { Scanner scan = new Scanner(System.in); //Receiving data from the keyboard //Receiving a string using the nextLine method System.out.println("Receiving using the nextLine method:"); //Determine whether there is still input if (scan.hasNextLine()) { String str2 = scan.nextLine(); System.out.println("The input data is:" + str2); } scan.close(); } }

The output after executing the above program is:

$ javac ScannerDemo.java
$ java ScannerDemo
nextLine方式接收：
example com
输入的数据为：example com

It can be seen that the com string is output.

Difference between next() and nextLine()

next():

  • 1. Input can only be terminated after reading valid characters.
  • 2. For whitespace encountered before valid input characters, the next() method automatically removes it.
  • 3. Only after entering valid characters will the whitespace after them be used as a separator or terminator.
  • next() cannot get a string with spaces.

nextLine()：

  • 1. Enter is the terminator, that is, the nextLine() method returns all characters before the Enter key is input.
  • 2. It can obtain whitespace.

If you want to enter data of type int or float, the Scanner class also supports it, but before input, it is best to use the hasNextXxx() method to validate, then use nextXxx() to read:

ScannerDemo.java file code:

import java.util.Scanner; public class ScannerDemo { public static void main(String[] args) { Scanner scan = new Scanner(System.in); //Receiving data from the keyboard int i = 0; float f = 0.0f; System.out.print("Enter an integer:"); if (scan.hasNextInt()) { //Determine whether the input is an integer i = scan.nextInt(); //Receive integer System.out.println("Integer data:" + i); } else { //Input error message System.out.println("The input is not an integer!"); } System.out.print("Enter a decimal:"); if (scan.hasNextFloat()) { //Determine whether the input is a decimal f = scan.nextFloat(); //Receive decimal System.out.println("Decimal data:" + f); } else { //Input error message System.out.println("The input is not a decimal!"); } scan.close(); } }

The output after executing the above program is:

$ javac ScannerDemo.java
$ java ScannerDemo
输入整数：12
整数数据：12
输入小数：1.2
小数数据：1.2

In the following example, we can enter multiple numbers and calculate their sum and average. Each number is confirmed by pressing Enter. End input by entering a non-number, then output the execution result:

ScannerDemo.java file code:

import java.util.Scanner; class ExampleTest { public static void main(String[] args) { System.out.println("Please enter a number:"); Scanner scan = new Scanner(System.in); double sum = 0; int m = 0; while (scan.hasNextDouble()) { double x = scan.nextDouble(); m = m + 1; sum = sum + x; } System.out.println(m + "The sum of the numbers is" + sum); System.out.println(m + "The average of the numbers is" + (sum / m)); scan.close(); } }

The output after executing the above program is (enter a non-number to end input):

$ javac ScannerDemo.java
$ java ScannerDemo
请输入数字：
12
23
15
21.4
end
4个数的和为71.4
4个数的平均值是17.85

Common methods

Method Description
Constructors
Scanner(File source) Create Scanner from a file
Scanner(InputStream source) Create Scanner from an input stream
Scanner(String source) Create Scanner from a string
Basic input methods
boolean hasNext() Check if there is a next token (separated by spaces)
String next() Read the next token (string)
boolean hasNextLine() Check if there is a next line
String nextLine() Read the content of the next line
Type checking methods
boolean hasNextInt() Check if the next token is an integer
boolean hasNextDouble() Check if the next token is a double
boolean hasNextBoolean() Check if the next token is a boolean
Type reading methods
int nextInt() Read the next integer
double nextDouble() Read the next double
boolean nextBoolean() Read the next boolean
long nextLong() Read the next long integer
float nextFloat() Read the next float
short nextShort() Read the next short integer
byte nextByte() Read the next byte
Delimiter control
Scanner useDelimiter(String pattern) Set the delimiter pattern
Scanner useDelimiter(Pattern pattern) Set the delimiter using a regex
String delimiter() Return the currently used delimiter pattern
Other methods
void close() Close the scanner
Scanner skip(Pattern pattern) Skip input matching the specified pattern
Scanner skip(String pattern) Skip input matching the specified string
String findInLine(Pattern pattern) Find the specified pattern in the current line
String findInLine(String pattern) Find the specified string in the current line
Scanner reset() Reset the scanner
Locale locale() Return the locale currently used by the scanner
Scanner useLocale(Locale locale) Set the scanner's locale

For more content, refer to the API documentation:https://www.example.com/manual/jdk11api/java.base/java/util/Scanner.html。

Other extensions