Re: pass by reference

Donkey Hot <spam@plc.is-a-geek.com> wrote:
 


This won't work, though. While the String object s' is passed by reference,
the _reference_ s is passed by value. Any changes made to the reference in
the subprogram -- such as changing the object it points to -- will not have
any effect on the main program.

public class Demo {
  private static final STRING_1 = "String_1";
  private static final STRING_2 = "String_2";

  public void outer( ) {
    String workString = STRING_1;
    change( workString );
    // workString is still pointing to STRING_1
  }

  public void change( String stringToChange ) {
    stringToChange = STRING_2;
  }
}

If the argument is a mutable type (unlike String), you can get the required
effect by modifying (rather than reassigning) the object:

public class Demo2 {
  private static final STRING_1 = "String_1";
  private static final STRING_2 = "String_2";

  public void outer( ) {
    StringBuffer workString = new StringBuffer( STRING_1 );
    change1( workString );
    // workString now equals "String_2"
    change2( workString );
    // workString _still_ equals "String_2"
  }
 
  public void change1( StringBuffer stringToChange ) {
    stringToChange.delete( 0, stringToChange.length( ) );
    stringToChange.append( STRING_2 );
  }

  public void change2( StringBuffer stringToChange ) {
    // This won't work, as the change to the reference is
    // lost when the method returns
    stringToChange = new StringBuffer( STRING_1 );
  }
} // end class

If you have an immutable type or need to create a whole new object for some
other reason, you'll need to use some kind of wrapper. Either a
data-structure or an array:

  public void change( String[] stringWrapper ) {
    stringWrapper[0] = STRING_2;
  }

  private static final class ChangeData {
    public String toBeChanged;
  }

  public void change( ChangeData data ) {
    data.toBeChanged = STRING_2;
  }

--
Leif Roar Moldskred
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