我目前正在尝试实现Hashtable集合-我已启动并运行了所有程序,但是在尝试为表定义自定义迭代器时遇到了一个概念性问题。我有一个名为``HashEntry''的内部类,它们是存储在数组中的实际对象-它们存储键,值和条目的状态,即空, Activity ,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-如果我希望能够遍历表,是否应该遍历(并因此返回)HashEntry对象,还是遍历表中存储的实际值的哈希表约定? ? HashEntry类是私有(private)的,因此我假设它的错误做法是返回它的实例...

但是,如果是这样,我如何创建一个遍历其HashEntrys对象的Hashtable迭代器?我必须在HashEntry类中定义一个迭代器/可迭代器吗?

最佳答案

一般而言,是的,如果您确实提供了一个在HashEntry上进行迭代的迭代器,则可能会更好,这样用户在进行迭代时会同时获得键和值(以及状态)。通常,没有键就没有意义,反之亦然。

您为什么不只将HashEntry类作为public static通用内部类,而将实现特定的东西设为private呢?您可能还需要使HashEntry通用,因为我假设您的父类(我们称其为MyHashTable)也是基于TKeyTValue的通用类。

所以,如果我是你,我会让你的HashEntryMyHashTable看起来像这样:

// 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,并且其构造函数现在是private。这保证了除外部类MyHashTable之外的任何人都无法实例化HashEntry,因为实例化哈希表之外的任何一个都没有意义(请参阅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。至于编写一个,这取决于您自己的哈希表实现,但是基本上,您需要一种以任意顺序获取下一个元素的方法: Iterator<E>.next()

例如,下面是一个iterator()方法的实现,它在一个简单的数组上进行迭代:
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)

希望这能有所帮助。

09-25 20:18