/**
 * The following class lazily generates the sequence of natural numbers.
 *_rest is implemented as one of two states: evaluated or unevaluated.
 * @author Alan Cox
 */
class NaturalNumbers {
    
    private int _first;
    
    private interface IState {
        public NaturalNumbers getRest(NaturalNumbers owner);
    }
    
    private static class EvaluatedState implements IState {
        
        private NaturalNumbers _successor;
        
        EvaluatedState(NaturalNumbers successor) {
            _successor = successor;
        }
        
        public NaturalNumbers getRest(NaturalNumbers owner) {
            return _successor;
        }
    }
    
    private static class UnevaluatedState implements IState {
        
        public final static UnevaluatedState Singleton = new UnevaluatedState();
        private UnevaluatedState() {
        }
        
        public NaturalNumbers getRest(NaturalNumbers owner) {
            IState successor = new EvaluatedState(new NaturalNumbers(owner.getFirst() + 1));
            owner.setSuccessor(successor);
            return successor.getRest(owner);
        }
    }
    
    private IState _successor;
    
    public final static NaturalNumbers Singleton = new  NaturalNumbers(1);
    
    private  NaturalNumbers(int first) {
        _first = first;
        _successor = UnevaluatedState.Singleton;
    }
    
    public final int getFirst() {
        return _first;
    }
    
    public final NaturalNumbers getSuccessor() {
        return _successor.getRest(this);
    }
    
    private final void setSuccessor(IState successor) {
        _successor = successor;
    }
    
    // Simple test code.
    public static void main(String args[]) {
        int i;
        NaturalNumbers n = Singleton;
        
        for (i = 0; i < 7; i++) {
            System.out.print(n.getFirst() + " ");
            n = n.getSuccessor();
        }
        System.out.println(n.getFirst());
        
        n = Singleton;
        
        for (i = 0; i < 11; i++) {
            System.out.print(n.getFirst() + " ");
            n = n.getSuccessor();
        }
        System.out.println(n.getFirst());
    }
}