import java.io.PrintStream;

public class Test {
  public static void main(String[] args) throws Exception {

    System.setOut(new PrintStream(System.out) {
       public void println(String s) {
         super.println(s.toUpperCase());
       }
    });

    System.out.println("hello world"); // output :  HELLO WORLD
 }
}

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

  public class TeeOutputStream extends OutputStream {
    private OutputStream os1;
    private OutputStream os2;

    public TeeOutputStream(OutputStream os1, OutputStream os2) {
      this.os1 = os1;
      this.os2 = os2;
    }

    public void write(int i) throws IOException {
      os1.write(i);
      os2.write(i);
    }

    public void flush() throws IOException {
      os1.flush();
      os2.flush();
    }
    public void close() throws IOException {
      os1.close();
      os2.close();
    }

    public static void main(String[] args) throws IOException {
      System.out.println("Starting...");
      // keep the original
      PrintStream originalOutStream = System.out;
      System.setOut(
           new PrintStream(
               new TeeOutputStream(
                   System.out,
                   new PrintStream("C:/temp/howto.out"))));
      // restore the original

      System.out.println("*");
      for (int i = 1; i<= 10; i++) {
        System.out.println(i);
      }
      System.out.println("*");

      System.setOut(originalOutStream);
      System.out.println("Done. Check the log");
    }
  }