java - 对 java 集合的每次删除和添加执行操作

Question

我找不到任何相关信息:

通常我有一个扩展 HashSet 的类。插入到该集合中的每个对象都有它的“所有者”，我想分别计算属于每个所有者的对象数量。所以我写了下面的代码:

public class Viruses extends HashSet<Virus> {
    private HashMap<RaceName, Integer> countsPerRace = new HashMap<RaceName, Integer>();

    @Override
    public boolean add(Virus virus) {
        if(super.add(virus)) {
            RaceName race = virus.getOwner().getRace().getName();
            if(countsPerRace.containsKey(race)) {
                countsPerRace.put(race, countsPerRace.get(race) + 1);
            } else {
                countsPerRace.put(race, 1);
            }
            return true;
        } else {
            return false;
        }
    }

    @Override
    public boolean remove(Object virus) {
        if(super.remove(virus)) {
            RaceName race = ((Virus)virus).getOwner().getRace().getName();
            if(countsPerRace.containsKey(race)) {
                countsPerRace.put(race, countsPerRace.get(race) - 1);
            } else {
                throw new Exception("This should not happen...");
            }
            return true;
        } else {
            return false;
        }
    }

    /**
     * Returns number of viruses of given race.
     * @param raceId raceName of the viruses, which is equivalent of an owner id as there should never be two owners with the same race
     * @return number of viruses of given race.
     */
    public int getCount(RaceName raceId) {
        return countsPerRace.containsKey(raceId) ? countsPerRace.get(raceId) : 0;
    }

    // I don't need these, so I thought the best idea will be just to throw an exception here.
    @Override
    public boolean removeAll(Collection<?> collection) {
        throw new EngineRuntimeException("Unsupported operation!");
    }

    @Override
    public boolean addAll(Collection<? extends Virus> collection) {
        throw new EngineRuntimeException("Unsupported operation!");
    }
}

问题是，如果我使用迭代器删除对象，则不会调用删除方法。有没有一种方法可以在每次在 Java 集合中添加或删除对象时执行操作？如果不是，我必须重写哪些方法或类才能确保我的集合保持一致，无论我以哪种方式删除或添加内容？

Answer 1

正如您所发现的，不能保证迭代器使用公共(public) remove 方法。

在这种情况下，我强烈建议您考虑使用组合而不是继承。

但是，如果您想继续使用继承解决方案，则必须执行以下操作:

@Override
public Iterator<Virus> iterator() {
    final Iterator<Virus> delegate = super.iterator();
    return new Iterator<Virus>() {
        @Override
        public boolean hasNext() {
            return delegate.hasNext();
        }
        @Override
        public void remove() {
            // put your custom remove logic here
            //  ...
            delegate.remove();
        }
        @Override
        public Virus next() {
            return delegate.next();
        }
    };
}