Re: pass by reference

"angelochen960@gmail.com" <angelochen960@gmail.com> writes:


  According to the JLS, methods are ?declared?.

      ?You can't return more than one value from a method. If
      you want to, you have to return a little array (unless one
      value is an int and the other is a Person!) or an object
      of some special little class made just for this purpose.
      When I was helping Bill Joy and Guy L. Steele Jr. by
      reviewing drafts of the original Java Language
      Specification, I was originally upset that there was no
      way to do this. So I set out to find a small example
      program that obviously demanded such a feature, to
      convince them that multiple value returns must be added. I
      was unable to come up with one, and I could see that
      Java's philosphy was to leave out things that are rarely
      used and not crucial, so finally didn't say anything.?

   Dan Weinreb's Weblog

http://dlweinreb.wordpress.com/category/java/

  I have written several programs myself to show how multiple
  returns might be emulated in Java. Each approach has its
  advantages and disadvantages. I might add that, whenever I
  want to achieve something with Java, the lack of explicit
  multiple return values is not a problem for me.

  So, here are my notes:

  I assume a simple multiple-return task such as, in pseudocode:

operation "sumdiff"
in x, y;
out sum, difference;
{ sum = x + y; difference = x - y; }

  Solution with public fields:

class Sumdiff
{ public Sumdiff( final int x, final int y )
  { this.sum = x + y; this.difference = x - y; }
  public final int sum; public final int difference; }

public class Main
{ public static void main( final java.lang.String[] args )
  { final Sumdiff result = new Sumdiff( 4, 2 );
    java.lang.System.out.println
    ( result.sum + ", " + result.difference ); }}

6, 2

  If you do not like public fields, you might use getters
  as well.

  A ?processor object? can be created once and be used
  several times:

public class Main
{ public static void main( final java.lang.String[] args )
  { final Processor processor = new Processor();
    processor.set( 4, 2 );
    processor.calculateSumDiff();
    java.lang.System.out.println( processor.getSum() );
    java.lang.System.out.println( processor.getDifference() );
    processor.set( 8, 4 );
    processor.calculateSumDiff();
    java.lang.System.out.println( processor.getSum() );
    java.lang.System.out.println( processor.getDifference() ); }}

class Processor
{ public void set( final int x, final int y )
  { this.x = x; this.y = y; }
  public void calculateSumDiff()
  { this.sum = x + y; this.difference = x - y; }
  public java.lang.Integer getSum(){ return sum; }
  public java.lang.Integer getDifference(){ return difference; }
  int x; int y; int sum; int difference; }

  To avoid allocation overhead of a result object,
  the client might provide and reuse such an object:

class Result { public int x; public int y; }

class Server
{ void run( final Result result, final int x, final int y )
  { result.x = x + y; result.y = x - y; }}

public final class Main
{ private static Result result = new Result(); /* single allocation */
  public static void main( final java.lang.String argv[] )
  { Server server = new Server();
    server.run( result, 1, 2 );
    java.lang.System.out.println( result.x );
    java.lang.System.out.println( result.y );
    server.run( result, 3, 4 );
    java.lang.System.out.println( result.x );
    java.lang.System.out.println( result.y ); }}

  One can also emulate multiple returns via multiple
  arguments, but only when adopting a ?continuation passing
  style?. In the next example, the server ?returns? a pair
  of random numbers to the client, by calling back a method
  provided by the client.

interface Client { void continuation( int x, int y ); }

class Server
{ static java.util.Random rand = new java.util.Random();
  static void getPair( final Client client )
  { client.continuation( rand.nextInt( 11 ), rand.nextInt( 21 )); }}

class Example implements Client
{ public void continuation( final int x, final int y )
  { java.lang.System.out.println( x + ", " + y ); }
  public void main()
  { Server.getPair( this ); }}

public class Main
{ public static void main( final java.lang.String[] args )
  { new Example().main(); }}

  But, as said, I rarely ever (actually: never) have needed any
  of these multiple return value emulations in my own projects.

  Possibly this is because I already have accounted for the
  properties and limitations of Java when I was designing
  my classes. So I have designed them from the start in such
  a way that multiple return values are not needed.