Re: Polymorphism in Java SE?

From:
Patricia Shanahan <pats@acm.org>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 22 Dec 2007 15:10:43 -0800
Message-ID:
<fkk5hj$rci$1@ihnp4.ucsd.edu>
Stefan Ram wrote:
....

  A near miss is the field ?System.in?, which indeed contains an
  object of a proper /subtype/ of the field's type. So it is
  somewhat polymorphic. You do not know the actual type until
  run-time. It is only boring insofar as it is always the same
  subtype.

....

Huh? Why do you say it is always the same subtype?

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStream;

public class SystemDotInClasses {
   public static void main(String[] args) throws IOException {
     System.out.println("Initial class: "
         + System.in.getClass().getName());
     String someData = "xyzzy";
     InputStream someStream = new ByteArrayInputStream(
         someData.getBytes());
     System.setIn(someStream);
     System.out.println("Second class: "
         + System.in.getClass().getName());
     int firstByte = System.in.read();
     if (firstByte == -1) {
       System.out.println("Empty input");
     } else {
       System.out.println("First byte of input: "
           + (char) firstByte);
     }
     ByteArrayOutputStream outBytes = new ByteArrayOutputStream();
     DataOutputStream out = new DataOutputStream(outBytes);
     out.writeDouble(Math.PI);
     someStream = new DataInputStream(
         new ByteArrayInputStream(outBytes.toByteArray()));
     System.setIn(someStream);
     System.out.println("Third class: "
         + System.in.getClass().getName());
     double d = ((DataInputStream) System.in).readDouble();
     System.out.println("Read from data stream " + d);
   }
}

Patricia

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