前言
前言点击此处查看:
http://blog.csdn.net/wang7807564/article/details/79113195
Set
Set的特点就是每个元素都是唯一的,Set集合内两两元素之间进行equals()对比,结果都为false.Set集合也可以包含null元素。
HashSet
HashSet是常用的一种实现了Set接口的类,其底层实现原理是基于哈希表的,其随机访问时的效率很高,但是HashSet不保证集合中数据存储的顺序。
HashSet的继承关系类图如下所示:
HashSet的底层是实现的哈希表,对应的数据存储实际上时使用了集合类中Map接口的一个实现类HashMap.HashMap本身就是一个哈希表的实现类。HashSet用作存储数据的HashMap的Key是被存储在HashSet中的元素,Value是一个Object对象,其源码比较简单,就是封装了对HashMap的操作,源码如下:
public class HashSet<E>
extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable
{
static final long serialVersionUID = -5024744406713321676L;
private transient HashMap<E,Object> map;
private static final Object PRESENT = new Object();
public HashSet() {
map = new HashMap<>();
}
public HashSet(Collection<? extends E> c) {
map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
addAll(c);
}
public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}
public HashSet(int initialCapacity) {
map = new HashMap<>(initialCapacity);
}
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}
public Iterator<E> iterator() {
return map.keySet().iterator();
}
public int size() {
return map.size();
}
public boolean isEmpty() {
return map.isEmpty();
}
public boolean contains(Object o) {
return map.containsKey(o);
}
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}
public void clear() {
map.clear();
}
@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);
}
}
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);
// 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);
}
}
TreeSet
与HashSet相比,TreeSet的实现要更为复杂一些。其底层实现的数据结构是一棵平衡树,这个平衡树就是红黑树。其底层依赖K-V对的数据结构,与HashSet类似,其将Map结构的Key用作存储元素的值,其Value都存储一个Object对象的引用。TreeSet的依赖的是后面将要涉及到的TreeMap.
TreeMap的实现就是红黑树数据结构,也就是一棵自平衡的排序二叉树,这样就可以保证当需要快速检索指定节点。
其底层实现原理也是在TreeMap的基础上实现的:封装原理与HashSet类似,底层内容参见TreeMap.
EnumSet
EnumSet是一个抽象类,它继承了AbstractSet,AbstractSet类实现了Set接口。
可以认为EnumSet是存储枚举类型的集合,例如下面的代码:
public static void main(String[] args){
EnumSet e = EnumSet.noneOf(Season.class);
System.out.println(e);
e.add(Season.AUTUMN);
e.add(Season.SPRING);
System.out.println(e);
}
其中,Season是一个枚举类型,定义在:
enum Season {
SPRING, SUMMER, AUTUMN, WINTER;
}
输出结果:
[]
[SPRING, AUTUMN]
这个类不可以通过构造器来实例化,可以通过工厂方法来获取到EnumSet的实例,例如noneOf()方法可以通过反射来获取到Enum,并且返回EnumSet子类的实例,这部分的源代码为:
public static <E extends Enum<E>> EnumSet<E> noneOf(Class<E> elementType) {
Enum<?>[] universe = getUniverse(elementType);
if (universe == null)
throw new ClassCastException(elementType + " not an enum");
if (universe.length <= 64)
return new RegularEnumSet<>(elementType, universe);
//继承自 EnumSet,下同。
else
return new JumboEnumSet<>(elementType, universe);
}