Java集合之Set

一 综述

Set集合类似 于数学中的集合，不允许有重复元素。它判断对象是否相等是根据equals方法。Set继承了Collection的方法，没有另外添加方法。经常使用的是HashSet、TreeSet。

1.HashSet ：按照hash算法存储元素，因此查找和存取元素性能较高。

（1）HashSet不是线程同步的；

 （2）元素顺序是变化的；

 （3）元素的值可以是null，但是只能有一个；

 （4）判断元素相等不能不仅仅是equals相等，还有hashcode相等。hashcode决定元素的存储位置。

HashSet有个子类LinkedHashSet，使用链表维护集合的次序。因此插入元素的性能低于HashSet，但是迭代元素的性能较高。LinkedHashSet元素的顺序总是于元素的添加顺序一致。如下面代码所示：

HashSet<String> hlSet = new LinkedHashSet<String>(); hlSet.add("a"); hlSet.add("b"); hlSet.add("c"); hlSet.add("d");  //输出：LinkedHashSet---->[a, b, c, d] System.out.println("LinkedHashSet---->"+hlSet);  HashSet<String> hSet = new HashSet<String>(); hSet.add("a"); hSet.add("b"); hSet.add("c"); hSet.add("d"); //输出：HashSet---->[d, b, c, a] System.out.println("HashSet---->"+hSet);

2.TreeSet中元素处于排序状态，它提供了几个额外的方法：

修饰符与类型方法与描述booleanadd(E e)

Adds the specified element to this set if it is not already present.

booleanaddAll(Collection<? extendsE> c)

Adds all of the elements in the specified collection to this set.

Eceiling(E e)

Returns the least element in this set greater than or equal to the given element, ornull if there is no such element.

voidclear()

Removes all of the elements from this set.

Objectclone()

Returns a shallow copy of this TreeSet instance.

Comparator<? superE>comparator()

Returns the comparator used to order the elements in this set, ornull if this set uses thenatural ordering of its elements.

booleancontains(Object o)

Returns true if this set contains the specified element.

Iterator<E>descendingIterator()

Returns an iterator over the elements in this set in descending order.

NavigableSet<E>descendingSet()

Returns a reverse order view of the elements contained in this set.

Efirst()

Returns the first (lowest) element currently in this set.

Efloor(E e)

Returns the greatest element in this set less than or equal to the given element, ornull if there is no such element.

SortedSet<E>headSet(E toElement)

Returns a view of the portion of this set whose elements are strictly less thantoElement.

NavigableSet<E>headSet(E toElement, boolean inclusive)

Returns a view of the portion of this set whose elements are less than (or equal to, ifinclusive is true)toElement.

Ehigher(E e)

Returns the least element in this set strictly greater than the given element, ornull if there is no such element.

booleanisEmpty()

Returns true if this set contains no elements.

Iterator<E>iterator()

Returns an iterator over the elements in this set in ascending order.

Elast()

Returns the last (highest) element currently in this set.

Elower(E e)

Returns the greatest element in this set strictly less than the given element, ornull if there is no such element.

EpollFirst()

Retrieves and removes the first (lowest) element, or returns null if this set is empty.

EpollLast()

Retrieves and removes the last (highest) element, or returns null if this set is empty.

booleanremove(Object o)

Removes the specified element from this set if it is present.

intsize()

Returns the number of elements in this set (its cardinality).

NavigableSet<E>subSet(E fromElement, boolean fromInclusive,E toElement, boolean toInclusive)

Returns a view of the portion of this set whose elements range fromfromElement totoElement.

SortedSet<E>subSet(E fromElement,E toElement)

Returns a view of the portion of this set whose elements range fromfromElement, inclusive, totoElement, exclusive.

SortedSet<E>tailSet(E fromElement)

Returns a view of the portion of this set whose elements are greater than or equal tofromElement.

NavigableSet<E>tailSet(E fromElement, boolean inclusive)

Returns a view of the portion of this set whose elements are greater than (or equal to, ifinclusive is true)fromElement

重要方法代码如下：

TreeSet<Integer> tSet = new TreeSet<Integer>();  tSet.add(1);  tSet.add(2);  tSet.add(3);  tSet.add(4);  tSet.add(5);  tSet.add(6);    System.out.println("first----->"+tSet.first());  //first----->1  System.out.println("last----->"+tSet.last());  //last----->6  System.out.println("lower----->"+tSet.lower(5)); //lower----->4  System.out.println("higher----->"+tSet.higher(2));  //higher----->3  System.out.println("subSet----->"+tSet.subSet(2, 5)); //subSet----->[2, 3, 4]  System.out.println("headSet----->"+tSet.headSet(3)); //headSet----->[1, 2]  System.out.println("tailSet----->"+tSet.tailSet(3));    //tailSet----->[3, 4, 5, 6]    //输出：----->6----->5----->4----->3----->2----->1  Iterator<Integer> itDes= tSet.descendingIterator();  while(itDes.hasNext())  {  System.out.print("----->"+itDes.next());  }  System.out.println();

TreeSet支持两种排序

（1）自然排序：调用compareTo方法来比较元素之间的大小。Java的常用类大都实现了Comparable接口，提供了比较大小的方法：

 1》BigDecimal、BigInteger及所有数值型类：按照对应的数值大小比较；

2》Character：按照Unicode码值进行比较；

3》Boolean：True大于False；

4》String：按照字符串中的Unicode值比较；

5》Date、Time：后面的日期时间比前面的大。

向TreeSet中插入元素时，只有第一个不需要实现Comparable接口，后面的都需要实现，否则会报ClassCastException。

（2）定制排序：定制排序需要实现Comparable接口，例如逆序。

public class TreeSetTest3 {        public static void main(String[] args) {          Set<String> set = new TreeSet<String>(new MyComparator());                    set.add("a");          set.add("b");          set.add("c");          set.add("d");          set.add("e");          set.add("A");                    for(Iterator<String> iterator = set.iterator();iterator.hasNext();){              System.out.print(iterator.next()+" "); //输出：<span style="font-size:18px;color:#3366ff;"><strong>e d c b a A </strong></span>         }      }  }    class MyComparator implements Comparator<String>{        @Override      public int compare(String o1, String o2) {                    return o2.compareTo(o1);//降序排列     }        }  

自定义类型的排序：

    public class TreeSetTest2 {                public static void main(String[] args) {              Set<Person> set = new TreeSet<Person>(new PersonComparator());                            Person p1 =  new Person(10);              Person p2 =  new Person(20);              Person p3 =  new Person(30);              Person p4 =  new Person(40);                            set.add(p1);              set.add(p2);              set.add(p3);              set.add(p4);                            for(Iterator<Person> iterator = set.iterator();iterator.hasNext();){                  System.out.print(iterator.next().score+" "); //输出：10,20,30,40             }          }      }            class Person{          int score;                    public Person(int score){              this.score = score;          }                    public String toString(){              return String.valueOf(this.score);          }      }            class PersonComparator implements Comparator<Person>{                @Override          public int compare(Person o1, Person o2) {                            return o1.score - o2.score;          }                }  

2、HashSet的性能总是要好于TreeSet，因为TreeSet要维护红黑树。HashSet、TreeSet不是线程同步的。