问题

  如何存储字典里的所有单词能够让搜索、更新的效率快?

哈希表的解决方案

 JDK中HashMap的实现就是参照了这种思路,我们可以自己写一个

package com.study.jdk;

import java.io.Serializable;

/**
 * @author mdl
 * @date 2019/12/10
 */
public class SimpleHashMap<K, V> implements Serializable {

    /**
     *
     */
    private static final long serialVersionUID = -7278101382866134374L;

    transient Node<K, V>[] table;// 存储头节点的数组
    private int size;// 元素个数
    private static int defaultCapacity = 16;// 默认容量
    private static float defaultLoadFactor = 0.75f;// 扩展因子

    public SimpleHashMap() {}

    public SimpleHashMap(int capacity, int loadFactor) {
        defaultCapacity = capacity;
        defaultLoadFactor = loadFactor;
    }

    public int hash(K key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

    public V get(K key) {
        int n = table.length;
        int index = n - 1 & hash(key);
        Node<K, V> node;
        if ((node = table[index]) != null) {
            while (node != null && !node.key.equals(key)) {
                node = node.next;
            }
            return null == node ? null : node.value;
        }
        return null;
    }

    @SuppressWarnings("unchecked")
    public V put(K key, V value) {
        // 初始化数组
        if (table == null) {
            // 并不是在构造函数中完成初始化,而是在put的时候
            table = new Node[defaultCapacity];
        }
        int n = table.length;
        int index = n - 1 & hash(key); // 计算存储数组的下标
        // 获取到数组角标元素,可视为头结点
        Node<K, V> node = table[index];
        // 遍历链表中节点对象
        while (node != null) {
            if (node.key.equals(key)) {
                // key存在相同的情况下,替换value,直接退出程序
                node.value = value;
                return value;
            } else {
                // 不同的话,找下一个结点
                node = node.next;
            }
        }
        // 走到这里,说明table[index]里没有找到相同key的链结点
        // 在这里做扩容是因为,要加入的元素是作为新增元素的,
        // 比如容量16,扩容因子0.75,当前table里元素为12时,再添加元素的话,就扩容量至32(16 * 2),
        // 下一次扩容在(32 * 0.75)后继续添加元素时
        if (size >= defaultCapacity * defaultLoadFactor) {
            resize();
        }
        // 将新添加的数据作为头结点添加到数组中
        table[index] = new Node<>(key, value, table[index]);
        size++;
        return value;
    }

    // 扩展数组
    @SuppressWarnings("unchecked")
    public void resize() {
        // 扩容后要对元素重新put(重新散列),所以要将size置为0
        size = 0;
        // 记录先之前的数组
        Node<K, V>[] oldNodes = table;
        defaultCapacity = defaultCapacity << 1;
        table = new Node[defaultCapacity];
        // 遍历散列表中每个元素
        for (int i = 0; i < oldNodes.length; i++) {
            // 扩容后hash值会改变,所以要重新散列
            // 当前数组的头结点
            Node<K, V> node = oldNodes[i];
            while (node != null) {
                Node<K, V> oldNode = node;
                put(node.key, node.value);// 重新散列
                node = node.next;// 指针往后移
                oldNode.next = null;// 将当前散列的节点next置为null
            }
        }
    }

    // 节点对象
    public class Node<K, V> {
        K key;
        V value;
        Node<K, V> next;

        public Node(K key, V value, Node<K, V> next) {
            this.key = key;
            this.value = value;
            this.next = next;
        }
    }

}

编写测试类

package com.study.jdk;

/**
 * @author mdl
 * @date 2019/12/10
 */
public class Dictionary {

    private String word;

    private String desc;

    /**
     *
     */
    public Dictionary() {
        super();
    }

    /**
     * @param word
     * @param desc
     */
    public Dictionary(String word, String desc) {
        super();
        this.word = word;
        this.desc = desc;
    }

    /**
     * @return the word
     */
    public String getWord() {
        return word;
    }

    /**
     * @param word
     *            the word to set
     */
    public void setWord(String word) {
        this.word = word;
    }

    /**
     * @return the desc
     */
    public String getDesc() {
        return desc;
    }

    /**
     * @param desc
     *            the desc to set
     */
    public void setDesc(String desc) {
        this.desc = desc;
    }

    /* (non-Javadoc)
     * @see java.lang.Object#toString()
     */
    @Override
    public String toString() {
        return "Dictionary [word=" + word + ", desc=" + desc + "]";
    }

}
View Code
package com.study.jdk;

/**
 * @author mdl
 * @date 2019/12/10
 */
public class Test {


    public static void main(String[] args) {
        Dictionary dictionary1 =new Dictionary("dog", "一种动物,汪汪叫");
        Dictionary dictionary2 =new Dictionary("elephant", "大象,长鼻子");
        Dictionary dictionary3 =new Dictionary("programmer", "程序员,特指秃顶,颈椎病,没女朋友那类人");

        SimpleHashMap<String, Dictionary> map =new SimpleHashMap<String, Dictionary>();
        map.put("dog", dictionary1);
        map.put("elephant", dictionary2);
        map.put("programmer", dictionary3);

        System.out.println(map.get("dog"));
        System.out.println(map.get("elephant"));
        System.out.println(map.get("programmer"));

    }
}
02-09 22:20