Question

执行程序时，它在第13行上始终显示“ null”，我想知道我的算法出了什么问题，因为它始终显示null。

private class SpadeIterator implements Iterator<Card>{
    private int nextCardSpade;
    private List<Card> cards;
    private int count=0;
    private SpadeIterator(List cards) {
        this.cards=cards;
        this.nextCardSpade = cards.size()-1;
    }

    @Override
    public boolean hasNext() {
        count++;
        if(nextCardSpade<0)
            return false;
        //nextCardSpade--;
        return true;
    }

   @Override
    public Card next() {

        int i=0;
        this.count=i;
        Card temp = cards.get(nextCardSpade);

        while(hasNext()){    //find SPADES
            temp=cards.get(nextCardSpade--);
            i++;

            if(temp.suit.value == Suit.SPADES.value)
                return temp;
        }
        //DONT MOVE
        return null;
        //nextCardSpade--;      //DONT DELETE

    }
}

Current Results

结果旨在显示13个黑桃，而最后不返回null。

Answer 1

您的next()方法不应包含任何会返回无效值的情况，例如null。如果没有下一个要返回的元素，则是hasNext()方法的 job 返回false，以防止您调用next()。

因此您的代码应更像

class SpadeIterator implements Iterator<Card>{
    private int spadesCounter = 0;
    private Iterator<Card> cardsIt;

    private SpadeIterator(List<Card> cards) {
        cardsIt = cards.iterator();
    }

    @Override
    public boolean hasNext() {
        return spadesCounter<13; // we can't put spacesCounter++ here because 
                                 // we should be able to call `hasNext()` many times
                                 // and still get same answer,
                                 // so `hasNext()` shouldn't change any state 
                                 // (at least one which could cause changing its result)
    }

    @Override
    public Card next() {
        Card temp = cardsIt.next(); //if our `hasNext()` returned `false` but we 
                                    //didn't check or ignored it, this will CORRECTLY 
                                    //throw NoSuchElementException 
        while(temp.suit.value != Suit.SPADES.value){
            temp = cardsIt.next();
        }
        spadesCounter++;
        return temp;
    }
}

或者，如果您只是想遍历列表并仅打印选定的元素，则可以使用具有过滤功能的流

List<Card> cards = ...//not really important how get it
cards.stream()
     .filter(card -> card.suit.value == Suit.SPADES.value)
     .forEach(card -> System.out.println(card));

或更简单

for (Card card : cards){
    if(card.suit.value == Suit.SPADES.value){
        System.out.println(card);
    }
}

Answer 2

还要检查module Services module Trips class TripPreviewResponseEntity < Grape::Entity expose :id expose :title expose :duration expose :total_price expose :description expose :details expose :destinations do |trip, _options| DestinationResponseEntity.represent(trip.destinations, trip_id: trip.id) end end end end是否等于0：

nextCardSpade

如何停止打印空值

2 个答案: