Adapter Design Pattern in Java

Another Adapter source code example

  1. Identify the desired interface.
  2. Design a "wrapper" class that can "impedance match" the old to the new.
  3. The adapter/wrapper class "has a" instance of the legacy class.
  4. The adapter/wrapper class "maps" (or delegates) to the legacy object.
  5. The client uses (is coupled to) the new interface.
/* The OLD */
class SquarePeg {
   private double width;
   public SquarePeg( double w )       { width = w; }
   public double getWidth()           { return width; }
   public void   setWidth( double w ) { width = w; }
}

/* The NEW */
class RoundHole {
   private int radius;
   public RoundHole( int r ) {
      radius = r;
      System.out.println( "RoundHole: max SquarePeg is " + r * Math.sqrt(2) );
   }
   public int getRadius() { return radius; }
}

// Design a "wrapper" class that can "impedance match" the old to the new
class SquarePegAdapter {

   // The adapter/wrapper class "has a" instance of the legacy class
   private SquarePeg sp;

   public SquarePegAdapter( double w ) { sp = new SquarePeg( w ); }

   // Identify the desired interface
   public void makeFit( RoundHole rh ) {
      // The adapter/wrapper class delegates to the legacy object
      double amount = sp.getWidth() - rh.getRadius() * Math.sqrt(2);
      System.out.println( "reducing SquarePeg " + sp.getWidth() + " by " + ((amount < 0) ? 0 : amount) + " amount" );
      if (amount > 0) {
         sp.setWidth( sp.getWidth() - amount );
         System.out.println( "   width is now " + sp.getWidth() );
      }
   }
}

class AdapterDemoSquarePeg {
   public static void main( String[] args ) {
      RoundHole        rh = new RoundHole( 5 );
      SquarePegAdapter spa;

      for (int i=6; i < 10; i++) {
         spa = new SquarePegAdapter( (double) i );
         // The client uses (is coupled to) the new interface
         spa.makeFit( rh );
      }
   }
}

Output

RoundHole: max SquarePeg is 7.0710678118
reducing SquarePeg 6.0 by 0.0 amount
reducing SquarePeg 7.0 by 0.0 amount
reducing SquarePeg 8.0 by 0.9289321881345245 amount
   width is now 7.0710678118654755
reducing SquarePeg 9.0 by 1.9289321881345245 amount
   width is now 7.0710678118654755

Code examples