Reading from standard input
Following the standard I/O model, Java has System.in, System.out, and System.err. Throughout this book, you’ve seen how to write to standard output using System.out, which is already prewrapped as a PrintStream object. System.err is likewise a PrintStream, but System.in is a raw InputStream with no wrapping. This means that although you can use System.out and System.err right away, System.in must be wrapped before you can read from it.
Typically, you’ll want to read input a line at a time using readLine( ), so you’ll want to wrap System.in in a BufferedReader. To do this, you must convert System.in to a Reader using InputStreamReader. Here’s an example that simply echoes each line that you type in:
//: c12:Echo.java
// How to read from standard input.
// {RunByHand}
import java.io.*;
public class Echo {
public static void main(String[] args)
throws IOException {
BufferedReader in = new BufferedReader(
new InputStreamReader(System.in));
String s;
while((s = in.readLine()) != null && s.length() != 0)
System.out.println(s);
// An empty line or Ctrl-Z terminates the program
}
} ///:~
The reason for the exception specification is that readLine( ) can throw an IOException. Note that System.in should usually be buffered, as with most streams.
