Set集合之LinkedHashSet

ゝ一世哀愁。 2023-10-04 12:05 49阅读 0赞

# 源码分析 #

package java.util;
    
    public class LinkedHashSet<E>
        extends HashSet<E>
        implements Set<E>, Cloneable, java.io.Serializable {
    
        private static final long serialVersionUID = -2851667679971038690L;
    
        
        public LinkedHashSet(int initialCapacity, float loadFactor) {
            super(initialCapacity, loadFactor, true);
        }
    
        
        public LinkedHashSet(int initialCapacity) {
            super(initialCapacity, .75f, true);
        }
    
       
        public LinkedHashSet() {
            super(16, .75f, true);
        }
    
       
        public LinkedHashSet(Collection<? extends E> c) {
            super(Math.max(2*c.size(), 11), .75f, true);
            addAll(c);
        }
    
        
        @Override
        public Spliterator<E> spliterator() {
            return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
        }
    }

> LinkedHashSet继承自HashSet，实现了Set,Cloneable,Serializable接口。
> 
> 从LinkedHashSet源码可以知道，LinkedHashSet的实现完全依赖于HashSet。因为LinkedHashSet没有提供关于Map集合的常用方法实现。
> 
> LinkedHashSet的构造器都依赖于HashSet的三个形参的构造器，即如下的HashSet构造器

/**
         * 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.
         *
         * @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);
        }

> 从HashSet该构造器可以看出，其内部实现调用了LinkedHashMap(int initialCapacity,float loadFactor)构造器

/**
         * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
         * with the specified initial capacity and load factor.
         *
         * @param  initialCapacity the initial capacity
         * @param  loadFactor      the load factor
         * @throws IllegalArgumentException if the initial capacity is negative
         *         or the load factor is nonpositive
         */
        public LinkedHashMap(int initialCapacity, float loadFactor) {
            super(initialCapacity, loadFactor);
            accessOrder = false;
        }

# 结论 #

> LinkedHashSet虽然名义上继承自HashSet，但是实质上底层却是accessOrder==false的LinkedHashMap。所以LinkedHashSet具有插入顺序的LinkedHashMap的特性。
> 
> 1.key值不可重复（即LinkedHashSet的集合元素不可重复）
> 
> 2.key值有序，为插入顺序
> 
> 3.LinkedHashSet适合整体遍历，不适合做频繁的插入和删除操作