java – 如何创建自定义哈希表迭代器？

2019年8月6日 208次阅读

我目前正在尝试实现一个Hashtable集合 – 我已经完成了所有工作,但是当我尝试为表定义一个自定义迭代器时,我遇到了一个概念问题.我有一个名为’HashEntry’的内部类,它是存储在数组中的实际对象 – 它们存储条目的键,值和状态,即空,活动,已删除.

private class HashEntry
{
    private TKey m_key;
    private TValue m_value;
    private EntryStatus status;

    //standard constructor
    public HashEntry(TKey key, TValue value)
    {
        m_key = key;
        m_value = value;
        status = EntryStatus.ACTIVE;
    }

    public HashEntry(TKey key, TValue value, EntryStatus i) {
        m_key = key;
        m_value = value;
        status = i;
    }

    //default 'empty' constructor
    public HashEntry()
    {
        //calls default constructor, creates placeholder entry
        m_key = null;
        m_value = null;
        status = EntryStatus.EMPTY;
    }

    //equals operator override, this override just compares compares
    // the objects held in the entry, so any object used with this
    // implementation must hae=ve its own equals override
    @Override
    public boolean equals(Object obj)
    {
        if (obj == null) { return false; }
        if (getClass() != obj.getClass()) { return false; }

        final HashEntry other = (HashEntry) obj;
        return (!((this.m_key == null) ? (other.m_key != null) : !this.m_key.equals(other.m_key)));
    }

    // override of the hashCode() function--just calls the hashCode
    // function of the embedded object, so that must be provided
    @Override
    public int hashCode()
    {
        return this.m_key.hashCode();
    }

    // toString override just returns the toString of the embedded object
    @Override
    public String toString()
    {
        StringBuilder sb = new StringBuilder();
        sb.append(m_key.toString()).append(m_value.toString());
        return sb.toString();
    }
}

这是我的问题的第一部分 – 如果我想能够遍历表,我应该迭代(并因此返回)HashEntry对象,还是哈希表约定迭代存储在表中的实际值？ HashEntry类是私有的,所以我假设它的错误做法是返回它的实例……

但如果是这样,我如何创建一个迭代HashEntrys对象的Hashtable迭代器？我是否必须在HashEntry类中定义迭代器/ iterable？

最佳答案 一般来说,是的,如果您提供迭代HashEntrys的迭代器可能会更好,因此用户在迭代时同时获得键和值(和状态).通常,没有密钥,价值就没有意义,反之亦然.

为什么不将HashEntry类作为公共静态泛型内部类,并使特定于实现的东西变为私有？您可能还需要使HashEntry成为通用的,因为我假设您的父类(我们只是称之为MyHashTable)也是基于TKey和TValue的通用.

所以,如果我是你,我会让你的HashEntry和MyHashTable看起来更像这样：

// Note: implements Iterable<E> now
public class MyHashTable<TKey, TValue> implements Iterable<MyHashTable.HashEntry<TKey, TValue>>
{
    public Iterator<MyHashTable.HashEntry<TKey, TValue>> iterator() {
        // ...
        // Make and return your iterator here
        // ...
    }

    // Note: public and generic now
    public static class HashEntry<TKey, TValue>
    {
        private TKey m_key;
        private TValue m_value;
        private EntryStatus status;

        //standard constructor
        // Note: private now
        public HashEntry(TKey key, TValue value)
        {
            m_key = key;
            m_value = value;
            status = EntryStatus.ACTIVE;
        }

        // Note: private now
        private HashEntry(TKey key, TValue value, EntryStatus i) {
            m_key = key;
            m_value = value;
            status = i;
        }

        //default 'empty' constructor
        // Note: private now
        public HashEntry()
        {
            //calls default constructor, creates placeholder entry
            m_key = null;
            m_value = null;
            status = EntryStatus.EMPTY;
        }

        public TKey getKey() {
            return m_key;
        }

        public TValue getValue() {
            return m_value;
        }

        public EntryStatus getEntryStatus() {
            return status;
        }

        //equals operator override, this override just compares compares
        // the objects held in the entry, so any object used with this
        // implementation must hae=ve its own equals override
        @Override
        public boolean equals(Object obj)
        {
            if (obj == null) { return false; }
            if (getClass() != obj.getClass()) { return false; }

            final HashEntry other = (HashEntry) obj;
            return (!((this.m_key == null) ? (other.m_key != null) : !this.m_key.equals(other.m_key)));
        }

        // override of the hashCode() function--just calls the hashCode
        // function of the embedded object, so that must be provided
        @Override
        public int hashCode()
        {
            return this.m_key.hashCode();
        }

        // toString override just returns the toString of the embedded object
        @Override
        public String toString()
        {
            StringBuilder sb = new StringBuilder();
            sb.append(m_key.toString()).append(m_value.toString());
            return sb.toString();
        }
    }
}

请注意,HashEntry现在是MyHashTable的内部类,它是通用的,它的构造函数现在是私有的.这保证除了外部类MyHashTable之外的任何人都可以实例化HashEntry,因为在哈希表外部实例化一个是没有意义的(参见this).但是,其他人可以通过getter访问条目的键和值.

迭代器本身将是Iterator< MyHashTable.HashEntry< TKey,TValue>>的实例.至于写一个,这取决于你自己的哈希表实现,但你基本上你需要一种方式获得任何序列的下一个元素：Iterator<E>.next().

例如,这是一个迭代一个简单数组的iterator()方法实现：

private Type[] arrayList;
private int currentSize;

@Override
public Iterator<Type> iterator() {
    Iterator<Type> it = new Iterator<Type>() {

        private int currentIndex = 0;

        @Override
        public boolean hasNext() {
            return currentIndex < currentSize && arrayList[currentIndex] != null;
        }

        @Override
        public Type next() {
            return arrayList[currentIndex++];
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }
    };
    return it;
}

(来源：https://stackoverflow.com/a/5849625/837703)

希望这有点帮助.

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