abstract class Cacheable {
  
  Object _cached;
 
  abstract Object retrieve();
  Object get() { if (_object == null) _cached = retrieve(); return _cached;}
  
}

class CacheableA {

  A retrieve() { return retrieveA();}
  String retrieveA() { return "asdfasdf";}
 }

class CachableB {

  B retrieve() { return retrieveB();}
  Integer retrieveB() { return new Integer(42);}
}

public class Foo {

  Cacheable _a = new CacheableA();
  Cacheable _b = new CacheableB();

  public String a() {

     return a.get();
  }

  public Integer b() {
 
     return b.get();
  }
}