Input in Java

Java has the following classes commonly used for input:

• Scanner
• FileReader
• BufferedReader

Input via Scanner §

Initialising §

import java.util.Scanner;
 
public class Test
{
	public static void main(String args[])
	{
		Scanner scan = new Scanner(System.in);
	}
 
}

We need to import the Scanner package before using scanner. Create a Scanner Object using the Scanner Constructor

Scanner() constructor takes either a FileStream or File as it’s arguments. Here, System.in is the default input stream from the terminal/console.

Methods §

• Scanner.nextLine(): Reads and returns an entire line, stopping only when it reaches a line terminator such as \n ,\t etc.
• Scanner.next[DATATYPE](): Scanner has many methods like nextInt(), nextDouble(), nextBoolean(), which all read until they find the specified data type, and then return said data.
• Scanner.hasNext(): Boolean that checks if any input that can be read is left.
• Scanner.hasNext[DATATYPE](): Boolean that only returns true if there exists a [DATATYPE] left to read. For example, Scanner.hasNextInteger() will only return true if an integer is given to it.

Input via FileReader §

Java implements all it’s classes related to file input and output in the java.io package.

The FileReader class to allow reading of simple .txt files. It is a very basic class, and only supports reading characters, which can be stored into an array. The FileReader object can be constructed with either a FILEPATH String or a File Object.

import java.io.FileReader;
 
//FileReader using String filepath
FileReader reader = new FileReader("testfile.txt");
 
File file = new File(); //Assume this File object is created properly
//FileReader using File object
FileReader reader2 = new FileReader(file);
 
//Array MUST be of type char
char arr[300];
//Reads data from testfile.txt and stores into arr array
reader.read(arr);
 
//Prevents further reading.
reader.close();

Best practice

The most common practice is to wrap another class such as Scanner (though this is slow for large files), or BufferedReader around FileReader.

Input via BufferedReader §

BufferedReader is a more powerful version of FileReader. It allows reading of Strings instead of just chars.

Generally, this class wraps around FileReader, as such:

BufferedReader br = new BufferedReader(new FileReader("file.txt"));

BufferedReader comes with many useful methods:

import java.io.BufferedReader;
 
BufferedReader br = new BufferedReader(new FileReader("file.txt"));
 
//Supports char reading, just like FileReader
char c = br.read();
 
//Stores the next line into the text array
String text = br.readLine(); 
 
//Can skip x characters as well!
br.skip(5);