How to Read Tokens From Scanners Including New Lines

Scanner! User Input and File Processing

Java uses an object called a Scanner to process program input. Programme input can be things users blazon or data read in from a file. Scanner objects first at the get-go of input and move frontward, reading input line by line, or word by give-and-take. Scanners don't move backward.

Consider this file:

              token token token token this is a new line with words not everything in the file is a word sometimes y'all tin can have as many as 99 numbers or even more, similar 400!  only when practise you really know when a file is done?

Scanners process files as "tokenized" input. The Scanner reads the file a small amount at a fourth dimension. It doesn't just read the entire file all at in one case. This is good! Files might be incredibly large. It can exist more efficient to read it a modest amount at a fourth dimension.

Scanners read file input line by line, and give-and-take by give-and-take. Words are almost synonymous with "tokens." A token is any group of characters all seperated by whitespace. In the file example above the first line has iv words, or four tokens.

Scanner uses the terminology "token" to disambiguate between a group of characters that's en English word, or something else like numbers or other data. A token is any grouping of characters seperated by whitespace.

Check for understanding:

How many tokens are on the first line?
How many tokens are on the last line?
How many tokens are on line iv?
Give an instance of a token that's not an English word.

Scanner Methods

The scanner has methods that return input to our program in a variety of means.

scanner.nextLine() - returns the entire next line of input
scanner.side by side() - returns ane word as a String
scanner.nextInt() - returns one word as an integer
scanner.nextDouble() - returns one word every bit an integer
scanner.nextFloat() - returns 1 word every bit an integer
scanner.nextBoolean() - returns one word as an integer

The Scanner too has "peek alee" methods that requite u.s. data about what's coming side by side in the input.

scanner.hasNextLine() - returns one word every bit a String
scanner.hasNext() - returns ane give-and-take as a String
scanner.hasNextInt() - returns one discussion equally an integer
scanner.hasnextDouble() - returns one word as an integer
scanner.hasnextFloat() - returns one word as an integer
scanner.hasnextBoolean() - returns one word as an integer

Here'due south how to create a Scanner that reads input from a file.

              try              {              String              filename              =                              "file.txt"              ;              File              file              =              new              File(filename);              Scanner              scanner              =              new              Scanner(file); }              catch              (IOException              eastward) {   e.printStackTrace();              Organisation              .out.println(                "Problem opening file."              ); }

Reading Tokens like Pac Human being

The Scanner reads a file similar Pac Human being but moving forward. Information technology eats tokens in the file token past token, or line by line. Our program controls the Scanner and tells it when to have it's side by side bite.

Permit's use the .side by side() method on the Scanner to process the file. Nosotros tin put it inside a for loop then the Scanner eats through the whole file token by token.

              while              (truthful) {              Cord              token              =              scanner.next();              System              .out.println(token); }

This plan will crash with a NoSuchElementException. The Scanner crashes when nosotros phone call .next() and information technology has zero left in the file to read.

Use the .hasNext() method to look one step alee earlier telling the Scanner to try to consume the side by side token.

              while              (scanner.hasNext()) {              Cord              token              =              scanner.adjacent();              System              .out.println(token); }

.hasNext() volition return truthful if that'southward anything left in the file. Using it as the condition for the while loop prevents the Scanner from running off the finish of the file and stops the plan from crashing.

Detecting Integers

Use the .hasNextInt() method to detect if the next token the Scanner will eat can be interpreted equally an integer. If it tin then you can call the .nextInt() method and have the Scanner interpret the token as an int and save information technology as a int variable in your plan.

Similar methods exist for other archaic information types (.hasNextBoolean(), .hasNextFloat(), .hasNextDouble).

Here's a elementary programme that iterates through a scanner, but looks for ints, and adds them all together.

              int total = 0; while (scanner.hasNext()) {   if (scanner.hasNextInt()) {     int n = scanner.nextInt();     total += n;   } else {     // motion the scanner forwards past anything that's not an integer.     // don't carp saving any of these tokens to a variable because     // they're not being used.     scanner.next();   } }

Zimmermann Fixecition