Scanner class

1
Scanner class

• Evan Korth
• NYU

2
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. So you must
  import java.util.Scanner

3
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
4
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
5
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

6
Systems 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.

8
Next Methods

• String 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. 

9
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

10
hasNext methods

• boolean 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
  "truefalse". 
• 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. 

11
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 ")

12
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)