何时以及为何使用Scanner？

Question

I am new to Java and was reading different ways to handle input and output. From what I've read, the various strategies boil down to two things ie

1. Reading / Writing by byte ( FileInputStream , FileOutputStream ).
2. Reading / Writing by character ( FileReader , FileWriter ).

But where does Scanner fit into the picture? An example use case would be good.

Answer 1

Use a Scanner if you want to read tokens

Use a FileReader when reading streams of characters

Use a FileInputReader when reading binary files.

Scanner:

A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace. The resulting tokens may then be converted into values of different types using the various next methods. Scanner - JavaDoc

And for Filereader and FileInputReader:

FileReader is meant for reading streams of characters. For reading streams of raw bytes, consider using a FileInputStream. FileReader -javadoc

Example for a scanner:

String input = "1 fish 2 fish red fish blue fish";
   Scanner s = new Scanner(input).useDelimiter("\\s*fish\\s*");
   System.out.println(s.nextInt());
   System.out.println(s.nextInt());
   System.out.println(s.next());
   System.out.println(s.next());
 s.close(); 

Answer 2

Scanner is a higher level class used to read input by tokens.

What is a token? For the class Scanner a token can be a primitive value and some special object:

• long (with nextLong)
• int (with nextInt)
• short (with nextShort)
• byte (with nextByte)
• boolean (with nextBoolean)
• BigDecimal (with nextBigDecimal)
• float (with nextFloat)
• double (with nextDouble)

In addition it is possible to read a whole line with nextLine .

Answer 3

The Scanner class is a class in java.util which allows the user to read values of various types. There are two constructors that are particularly useful: one takes an InputStream object as a parameter and the other takes a FileReader object as a parameter.

Scanner in = new Scanner(System.in);  // System.in is an InputStream
Scanner inFile = new Scanner(new FileReader("myFile"));// If the file ≥myFile≤ is not found, a FileNotFoundException is thrown. 

Various Methods:

• int nextInt() : Returns the next token as an int. If the next token is not an integer, InputMismatchException is thrown.
• long nextLong() : Returns the next token as a long. If the next token is not an integer, InputMismatchException is thrown.
• float nextFloat() : Returns the next token as a float. If the next token is not a float or is out of range, InputMismatchException is thrown.
• double nextDouble() : Returns the next token as a long. If the next token is not a float or is out of range, InputMismatchException is thrown.
• String next(): Finds and returns the next complete token from this scanner and returns it as a string; a token is usually ended by whitespace such as a blank or line break. If not token exists, NoSuchElementException is thrown.
• String nextLine() : Returns the rest of the current line, excluding any line separator at the end.
• void close() : Closes the scanner.

The Scanner looks for tokens in the input. A token is a series of characters that ends with what Java calls whitespace. Thus, if we read a line that has a series of numbers separated by blanks, the scanner will take each number as a separate token.