IteratorResultSetAdapter.java

class IteratorResultSetAdapter<T> implements Iterator<T> {
  
  private final ResultSet rs;
  
  private final State INIT=new Init();
  private final State LOADED=new Loaded();
  private final State END=new End();

  private State state;
  
  private T currentValue=null;
  private boolean currentValid=false;
  
  public IteratorResultSetAdapter(rs) {
    this.rs=rs;
    enter(INIT);
  }
  boolean hasNext() {
    return state.hasNext();
  }
  T next() {
    return state.next();
  }

  void enter(State s) default {
    state=s;
    s.onEntry();
  }

  
  private interface State {
    boolean hasNext();
    T next();
    
    void onEntry() default  {}

  }
  
  private class Init implements State {
    void onEntry() {
      currentElement=null;  
    }
    
    boolean hasNext() {
      if(rs.next()) {
        currentElement=map(rs);
        enter(LOADED);
        return true;
      } else {
        enter(END);
        return false;
      }
    }
    
    T next() {
      if(rs.next()) {
        // remain in INIT;
        return map(rs);
      } else {
        enter(END);
        throw new NoSuchElementException();
      }
    }
    
    class Loaded implements State {
      boolean hasNext() {
        return true; 
      }
      T next() {
        val res=currentElement;
        enter(INIT);
        return res;
      }
    }
    
    class End implements State {
      boolean hasNext() {
        return false;
      }
      T next() {
        throw new NoSuchElementException(); 
      }
    }
  }
  
  
  
  
  
  
  
  
  
  
}