Java集合框架学习（十一） Hashtable详解

Hashtable介绍

1. 继承自Dictionary;

2. 线程安全；

3. 支持Iterator和Enumeration；

4. key和value都不可为null;

5. 一般用于多线程环境中。

类定义

public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, Serializable

例子介绍

package com.dylan.collection;

import java.util.Enumeration;
import java.util.Hashtable;

/**
 * @author xusucheng
 * @create 2018-02-03
 **/
public class HashtableExample {
    public static void main(String[] args) {

        Enumeration names;
        String key;

        // Creating a Hashtable
        Hashtable<String, String> hashtable =
                new Hashtable<String, String>();

        // Adding Key and Value pairs to Hashtable
        hashtable.put("Key1","Chaitanya");
        hashtable.put("Key2","Ajeet");
        hashtable.put("Key3","Peter");
        hashtable.put("Key4","Ricky");
        hashtable.put("Key5","Mona");

        names = hashtable.keys();
        while(names.hasMoreElements()) {
            key = (String) names.nextElement();
            System.out.println("Key: " +key+ " & Value: " +
                    hashtable.get(key));
        }
    }
}

使用Iterator迭代HashTable

package com.dylan.collection;

import java.util.Hashtable;
import java.util.Iterator;
import java.util.Set;

/**
 * @author xusucheng
 * @create 2018-02-03
 **/
public class IterateHashtable {
    public static void main(String[] args) {

        String str;

        // Creating a Hashtable instance
        Hashtable<String, String> hashtable = new Hashtable<String, String>();

    /* Adding key-value pairs to Hashtable
     * public V put(K key, V value): Maps the specified key to the
     * specified value in this hashtable. Neither the key nor the
     * value can be null. The value can be retrieved by calling the
     * get method with a key that is equal to the original key.
     */
        hashtable.put("A", "Apple");
        hashtable.put("B", "Orange");
        hashtable.put("C", "Mango");
        hashtable.put("D", "Banana");
        hashtable.put("E", "Grapes");

        System.out.println("Hashtable contains:");

    /* public Set<K> keySet(): Returns a Set view of the keys
     * contained in this map. The set is backed by the map,
     * so changes to the map are reflected in the set, and
     * vice-versa.
     */
        Set<String> keys = hashtable.keySet();

        //Obtaining iterator over set entries
        Iterator<String> itr = keys.iterator();

        //Displaying Key and value pairs
        while (itr.hasNext()) {
            // Getting Key
            str = itr.next();

       /* public V get(Object key): Returns the value to which
        * the specified key is mapped, or null if this map
        * contains no mapping for the key.
        */
            System.out.println("Key: "+str+" & Value: "+hashtable.get(str));
        }
    }
}