Test.java

import java.io.OutputStream;
import java.io.IOException;

class BufferOutput {
   final private OutputStream o;
   BufferOutput (OutputStream o) { this.o = o; }
   final protected byte[] buf = new byte[512];
   protected int pos = 0;
   public void putchar (char c) throws IOException {
      if (pos == buf.length) flush();
      buf[pos++] = (byte)c;
   }
   public void putstr (String s) throws IOException {
      for (int i = 0; i < s.length(); i++) {
         putchar (s.charAt(i));  // dynamic dispatch on "this"
      }
   }
   public void flush() throws IOException {
      if (pos>0) o.write (buf, 0, pos);
      pos = 0;
   }
}

class LineBufferOutput extends BufferOutput {
   LineBufferOutput (OutputStream o) { super(o); }
   @Override
   public void putchar (char c) throws IOException {
      super.putchar(c);
      if (c == '\n') flush();
   }
}

public class Test {
   public static void main (String[] args) throws IOException {
      LineBufferOutput lbo = new LineBufferOutput(System.out);
      lbo.putstr ("lbo\nlbo");
      System.out.print ("print\n");
      lbo.putstr ("\n");
   }
}