HashSet源码详解(基于jdk1.8.0_261)

HashSet简介

  • HashSet继承AbstractSet,实现了Set,Cloneable,java.io.Serializable接口,故可拷贝,可序列化;
  • HashSet依赖于HashMap,在HashMap中放入元素实际上都是放入HashMap 的实例 map中作为key,每个value都使用new Object()填充,由于HashMap的key是唯一的,故HashSet中的元素是唯一的;
  • HashSet在多线程环境下是线程不安全的,可以外部加锁或使用其他并发线程安全类代替或Set s = Collections.synchronizedSet(new HashSet(...));
  • HashSet的迭代器也具有fail-fast机制;
  • 可以存null;(因为HashMap的key是唯一的)

为什么HashMap的key是唯一的?
因为 当一个元素准备加入map中时,先通过HashMap的hash函数计算出其hash值,在通过 (n -1 ) & hash 得到他的索引,若该位置为空,立即将元素的key value放入,否则若key相等,则替换value值,若key不等,启动哈希冲突方法,或用链表存,或用红黑树存冲突的节点。相同的key一定得到一样的hash,故存放的位置一定相同,只替换value,不同的key,hash值可能相同,但存放的位置一定不同。故HashMap中key是唯一的,故HashSet中元素是唯一的;

HashSet UML简图

HashSet API概述

字段

static final long serialVersionUID = -5024744406713321676L;
private transient HashMap<E,Object> map;
private static final Object PRESENT = new Object();

map 字段主要用于存储数据,HashSet本质上的数据结构就是HashMap,HashMap的key存储实际元素,PRESENT字段填充所有k-v对的value值

构造函数

public HashSet()
public HashSet(Collection<? extends E> c)
public HashSet(int initialCapacity, float loadFactor)
public HashSet(int initialCapacity)
HashSet(int initialCapacity, float loadFactor, boolean dummy) //为LinkedHashMap服务,多一个dummy参数是为了和两个参数的构造函数区别开

override或新增的方法

public Iterator<E> iterator() //返回元素的迭代器
public int size()
public boolean isEmpty()
public boolean contains(Object o)
public boolean add(E e)
public boolean remove(Object o)
public void clear()
public Object clone()
public Spliterator<E> spliterator()

private方法

private void writeObject(java.io.ObjectOutputStream s)
private void readObject(java.io.ObjectInputStream s)

直接继承或来自接口的default方法

--------------继承自AbstractSet的方法----------------
public boolean equals(Object o)
public int hashCode()
public boolean removeAll(Collection<?> c) 

--------------继承自AbstrctCollection的方法----------
public boolean addAll(Collection<? extends E> c) 
public boolean retainAll(Collection<?> c) 
public String toString()
public boolean containsAll(Collection<?> c) 
public <T> T[] toArray(T[] a) 
public Object[] toArray()
------------来自接口的defalut的方法------------------
default void forEach(Consumer<? super T> action) //来自Iterable接口
default Stream<E> stream()  //来自Colletion接口
default Stream<E> parallelStream()   //来自Colletion接口

HashSet源码详解

package java.util;

import java.io.InvalidObjectException;
import sun.misc.SharedSecrets;


public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;
    //存储HashSet的元素的数据结构,可以看出HashSet 和 HashMap 存在 has-a 关系
    //HashSet中元素都放入map中当key
    private transient HashMap<E,Object> map;
    //所有的value都用PRESENT代替
    private static final Object PRESENT = new Object();
   // 构造一个空的HashSet实例,初始化容量为16,load factor = 0.75
    public HashSet() {
        map = new HashMap<>();
    }
    // 用一个集合c填充HashSet,load factor = 0.75,容量为 Math.max((int)(c.size()/.75f) +1 ,16)
    // c = null时 会抛异常
    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }
    //构造一个空的HashSet.capacity,load factor自定义 
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }
    //构造一个空的HashSet,load factor = 0.75f,容量自定义
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }
    //包权限的HashSet构造函数,被用于LinkedHashSet类,dummy参数是为了和上述两个参数的构造函数区别开
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

    //返回HashMap的keySet的迭代器,即为HashSet元素的迭代器
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }
    //返回集合中的元素个数
    public int size() {
        return map.size();
    }
    //判空
    public boolean isEmpty() {
        return map.isEmpty();
    }

    /**
     * Returns <tt>true</tt> if this set contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this set
     * contains an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this set is to be tested
     * @return <tt>true</tt> if this set contains the specified element
     */
    //判断HashSet中是否包含对象o
    public boolean contains(Object o) {
        return map.containsKey(o);
    }
    //添加一个元素,若添加成功,返回true
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }
    //移除对象o
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }
    //移除所有的元素
    public void clear() {
        map.clear();
    }

    //返回HashSet的浅拷贝
    @SuppressWarnings("unchecked")
    public Object clone() {
        try {
            HashSet<E> newSet = (HashSet<E>) super.clone();
            newSet.map = (HashMap<E, Object>) map.clone();
            return newSet;
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }
    //序列化
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();

        // Write out HashMap capacity and load factor
        s.writeInt(map.capacity());
        s.writeFloat(map.loadFactor());

        // Write out size
        s.writeInt(map.size());

        // Write out all elements in the proper order.
        for (E e : map.keySet())
            s.writeObject(e);
    }
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in any hidden serialization magic
        s.defaultReadObject();

        // Read capacity and verify non-negative.
        int capacity = s.readInt();
        if (capacity < 0) {
            throw new InvalidObjectException("Illegal capacity: " +
                                             capacity);
        }

        // Read load factor and verify positive and non NaN.
        float loadFactor = s.readFloat();
        if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
            throw new InvalidObjectException("Illegal load factor: " +
                                             loadFactor);
        }

        // Read size and verify non-negative.
        int size = s.readInt();
        if (size < 0) {
            throw new InvalidObjectException("Illegal size: " +
                                             size);
        }
        // Set the capacity according to the size and load factor ensuring that
        // the HashMap is at least 25% full but clamping to maximum capacity.
        capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
                HashMap.MAXIMUM_CAPACITY);

        // Constructing the backing map will lazily create an array when the first element is
        // added, so check it before construction. Call HashMap.tableSizeFor to compute the
        // actual allocation size. Check Map.Entry[].class since it's the nearest public type to
        // what is actually created.

        SharedSecrets.getJavaOISAccess()
                     .checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));

        // Create backing HashMap
        map = (((HashSet<?>)this) instanceof LinkedHashSet ?
               new LinkedHashMap<E,Object>(capacity, loadFactor) :
               new HashMap<E,Object>(capacity, loadFactor));

        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
            @SuppressWarnings("unchecked")
                E e = (E) s.readObject();
            map.put(e, PRESENT);
        }
    }
    public Spliterator<E> spliterator() {
        return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
    }
}

HashSet示例

package collectionlearn;

import java.lang.reflect.Array;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;
import java.util.stream.Collectors;

public class HashSetTest {
    public static void main(String[] args) {
        HashSet<Integer> hashSet = new HashSet<>();
        hashSet.addAll(Arrays.asList(1,4,7,9,5,6,8));
        System.out.println(hashSet);
        hashSet.add(100);
        hashSet.add(200);
        hashSet.add(300);
        if(! hashSet.add(300)) {
            System.out.println("元素已存在");
            hashSet.remove(300);
            System.out.println("元素已移除");
        }
        System.out.println(hashSet);
        if(hashSet.contains(4)){
            System.out.println("4已存在");
        }
        HashSet<Integer> copyhashSet = null;
        if(! hashSet.isEmpty()){
            copyhashSet = (HashSet<Integer>) hashSet.clone();
            hashSet.clear();
            System.out.println("hashSet已清空");
            System.out.println(hashSet);
        }
        System.out.println("copyHashSet :" + copyhashSet);
        Set<Integer> set =  copyhashSet.stream().filter(x -> x > 10).collect(Collectors.toSet());
        System.out.println(set);
    }
}

  • 结果
[1, 4, 5, 6, 7, 8, 9]
元素已存在
元素已移除
[1, 4, 100, 5, 6, 7, 8, 200, 9]
4已存在
hashSet已清空
[]
copyHashSet :[1, 4, 100, 5, 6, 7, 8, 200, 9]
[100, 200]

面试session

  • 谈谈你对HashSet的认识?
    HashSet本质上的数据结构就是HashMap,Hashset中加入的每个元素都为map的key,所有value值都利用PRESENT字段填充,PSESENT字段是个static final 的Object实例。由于HashMap key的唯一性保证了HashSet中元素的唯一性。

初稿完

posted @ 2020-07-17 02:34  ahpuched  阅读(351)  评论(0编辑  收藏  举报