Reading in from System.in - Java

Question

I am not sure how you are supposed to read in from system input from a Java file.

I want to be able to call java myProg < file

Where file is what I want to be read in as a string and given to myProg in the main method.

Any suggestions?

Answer 1

You can use System.in to read from the standard input. It works just like entering it from a keyboard. The OS handles going from file to standard input.

import java.util.Scanner;
class MyProg {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("Printing the file passed in:");
        while(sc.hasNextLine()) System.out.println(sc.nextLine());
    }
}

Answer 2

Well, you may read System.in itself as it is a valid InputStream. Or also you can wrap it in a BufferedReader:

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

Answer 3

In Java, console input is accomplished by reading from System.in. To obtain a character based stream that is attached to the console, wrap System.in in a BufferedReader object. BufferedReader supports a buffered input stream. Its most commonly used constructor is shown here:

BufferedReader(Reader inputReader)

Here, inputReader is the stream that is linked to the instance of BufferedReader that is being created. Reader is an abstract class. One of its concrete subclasses is InputStreamReader, which converts bytes to characters.

To obtain an InputStreamReader object that is linked to System.in, use the following constructor:

InputStreamReader(InputStream inputStream)

Because System.in refers to an object of type InputStream, it can be used for inputStream. Putting it all together, the following line of code creates a BufferedReader that is connected to the keyboard:

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

After this statement executes, br is a character-based stream that is linked to the console through System.in.

This is taken from the book Java- The Complete Reference by Herbert Schildt

Answer 4

Use System.in, it is an InputStream which just serves this purpose

Answer 5

You would read from System.in just like you would for keyboard input using, for example, InputStreamReader or Scanner.

Answer 6

You can call java myProg arg1 arg2 ... :

public static void main (String args[]) {
    BufferedReader in = new BufferedReader(new FileReader(args[0]));
}

Answer 7

You probably looking for something like this.

FileInputStream in = new FileInputStream("inputFile.txt");
BufferedReader br = new BufferedReader(new InputStreamReader(in));

Answer 8

class myFileReaderThatStarts with arguments
{

 class MissingArgumentException extends Exception{      
      MissingArgumentException(String s)
  {
     super(s);
  }

   }    
public static void main(String[] args) throws MissingArgumentException
{
//You can test args array for value 
if(args.length>0)
{
    // do something with args[0]
}
else
{
// default in a path 
// or 
   throw new MissingArgumentException("You need to start this program with a path");
}
}