Custom delimiters, Reading system input and Reading file input using Scanner

Using custom delimiters

You can use custom delimiters (regular expressions) with Scanner, with .useDelimiter(","), to determine how the input is read. This works similarly to String.split(...). For example, you can use Scanner to read from a list of comma separated values in a String: 

Scanner scanner = null;
try{
	scanner = new Scanner("i,like,unicorns").useDelimiter(",");;
	while(scanner.hasNext()){
		System.out.println(scanner.next());
	}
}catch(Exception e){
	e.printStackTrace();
}finally{
	if (scanner != null)
		scanner.close();
}

This will allow you to read every element in the input individually. Note that you should not use this to parse CSV data, instead, use a proper CSV parser library, see CSV parser for Java for other possibilities.

Reading system input using Scanner

Scanner scanner = new Scanner(System.in); //Scanner obj to read System input
String inputTaken = new String();
while (true) {
	String input = scanner.nextLine(); // reading one line of input
	if (input.matches("\\s+")) // if it matches spaces/tabs, stop reading
		break;
	inputTaken += input + " ";
}
System.out.println(inputTaken);

The scanner object is initialized to read input from keyboard. So for the below input from keyboar, it'll produce the output as Reading from keyboard 

Reading
from
keyboard
//space

Reading file input using Scanner

Scanner scanner = null;
try {
	scanner = new Scanner(new File("Names.txt"));
	while (scanner.hasNext()) {
		System.out.println(scanner.nextLine());
	}
} catch (Exception e) {
	System.err.println("Exception occurred!");
} finally {
	if (scanner != null)
		scanner.close();
}

Here a Scanner object is created by passing a File object containing the name of a text file as input. This text file will be opened by the File object and read in by the scanner object in the following lines. scanner.hasNext() will check to see if there is a next line of data in the text file. Combining that with a while loop will allow you to iterate through every line of data in the Names.txt file. To retrieve the data itself, we can use methods such as nextLine(),nextInt(),nextBoolean(), etc. In the example above, scanner.nextLine()is used. nextLine() refers to the following line in a text file, and combining it with a scanner object allows you to print the contents of the line. To close a scanner object, you would use .close().

Using try with resources (from Java 7 onwards), the above mentioned code can be written elegantly as below.

try (Scanner scanner = new Scanner(new File("Names.txt"))) {
	while (scanner.hasNext()) {
		System.out.println(scanner.nextLine());
	}
} catch (Exception e) {
	System.err.println("Exception occurred!");
}

Read the entire input as a String using Scanner

You can use Scanner to read all of the text in the input as a String, by using \Z (entire input) as the delimiter. For example, this can be used to read all text in a text file in one line: 

String content = new Scanner(new File("filename")).useDelimiter("\\Z").next();
System.out.println(content);

Remember that you'll have to close the Scanner, as well as catch the IoException this may throw, as described in the example Reading file input using Scanner.

Carefully Closing a Scanner

it can happen that you use a scanner with the System.in as parameter for the constructor, then you need to be aware that closing the scanner will close the InputStream too giving as next that every try to read the input on that (Or any other scanner object) will throw an java.util.NoSuchElementException or an java.lang.IllegalStateException

example:

Scanner sc1 = new Scanner(System.in);
Scanner sc2 = new Scanner(System.in);
int x1 = sc1.nextInt();
sc1.close();
// java.util.NoSuchElementException
int x2 = sc2.nextInt();
// java.lang.IllegalStateException
x2 = sc1.nextInt()

Read an int from the command line

import java.util.Scanner;
Scanner s = new Scanner(System.in);
int number = s.nextInt();

If you want to read an int from the command line, just use this snippet. First of all, you have to create a Scanner object, that listens to System.in, which is by default the Command Line, when you start the program from the command line. After that, with the help of the Scanner object, you read the first int that the user passes into the command line and store it in the variable number. Now you can do whatever you want with that stored int.

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