Evan Korth

Scanner class

Evan Korth

NYU

Road Map

• Scanner– Constructors– Terminolgy

• Token• Delimiter

– Static fields– next methods– hasNext methods

• Reading: – Liang 6: chapter: 2.13– Liang 7: chapter: 2.11

Evan Korth

Evan Korth

Java 1.5 (5.0)’s Scanner class

• Prior to Java 1.5 getting input from the console involved multiple steps.

• Java 1.5 introduced the Scanner class which simplifies console input. It can also be used to read from files and Strings (among other sources).

• It also can be used for powerful pattern matching – we will not cover pattern matching in this class.

• Scanner is in the Java.util package. You must import the class by including this line:

import java.util.Scanner;

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Creating Scanner objects

• We can create a Scanner object by invoking several different constructors.– Scanner(File source)

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

– Scanner(InputStream source)           Constructs a new Scanner that produces values scanned from the specified input stream.

– Scanner(Readable source)           Constructs a new Scanner that produces values scanned from the specified source.

– Scanner(String source)           Constructs a new Scanner that produces values scanned from the specified string.

I have included 4 of the 8 constructors provided by the API – Source java.sun.com

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Terminology

• Token: In computing, a token is a categorized block of text, usually consisting of indivisible characters known as lexemes. A token can look like anything: English, gibberish symbols, anything; it just needs to be a useful part of the structured text.

• Delimiter: A delimiter is a character used to separate items of data stored on a computer. It is used to tell computers to finish processing one piece of data and move on to the next one. Most delimiters are characters that will not be used in the data, such as spaces or commas. It must also be consistent throughout the data.

Source: wikipedia

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Scanner class

• The Scanner class basically parses input from the source into tokens by using delimiters to identify the token boundaries.

• The default delimiter is whitespace:– [ \t\n\x0B\f\r]

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System’s static fields

• static PrintStream err           The "standard" error output stream.

• static InputStream in           The "standard" input stream.

• static PrintStream out           The "standard" output stream.

• Remember: You have been using System.out since your first “Hello World” program.

• Now we see System.in is an InputStream

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Scanner

• Scanner will read a line of input from its source (our examples will be from System.in but we have already seen other sources are possible)

• Example:

Scanner sc = new Scanner (System.in);

int i = sc.nextInt();

System.out.println("You entered " + i);

• This example reads a single int from System.in and outputs it to System.out. It does not check that the user actually entered an int.

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Next MethodsString next()

Finds and returns the next complete token from this scanner. 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. 

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. 

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. 

short nextShort() Scans the next token of the input as a short. 

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InputMismatchExceptions

• InputMismatchException: This exception can be thrown if you try to get the next token using a next method that does not match the type of the token

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hasNext methodsboolean hasNext()

Returns true if this scanner has another token in its input. boolean hasNextBoolean()

Returns true if the next token in this scanner's input can be interpreted as a boolean value using a case insensitive pattern created from the string "true|false". 

boolean hasNextByte() Returns true if the next token in this scanner's input can be interpreted as a byte value in the default 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. 

booleanhasNextLine() Returns true if there is another line in the input of this scanner. 

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 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. 

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Another example

Scanner sc = new Scanner (System.in);System.out.print ("Enter first int: ");while (sc.hasNextInt()){int i = sc.nextInt();System.out.println("You entered " + i);System.out.print ("Enter another int: ");

}

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Scanner

• The Scanner class has more powerful abilities. For example, you can:– Change the delimiters– Use regular expressions– Read input in different radixes (radices or

bases)