LinkedHashSet一文同

JAVA COLLECTIONS 源码

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1.什么是LinkedHashSet

• 迭代有序，非重复，非线程安全的HashSet，底层数据结构为LinkedHashMap
• 相比于HashSet，LinkedHashSet多维护着一个运行于所有条目的双重链接列表（doubly-linked list ）
• 此链接列表定义了迭代顺序，该迭代顺序可为插入顺序或是访问顺序（参见LinkedHashMap）
• 适用于存储有序的不重复值的场景，同时也是一个多态的典型实例

2.LinkedHashSet的数据结构

• 类定义

public class LinkedHashSet<E>
    extends HashSet<E>
    implements Set<E>, Cloneable, java.io.Serializable

• HashSet的friendly构造器 -- 重点： LinkedHashSet的实现原理

/**
  * 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.
  * LinkedHashSet独享该构造器，同时底层维护一个LinkedHashMap：
  * -- 借用其的迭代有序和key不重复特性
  * 因此在LinkedHashSet中只有其构造器的实现
  * @param      initialCapacity   the initial capacity of the hash map
  * @param      loadFactor        the load factor of the hash map
  * @param      dummy             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);
}

• 构造器

/**
  * Constructs a new, empty linked hash set with the specified initial
  * capacity and load factor.
  * 底层会调用父类的构造方法，构造一个有指定初始容量和加载因子的LinkedHashMap实例
  * @param      initialCapacity the initial capacity of the linked hash set
  * @param      loadFactor      the load factor of the linked hash set
  * @throws     IllegalArgumentException  if the initial capacity is less
  *               than zero, or if the load factor is nonpositive
  */
public LinkedHashSet(int initialCapacity, float loadFactor) {
    super(initialCapacity, loadFactor, true);
}
/**
  * Constructs a new, empty linked hash set with the specified initial
  * capacity and the default load factor (0.75).
  * 底层会调用父类的构造方法，构造一个带指定初始容量和默认加载因子0.75的LinkedHashMap实例
  * @param   initialCapacity   the initial capacity of the LinkedHashSet
  * @throws  IllegalArgumentException if the initial capacity is less
  *              than zero
  */
public LinkedHashSet(int initialCapacity) {
    super(initialCapacity, .75f, true);
}
/**
  * Constructs a new, empty linked hash set with the default initial
  * capacity (16) and load factor (0.75).
  * 底层会调用父类的构造方法，构造一个带默认初始容量16和加载因子0.75的LinkedHashMap实例
  */
public LinkedHashSet() {
    super(16, .75f, true);
}
/**
  * Constructs a new linked hash set with the same elements as the
  * specified collection.  The linked hash set is created with an initial
  * capacity sufficient to hold the elements in the specified collection
  * and the default load factor (0.75).
  * 底层会调用父类的构造方法，构造一个足以包含指定collection中所有元素的初始容量和加载因子为0.75的LinkedHashMap实例
  * @param c  the collection whose elements are to be placed into
  *           this set
  * @throws NullPointerException if the specified collection is null
  */
public LinkedHashSet(Collection<? extends E> c) {
    super(Math.max(2*c.size(), 11), .75f, true);
    addAll(c);
}