Java集合框架（九）— HashTable

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Hashtable定义

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

Hashtable 也是一个散列表，它存储的内容是键值对(key-value)映射。

Hashtable 的函数都是同步的，这意味着它是线程安全的。它的key、value都不可以为null。此外，Hashtable中的映射不是有序的。

Hashtable是线程安全的。

Hashtable的所有API

synchronized void                clear()
synchronized Object              clone()
             boolean             contains(Object value)
synchronized boolean             containsKey(Object key)
synchronized boolean             containsValue(Object value)
synchronized Enumeration<V>      elements()
synchronized Set<Entry<K, V>>    entrySet()
synchronized boolean             equals(Object object)
synchronized V                   get(Object key)
synchronized int                 hashCode()
synchronized boolean             isEmpty()
synchronized Set<K>              keySet()
synchronized Enumeration<K>      keys()
synchronized V                   put(K key, V value)
synchronized void                putAll(Map<? extends K, ? extends V> map)
synchronized V                   remove(Object key)
synchronized int                 size()
synchronized String              toString()
synchronized Collection<V>       values()

Hashtable构造函数

public Hashtable() {
        this(11, 0.75f);
    }

public Hashtable(int initialCapacity) {
        this(initialCapacity, 0.75f);
    }

public Hashtable(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal Load: "+loadFactor);

        if (initialCapacity==0)
            initialCapacity = 1;
        this.loadFactor = loadFactor;
        table = new Entry<?,?>[initialCapacity];
        threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    }

public Hashtable(Map<? extends K, ? extends V> t) {
        this(Math.max(2*t.size(), 11), 0.75f);
        putAll(t);
    }

Hashtable遍历：四种

package HashTableDemo;

import java.util.*;

/**
 * @ClassName: HashTableTest
 * @Author Sandy
 * @Date: 2018/12/4
 * @Version v1.0.0
 * @Description: //TODO
 */

public class HashtableTest {

    public static void iteratorHashtableByEntrySet(Hashtable hashtable){

        Iterator iterator = hashtable.entrySet().iterator();
        while(iterator.hasNext()){
            Map.Entry entry = (Map.Entry) iterator.next();
            Object key = entry.getKey();
            Object value = entry.getValue();
            System.out.println("key:  "+ key +" value: "+ value);
        }
        System.out.println();
    }

    public static void iteratorHashtableByEntryKey(Hashtable hashtable){

        Iterator iterator = hashtable.keySet().iterator();
        while(iterator.hasNext()){
            Object key = iterator.next();
            Object value = hashtable.get(key);
            System.out.println("key:  "+ key +" value: "+ value);
        }
        System.out.println();
    }

    public  static void iteratorHashtableByValueColl(Hashtable hashtable){
        Iterator iterator = hashtable.values().iterator();
        while (iterator.hasNext()){
            Object value = iterator.next();
            System.out.println("value: "+ value);
        }
        System.out.println();
    }

    public static void iteratorHashtableByEnumeration(Hashtable hashtable){
        Enumeration enumeration = hashtable.elements();
        while (enumeration.hasMoreElements()){
            System.out.println("element(value): " + enumeration.nextElement());
        }
        System.out.println();
    }

    public static void main(String[] args) {

        int val;
        Object key;
        Object value;
        Hashtable hashtable = new Hashtable();
        Random r = new Random();

        for (int i=0; i<5; i++) {
            // 随机获取一个[0,100)之间的数字
            val = r.nextInt(200);
            key = String.valueOf(val);
            value = r.nextInt(5);
            // 添加到HashMap中
            hashtable.put(key, value);
        }

        iteratorHashtableByEntrySet(hashtable);
        iteratorHashtableByEntryKey(hashtable);
        iteratorHashtableByValueColl(hashtable);
        iteratorHashtableByEnumeration(hashtable);
    }
}

输出：

key:  75 value: 1
key:  87 value: 1
key:  2 value: 0
key:  68 value: 2
key:  8 value: 1

key:  75 value: 1
key:  87 value: 1
key:  2 value: 0
key:  68 value: 2
key:  8 value: 1

value: 1
value: 1
value: 0
value: 2
value: 1

element(value): 1
element(value): 1
element(value): 0
element(value): 2
element(value): 1