Java-Collections Framework学习与总结-HashSet和LinkedHashSet

本篇总结一下两个常用的集合类HashSet和LinkedHashSet。

它们都实现了相同接口java.util.Set。Set表示一种元素无序且不可重复的集合；之前总结过的java.util.List表示一种元素可重复且有序的集合。它们都扩展自java.util.Collection。

Java代码


public interface Set<E> extends Collection<E> {
int size();
boolean isEmpty();
boolean contains(Object o);
Iterator<E> iterator();
Object[] toArray();
<T> T[] toArray(T[] a);
boolean add(E e);
boolean remove(Object o);
boolean containsAll(Collection<?> c);
boolean addAll(Collection<? extends E> c);
boolean retainAll(Collection<?> c);
boolean removeAll(Collection<?> c);
void clear();
boolean equals(Object o);
int hashCode();

可以看到Set的接口描述，和List比，只少了一些与下标有关的特性。

先来看一下HashSet的实现方式。

Java代码


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

HashSet是可克隆、可序列化的；实现了java.util.Set；继承了java.util.AbstractSet，AbstractSet包含一些骨架方法，很容易看懂。继续往下看。

Java代码


private transient HashMap<E,Object> map;

// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();

看到这里就基本上明白是怎么实现的了吧！原来HashSet内部是一个HashMap，难怪HashSet用起来很像一个HashMap，只是没有value。所以PRESENT这个对象就用来填充value值了。有了前面HashMap的分析，后面的基本上不用看了。

不过有一个小地方要注意一下。

Java代码


/**
* 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<E,Object>(initialCapacity, loadFactor);
}

这个构造方法和其他的不一样。里面的Map实现是LinkedHashMap。从注释上可以看到，这个方法只是由LinkedHashSet使用，方法的第三个参数也只是用来与其他构造方法进行区分，代码中并没有实际用到。

接下来看一下java.util.LinkedHashSet的实现吧。

Java代码


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

private static final long serialVersionUID = -2851667679971038690L;

/**
* Constructs a new, empty linked hash set with the specified initial
* capacity and load factor.
*
* @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).
*
* @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).
*/
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).
*
* @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);
}
}

以上便是LinkedHashSet的全部代码了，基本上没什么可说的了，参考下LinkedHashMap的总结吧