java集合(二)HashSet和LinkHashSet源码分析

简述

HashSet实现了set接口, 底层是hashMap. (数组+单链表+红黑树)
LinkedHashSet是hashSet的一个子类
《java集合(二)HashSet和LinkHashSet源码分析》

HashSet源码分析

package com.tgb.kwy;

import sun.misc.SharedSecrets;

import java.io.InvalidObjectException;
import java.util.*;

/** * Description * * @author kongwy 15732621629@163.com * @version 1.0 * @date 2018-06-06-19 -58 */
public class HashSet<E>
        extends AbstractSet<E>
        implements Set<E>, Cloneable, java.io.Serializable
{

    /*类A序列化 类B反序列化. 1.A和B的serialVersionUID不一致, 如果修改了A的serialVersion值, 执行B,会报错 2.如果A,B都没显示写id,实体类没有改动, 序列化反序列化正常 3.A,B id一致,a增加字段,B不变, 序列化和反序列化都正常,A增加的字段丢失 4.A,B id一致,B减少字段,a不变, A端多的字段值丢失 5.A,B id一致,A减少一个字段,b不变. 序列反序列正常,b比a多的字段,赋类型默认值*/
    static final long serialVersionUID = -5024744406713321676L;

    /*底层使用HashMap来保存HashSet中所有元素。 */
    private transient HashMap<E,Object> map;

    /*定义一个虚拟的Object对象作为HashMap的value,将此对象定义为static final PRESENT是map中出入key-value中对应的value,所以向map中添加键值对时,键值对对应的值, 固定是PRESENT * 关于final,看标注1*/
    private static final Object PRESENT = new Object();

    /** * 空构造函数,底层默认空的hashMap,初始容量16,加载因子0.75 */
    public HashSet() {
        map = new HashMap<>();
    }

    /** * 带集合的构造函数 * * @param c the collection whose elements are to be placed into this set * @throws NullPointerException if the specified collection is null */
    public HashSet(Collection<? extends E> c) {
        /*(c.size()/.75f)+1, hashmap默认加载因子是0.75;当map的阈值<map实际大小,hashMap容量就要翻倍. 这段就是计算总的空间大小*/
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        // 将集合(c)中的全部元素添加到HashSet中
        addAll(c);
    }

    /** * 指定HashSet初始容量和加载因子的构造函数 * @param initialCapacity 初始容量 * @param loadFactor 加载因子 * @throws IllegalArgumentException if the initial capacity is less * than zero, or if the load factor is nonpositive */
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    /** * 空的hashMap. 初始容量指定,加载因子默认0.75 * @param initialCapacity 初始容量 * @throws IllegalArgumentException if the initial capacity is less * than zero */
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

    /** * Constructs a new, empty linked hash set. (This package private * constructor is only used by LinkedHashSet.) The backing * HashMap instance is a LinkedHashMap with the specified initial * capacity and the specified load factor. * 构造一个指定初始容量和加载因子的hash集合.这个构造函数是包访问权限,不对外公开,实际只是对linkedHashSet的支持. * * @param initialCapacity 初始容量 * @param loadFactor 加载因子 * @param dummy 标记ignored (distinguishes this * constructor from other int, float constructor.) * @throws IllegalArgumentException if the initial capacity is less * than zero, or if the load factor is nonpositive */
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

    /** * Returns an iterator over the elements in this set. The elements * are returned in no particular order. *返回对此set中元素进行迭代的迭代器,返回元素不是特点的. * 底层依旧是hashMa. 使用其keySet返回所有的key * * @return an Iterator over the elements in this set * @see ConcurrentModificationException */
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

    /** * Returns the number of elements in this set (its cardinality). * 返回set中元素的数量 * 底层是map的size()方法返回Entry的数量 * @return the number of elements in this set (its cardinality) */
    public int size() {
        return map.size();
    }

    /** * Returns <tt>true</tt> if this set contains no elements. * 若不包含任何元素,返回true * 底层是map的isEmpty()方法 * @return <tt>true</tt> if this set contains no elements */
    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>. *如果set中包含指定元素,就返回true.更确切说: 当且仅当set包含这样一个元素e,才返回true. 条件是(o==null ? e==null :o.equals(e)) * @param o element whose presence in this set is to be test * @return <tt>true</tt> if this set contains the specified element */
    public boolean contains(Object o) {
        return map.containsKey(o);
    }

    /** * Adds the specified element to this set if it is not already present. * More formally, adds the specified element <tt>e</tt> to this set if * this set contains no element <tt>e2</tt> such that * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>. * If this set already contains the element, the call leaves the set * unchanged and returns <tt>false</tt>. * 我们知道,set是不允许有重复元素的,set每次添加元素,就是调用的这个方法. * 底层是将元素直接作为map的key,放入HahsMap的;具体这里, 等我写haspMap的源码分析再详细讲 * @param e element to be added to this set * @return <tt>true</tt> if this set did not already contain the specified * element */
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

    /** * Removes the specified element from this set if it is present. * More formally, removes an element <tt>e</tt> such that * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, * if this set contains such an element. Returns <tt>true</tt> if * this set contained the element (or equivalently, if this set * changed as a result of the call). (This set will not contain the * element once the call returns.) * 如果指定元素存在于此set中,则将其移除。 * 更确切地讲,如果此set包含一个满足(o==null ? e==null : o.equals(e))的元素e,那么就把这个元素移除掉,并返回true *底层是map的remove方法,删除指定的entry * @param o object to be removed from this set, if present * @return <tt>true</tt> if the set contained the specified element */
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

    /** * Removes all of the elements from this set. * The set will be empty after this call returns. * 移除所有的元素, 这个方法被调用之后, set就是空了,底层是清空了map中entry所有的元素 */
    public void clear() {
        map.clear();
    }

    /** * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements * themselves are not cloned. * 浅复制,只是引用,不是元素本身. * @return a shallow copy of this set */
    @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);
        }
    }
/*使用下面两个方法时,避免直接或间接地在正在进行反序列化的对象上调用任何方法* /** * Save the state of this <tt>HashSet</tt> instance to a stream (that is, * serialize it). * 序列化写入函数 * hashset总容量,加载因子,实际容量,所有元素,都写入到输出流中 * @serialData The capacity of the backing <tt>HashMap</tt> instance * (int), and its load factor (float) are emitted, followed by * the size of the set (the number of elements it contains) * (int), followed by all of its elements (each an Object) in * no particular order. */
    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);
    }

    /** * Reconstitute the <tt>HashSet</tt> instance from a stream (that is, * deserialize it). * 序列化读取函数 * 将HashSet的“总的容量,加载因子,实际容量,所有的元素”依次读出 */
    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);
        }
    }

    /** * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em> * and <em>fail-fast</em> {@link Spliterator} over the elements in this * set. * * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and * {@link Spliterator#DISTINCT}. Overriding implementations should document * the reporting of additional characteristic values. * * @return a {@code Spliterator} over the elements in this set * @since 1.8 */
    public Spliterator<E> spliterator() {
        return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
    }
}

LinkHashSet源码

LinkHashSet继承了HashSet,所以Link的更简单
set的时候, 有提到一个default权限的构造方法,这个方法内部调用的就是LinkHashMap的构造方法

/** * Constructs a new, empty linked hash set. (This package private * constructor is only used by LinkedHashSet.) The backing * HashMap instance is a LinkedHashMap with the specified initial * capacity and the specified load factor. * 构造一个指定初始容量和加载因子的hash集合.这个构造函数是包访问权限,不对外公开,实际只是对linkedHashSet的支持. * * @param initialCapacity 初始容量 * @param loadFactor 加载因子 * @param dummy 标记ignored (distinguishes this * constructor from other int, float constructor.) * @throws IllegalArgumentException if the initial capacity is less * than zero, or if the load factor is nonpositive */
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

LinkHashSet,构造方法就4个,统一调用了父类的这个构造方法

HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

源码不放了,就是四个构造函数…

附: fina
《java集合(二)HashSet和LinkHashSet源码分析》

    原文作者:java集合源码分析
    原文地址: https://blog.csdn.net/kwy15732621629/article/details/80600704
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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