结构型模式：Adapter（适配器）

/* 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 );
}
}
}

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