Dictionary.java

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package java.util;

/**
 * The {@code Dictionary} class is the abstract parent of any
 * class, such as {@code Hashtable}, which maps keys to values.
 * Every key and every value is an object. In any one {@code Dictionary}
 * object, every key is associated with at most one value. Given a
 * {@code Dictionary} and a key, the associated element can be looked up.
 * Any non-{@code null} object can be used as a key and as a value.
 * <p>
 * As a rule, the {@code equals} method should be used by
 * implementations of this class to decide if two keys are the same.
 * <p>
 * <strong>NOTE: This class is obsolete.  New implementations should
 * implement the Map interface, rather than extending this class.</strong>
 *
 * @author unascribed
 * @see java.util.Map
 * @see java.lang.Object#equals(java.lang.Object)
 * @see java.lang.Object#hashCode()
 * @see java.util.Hashtable
 * @since 1.0
 */
// 字典接口，类似于Map，也是一个关于键值对的操作规范
public abstract class Dictionary<K, V> {
    
    /**
     * Sole constructor.  (For invocation by subclass constructors, typically
     * implicit.)
     */
    public Dictionary() {
    }
    
    /**
     * Maps the specified {@code key} to the specified
     * {@code value} in this dictionary. Neither the key nor the
     * value can be {@code null}.
     * <p>
     * If this dictionary already contains an entry for the specified
     * {@code key}, the value already in this dictionary for that
     * {@code key} is returned, after modifying the entry to contain the
     * new element. <p>If this dictionary does not already have an entry
     * for the specified {@code key}, an entry is created for the
     * specified {@code key} and {@code value}, and {@code null} is
     * returned.
     * <p>
     * The {@code value} can be retrieved by calling the
     * {@code get} method with a {@code key} that is equal to
     * the original {@code key}.
     *
     * @param key   the hashtable key.
     * @param value the value.
     *
     * @return the previous value to which the {@code key} was mapped
     * in this dictionary, or {@code null} if the key did not
     * have a previous mapping.
     *
     * @throws NullPointerException if the {@code key} or
     *                              {@code value} is {@code null}.
     * @see java.lang.Object#equals(java.lang.Object)
     * @see java.util.Dictionary#get(java.lang.Object)
     */
    // 将指定的元素（key-value）存入字典，并返回旧值，允许覆盖
    public abstract V put(K key, V value);
    
    /**
     * Returns the value to which the key is mapped in this dictionary.
     * The general contract for the {@code isEmpty} method is that if this
     * dictionary contains an entry for the specified key, the associated
     * value is returned; otherwise, {@code null} is returned.
     *
     * @param key a key in this dictionary.
     *            {@code null} if the key is not mapped to any value in
     *            this dictionary.
     *
     * @return the value to which the key is mapped in this dictionary;
     *
     * @throws NullPointerException if the {@code key} is {@code null}.
     * @see java.util.Dictionary#put(java.lang.Object, java.lang.Object)
     */
    // 根据指定的key获取对应的value，如果不存在，则返回null
    public abstract V get(Object key);
    
    /**
     * Removes the {@code key} (and its corresponding
     * {@code value}) from this dictionary. This method does nothing
     * if the {@code key} is not in this dictionary.
     *
     * @param key the key that needs to be removed.
     *
     * @return the value to which the {@code key} had been mapped in this
     * dictionary, or {@code null} if the key did not have a
     * mapping.
     *
     * @throws NullPointerException if {@code key} is {@code null}.
     */
    // 移除拥有指定key的元素，并返回刚刚移除的元素的值
    public abstract V remove(Object key);
    
    /**
     * Returns an enumeration of the keys in this dictionary. The general
     * contract for the keys method is that an {@code Enumeration} object
     * is returned that will generate all the keys for which this dictionary
     * contains entries.
     *
     * @return an enumeration of the keys in this dictionary.
     *
     * @see java.util.Dictionary#elements()
     * @see java.util.Enumeration
     */
    // 返回当前字典中key的枚举器（具体含义需参见子类的实现）
    public abstract Enumeration<K> keys();
    
    /**
     * Returns an enumeration of the values in this dictionary. The general
     * contract for the {@code elements} method is that an
     * {@code Enumeration} is returned that will generate all the elements
     * contained in entries in this dictionary.
     *
     * @return an enumeration of the values in this dictionary.
     *
     * @see java.util.Dictionary#keys()
     * @see java.util.Enumeration
     */
    // 返回当前字典中value的枚举器（具体含义需参见子类的实现）
    public abstract Enumeration<V> elements();
    
    /**
     * Returns the number of entries (distinct keys) in this dictionary.
     *
     * @return the number of keys in this dictionary.
     */
    // 获取当前字典中的元素数量
    public abstract int size();
    
    /**
     * Tests if this dictionary maps no keys to value. The general contract
     * for the {@code isEmpty} method is that the result is true if and only
     * if this dictionary contains no entries.
     *
     * @return {@code true} if this dictionary maps no keys to values;
     * {@code false} otherwise.
     */
    // 判断当前字典是否为空集
    public abstract boolean isEmpty();
    
}