java.util.Scanner

Description

This java tutorial focuses on the usage of the Scanner class of java.util package. We will be showing the basic usage of Scanner class until the most advanced features of this class using examples. As we move along on this document, we will be showing java scanner examples more importantly on the usage of Scanner methods.

The Scanner class has a rich set of API which generally used to break down the input to Scanner constructor into tokens. It can parse the tokens into primitive data types using java regular expressions. The input is broke down into tokens using the delimiter defined on the Scanner class using the radix method or can be define as well as method argument on some of the methods available on this scanner class.

Scanner Class Declaration

public final class Scanner
extends Object
implements Iterator, Closeable

Scanner Class Constructors

public Scanner(Readable source)

Constructs a new Scanner that produces values scanned from the specified source.

Parameters:

source – A character source implementing the Readable interface

Example:

Don’t get confused on the Object type Readable as constructor parameter. Readable is an interface which has been implemented by BufferedReader, CharArrayReader, CharBuffer, FileReader, FilterReader, InputStreamReader, LineNumberReader, PipedReader, PushbackReader, Reader, StringReader. Which means, we can use any of these classes in instantiating the Scanner object.

package com.javatutorialhq.java.examples;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.util.Scanner;

public class ScannerExample1 {

	public static void main(String[] args) {
		
		try {
			FileReader reader = new FileReader(new File("D:temptest.txt"));
			Scanner scan = new Scanner(reader);
			while(scan.hasNextLine()){
				System.out.println(scan.nextLine());
			}
			scan.close();
		} catch (FileNotFoundException e) {
			System.out.println("Error Reading File");
			e.printStackTrace();
		}

	}

}

public Scanner(InputStream source)

Constructs a new Scanner that produces values scanned from the specified input stream. Bytes from the stream are converted into characters using the underlying platform’s default charset.

Parameters:

source – An input stream to be scanned

Example:

The method input on this constructor is InputStream. The InputStream class is one of the top level classes in the java.io package and using it would be a problem. However we can use the subclasses of InputStream as shown below. We have used the FileInputStream for below example as it is a subclass of InputStream, there would not be an issue in using it.

package com.javatutorialhq.java.examples;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class ScannerExample2 {

	public static void main(String[] args) {
		
		try {
			File input = new File("D:temptest.txt");
			FileInputStream fis = new FileInputStream(input);
			Scanner scan = new Scanner(fis);
			while(scan.hasNextLine()){
				System.out.println(scan.nextLine());
			}
			scan.close();
		} catch (FileNotFoundException e) {
			System.out.println("Error Reading File");
			e.printStackTrace();
		}

	}

}

public Scanner(File source) throws FileNotFoundException

Constructs a new Scanner that produces values scanned from the specified file. Bytes from the file are converted into characters using the underlying platform’s default charset.

Parameters:

source – A file to be scanned

Example:

This constructor is very straightforward. It just require a File source. So it means that this constructor sole purpose is instantiate a Scanner object to scan through a File.

package com.javatutorialhq.java.examples;

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class ScannerExample3 {

	public static void main(String[] args) {
		
		try {
			File input = new File("D:temptest.txt");
			Scanner scannner = new Scanner(input);
			while(scannner.hasNextLine()){
				System.out.println(scannner.nextLine());
			}
			scannner.close();
		} catch (FileNotFoundException e) {
			System.out.println("Error Reading File");
			e.printStackTrace();
		}

	}

}

public Scanner(Path source) throws IOException

Constructs a new Scanner that produces values scanned from the specified file. Bytes from the file are converted into characters using the underlying platform’s default charset.

Parameters:

source – the path to the file to be scanned

Example:

This constructor is not a straightforward approach because the constructor parameter requires a Path source which is rarely used. Below is an example code on how to use this constructor.

package com.javatutorialhq.java.examples;

import java.io.IOException;
import java.nio.file.FileSystems;
import java.nio.file.Path;
import java.util.Scanner;

public class ScannerExample4 {

	public static void main(String[] args) {
		
		try {
			Path path = FileSystems.getDefault().getPath("access.log");
			System.out.println(path.toUri());
			Scanner scannner = new Scanner(path);
			while(scannner.hasNextLine()){
				System.out.println(scannner.nextLine());
			}
			scannner.close();
		} catch (IOException e) {
			System.out.println("Error Reading the path");
			e.printStackTrace();
		}

	}

}

public Scanner(String source)

Constructs a new Scanner that produces values scanned from the specified string.

Parameters:

source – A string to scan

Example:

This constructor might be the easiest to use since it only requires a string parameter however this is strictly used to scan the string input. Let me show you what I mean below.

package com.javatutorialhq.java.examples;

import java.util.Scanner;

public class ScannerExample5 {

	public static void main(String[] args) {

		String source = "This is a test";
		Scanner scannner = new Scanner(source);
		while (scannner.hasNext()) {
			System.out.println(scannner.next());
		}
		scannner.close();

	}

}

Scanner used to Read File

Reading a file is very easy using the Scanner class. We just have to declare it using the Scanner constructor like for example:

Scanner scan = new Scanner(new File("C:/test.txt"));

If we want to have a locale specific we just have to declare it using one of the Scanner method

scan.useLocale(Locale.ENGLISH);

The trick in iterating through the Scanner token is to use those methods which starts with has like hasNext, hasNextInt, etc. For the purpose of discussion, let’s settle first in reading the file line by line.

while(scan.hasNextLine()){
	System.out.println(scan.nextLine());
}
scan.close();

From the java code snippet above we have used the scan.hasNextLine() flag as a means to check if there are token which in this example available on the scanner input. The nextLine() method returns the current token and advances to the next token. The combination of the hasNextLine() and the nextLine() combination is widely used to get all the tokens on the scanner input. Afterwards we call the close() method to close the Scanner object and thereby avoid memory leak.

Read string from Console Input using Scanner Class Example

The Scanner class accepts also an InputStream on one of its constructor. Thus in reading console input can be done using:

Scanner scan = new Scanner(System.in);

After putting the console input to our scanner object, we now have the access to a readable and convertible stream which means we can manipulate or either break the input into tokens. Using the rich set of Scanner API we can read the tokens depending on the delimiter which we want to use into specific data type.

Scanner Tips

I have recently discovered one problem in using Scanner. Let’s say for example, we are asking the user to input employee id and employee name from the user console. After which we will read the input from the console using Scanner. The employee id would be read using nextInt() and the employee name would be read as nextLine(). It’s pretty straightforward isn’t it? Well this won’t work. Try below example:

package com.javatutorialhq.java.examples;

import java.util.Scanner;

/*
 * Java Example to get employee id
 * and employee name from the console
 * and get it using Scanner
 */
		

public class Demo {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		System.out.print("Enter employee id:");
		int empId = scan.nextInt();
		System.out.print("Enter employee name:");
		String empName = scan.nextLine();
		scan.close();		
		

	}

}

Above example would not work because the nextInt method does not read the last newline character of your input, and thus that newline is consumed in the next call to nextLine. To resolve this just use next instead of nextline but if you insist of using nextLine add another scan.nextLine() after nextInt. Take a look below snippet

		Scanner scan = new Scanner(System.in);
		System.out.print("Enter employee id:");
		int empId = scan.nextInt();
		scan.nextLine();
		System.out.print("Enter employee name:");
		String empName = scan.nextLine();
		scan.close();

 

Java Scanner Method Examples

The following are the list of Scanner methods which we can use to work out in complicated parsing of input.

return Method Description
void close() Closes the scanner object.
Pattern delimiter() Returns the Pattern the Scanner object is currently using to match delimiters.
String findInLine(Pattern pattern) This method returns a String object that satisfies the Pattern object specified as method argument.
String findInLine(String pattern) Attempts to find the next occurrence of a pattern constructed from the specified string, ignoring delimiters.
String findWithinHorizon(Pattern pattern, int horizon) Attempts to find the next occurrence of the specified pattern.
String findWithinHorizon(String pattern, int horizon) This method simply attempts to find the next occurrence of a pattern input ignoring delimiter
boolean hasNext() Returns true if this scanner has another token in its input.
boolean hasNext(Pattern pattern) Returns true if the next complete token matches the specified pattern.
boolean hasNext(String pattern) Returns true if the next token matches the pattern constructed from the specified string.
boolean hasNextBigDecimal() Returns true if the next token in this scanner's input can be interpreted as a BigDecimal using the nextBigDecimal() method.
boolean hasNextBigInteger() Returns true if the next token in this scanner's input can be interpreted as a BigInteger in the default radix using the nextBigInteger() method.
boolean hasNextBigInteger(int radix) Returns true if the next token in this scanner's input can be interpreted as a BigInteger in the specified radix using the nextBigInteger() method.
boolean hasNextBoolean() This method checks if the Scanner object has boolean data type on its buffer.
boolean hasNextByte() This method returns true if the next byte on the scanner buffer can be translated to byte data type otherwise false.
boolean hasNextByte(int radix) Returns true if the next token in this scanner's input can be interpreted as a byte value in the specified radix using the nextByte() method.
boolean hasNextDouble() Returns true if the next token in this scanner's input can be interpreted as a double value using the nextDouble() method.
boolean hasNextFloat() Returns true if the next token in this scanner's input can be interpreted as a float value using the nextFloat() method.
boolean hasNextInt() Returns true if the next token in this scanner's input can be interpreted as an int value in the default radix using the nextInt() method.
boolean hasNextInt(int radix) This method returns boolean, true if  the token can be interpreted as int data type with respect to the radix used by the scanner object otherwise false.
boolean hasNextLine() This method returns a boolean data type which corresponds to the existence of new line on the String tokens which the Scanner object holds.
boolean hasNextLong() Returns true if the next token in this scanner's input can be interpreted as a long value in the default radix using the nextLong() method.
boolean hasNextLong(int radix) Returns true if the next token in this scanner's input can be interpreted as a long value in the specified radix using the nextLong() method.
boolean hasNextShort() Returns true if the next token in this scanner's input can be interpreted as a short value in the default radix using the nextShort() method.
boolean hasNextShort(int radix) This method returns boolean, true if  the token can be interpreted as short data type with respect to the radix used by the scanner object otherwise false.
IOException ioException() Returns the IOException last thrown by this Scanner's underlying Readable.
Locale locale() This method returns a Locale which the Scanner class is using.
MatchResult match() This method returns a MatchResult object which corresponds to the result of the last operation by the scanner object.
String next() Finds and returns the next complete token from this scanner.
String next(Pattern pattern) Returns the next token if it matches the specified pattern.
String next(String pattern) Returns the next token if it matches the pattern constructed from the specified string.
BigDecimal nextBigDecimal() Scans the next token of the input as a BigDecimal.
BigInteger nextBigInteger() Scans the next token of the input as a BigInteger.
BigInteger nextBigInteger(int radix) Scans the next token of the input as a BigInteger.
boolean nextBoolean() Scans the next token of the input into a boolean value and returns that value.
byte nextByte() Scans the next token of the input as a byte.
byte nextByte(int radix) Scans the next token of the input as a byte.
double nextDouble() Scans the next token of the input as a double.
float nextFloat() Scans the next token of the input as a float.
int nextInt() Scans the next token of the input as an int.
int nextInt(int radix) Scans the next token of the input as an int.
String nextLine() Advances this scanner past the current line and returns the input that was skipped.
long nextLong() Scans the next token of the input as a long.
long nextLong(int radix) Scans the next token of the input as a long.
short nextShort() Scans the next token of the input as a short.
short nextShort(int radix) Scans the next token of the input as a short.
int radix() Returns this scanner's default radix.
void remove() The remove operation is not supported by this implementation of Iterator.
Scanner reset() Resets this scanner.
Scanner skip(Pattern pattern) Skips input that matches the specified pattern, ignoring delimiters.
Scanner skip(String pattern) Skips input that matches a pattern constructed from the specified string.
String toString() Returns the string representation of this Scanner.
Scanner useDelimiter(Pattern pattern) Sets this scanner's delimiting pattern to the specified pattern.
Scanner useDelimiter(String pattern) Sets this scanner's delimiting pattern to a pattern constructed from the specified String.
Scanner useLocale(Locale locale) Sets this scanner's locale to the specified locale.
Scanner useRadix(int radix) Sets this scanner's default radix to the specified radix.

Java Scanner FAQ’s

The following Questions are gathered around the web and from the mails of our beloved which I have compiled and answer them one by one. This is to further discuss in depth what is scanner in java.