Java集合框架（八）— HashMap

image.png

HashMap定义

public class HashMap<K, V> extends AbstractMap<K, V> implements Map<K, V>, Cloneable, Serializable

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

HashMap 继承于AbstractMap，实现了Map、Cloneable、java.io.Serializable接口。
HashMap 的实现不是同步的，这意味着它不是线程安全的。它的key、value都可以为null。此外，HashMap中的映射不是有序的。

HashMap 的实例有两个参数影响其性能：“初始容量” 和 “加载因子”。默认加载因子是 0.75，如果初始容量大于最大条目数除以加载因子，则不会发生 rehash 操作。

HashMap的API

void                 clear()
Object               clone()
boolean              containsKey(Object key)
boolean              containsValue(Object value)
Set<Entry<K, V>>     entrySet()
V                    get(Object key)
boolean              isEmpty()
Set<K>               keySet()
V                    put(K key, V value)
void                 putAll(Map<? extends K, ? extends V> map)
V                    remove(Object key)
int                  size()
Collection<V>        values()

HashMap的构造函数

public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        this.loadFactor = loadFactor;
        this.threshold = tableSizeFor(initialCapacity);
    }

public HashMap(Map<? extends K, ? extends V> m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

HashMap遍历

package HashMapDemo;

import java.util.*;

/**
 * @ClassName: HashMapTest
 * @Author sandy
 * @Date: 2018/12/3
 * @Version v1.0.0
 * @Description: //TODO
 */

public class HashMapTest {

    public static void iteratorHashMapByEntrySet(HashMap hashMap){

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

    public static void iteratorHashMapByValueColl(HashMap hashMap){
        Iterator iterator = hashMap.values().iterator();
        while (iterator.hasNext()){
            Object key = iterator.next();
            System.out.println("value: "+key);
        }
        System.out.println();
    }

    public static void main(String[] args) {

        int val;
        Object key;
        Object value;
        HashMap hashMap = new HashMap();
        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(100);

            key = String.valueOf(val);
            value = r.nextInt(5);
            // 添加到HashMap中
            hashMap.put(key, value);
        }

        iteratorHashMapByEntrySet(hashMap);
        iteratorHashMapByKeySet(hashMap);
        iteratorHashMapByValueColl(hashMap);

    }
}

运行结果：

key: 88 value: 4
key: 13 value: 2
key: 80 value: 3
key: 42 value: 4
key: 32 value: 3

key: 88 value: 4
key: 13 value: 2
key: 80 value: 3
key: 42 value: 4
key: 32 value: 3

value: 4
value: 2
value: 3
value: 4
value: 3