HashSet
此類(lèi)實(shí)現(xiàn) Set 接口，由哈希表（實(shí)際上是一個(gè) HashMap 實(shí)例）支持。它不保證集合的迭代順序；特別是它不保證該順序恒久不變。此類(lèi)允許使用 null 元素。
此類(lèi)為基本操作提供了穩(wěn)定性能，這些基本操作包括 add、remove、contains 和 size，假定哈希函數(shù)將這些元素正確地分布在桶中。對(duì)此集合進(jìn)行迭代所需的時(shí)間與 HashSet 實(shí)例的大?。ㄔ氐臄?shù)量）和底層 HashMap 實(shí)例（桶的數(shù)量）的“容量”的和成比例。因此，如果迭代性能很重要，則不要將初始容量設(shè)置得太高（或?qū)⒓虞d因子設(shè)置得太低）。
我們應(yīng)該為要存放到散列表的各個(gè)對(duì)象定義hashCode()和equals();
import java.util.HashSet;
import java.util.Iterator;

public class HashSetTest {
           public static void main(String[] args)
           {
                   HashSet hs=new HashSet();
                   /*hs.add("one");
                   hs.add("two");
                   hs.add("three");
                   hs.add("four");*/
                   hs.add(new Student(1,"zhangsan"));
                   hs.add(new Student(2,"lishi"));
                   hs.add(new Student(3,"wangwu"));
                   hs.add(new Student(1,"zhangsan"));
                
                   Iterator it=hs.iterator();
                   while(it.hasNext())
                   {
                           System.out.println(it.next());
                   }
           }
}

//HashSet要重寫(xiě)hashCode和equals方法
class Student        

{
           int num;
           String name;
           Student(int num,String name)
           {
                   this.num=num;
                   this.name=name;
           }
           public String toString()
           {
                   return "num :"+num+" name:"+name;
           }
           public int hashCode()
           {
                   return num*name.hashCode();
           }
           public boolean equals(Object o)
           {
                   Student s=(Student)o;
                   return num==s.num && name.equals(s.name);
           }
}

TreeSet
此類(lèi)實(shí)現(xiàn) Set 接口，該接口由 TreeMap 實(shí)例支持。此類(lèi)保證排序后的 set 按照升序排列元素，根據(jù)使用的構(gòu)造方法不同，可能會(huì)按照元素的自然順序 進(jìn)行排序，或按照在創(chuàng)建 set 時(shí)所提供的比較器進(jìn)行排序。
是一個(gè)有序集合,元素中安升序排序,缺省是按照自然順序進(jìn)行排序,意味著TreeSet中元素要實(shí)現(xiàn)Comparable接口;
我們可以構(gòu)造TreeSet對(duì)象時(shí),傳遞實(shí)現(xiàn)了Comparator接口的比較器對(duì)象.

import java.util.*;

public class TreeSetTest {
           public static void main(String[] args)
           {
                   //TreeSet ts=new TreeSet();
                   TreeSet ts=new TreeSet(new Students.compareToStudent());
                   ts.add(new Students(2,"zhangshan"));
                   ts.add(new Students(3,"lishi"));
                   ts.add(new Students(1,"wangwu"));
                   ts.add(new Students(4,"maliu"));
                
                   Iterator it=ts.iterator();
                   while(it.hasNext())
                   {
                           System.out.println(it.next());
                   }                
           }

}

class Students implements Comparable
{
           int num;
           String name;
           Students(int num,String name)
           {
                   this.num=num;
                   this.name=name;
           }

//定義一個(gè)內(nèi)部類(lèi)來(lái)實(shí)現(xiàn)比較器

           static class compareToStudent implements Comparator        

           {
                   public int compare(Object o1, Object o2) {
                           Students s1=(Students)o1;
                           Students s2=(Students)o2;
                           int rulst= s1.num > s2.num ? 1 : (s1.num==s2.num ? 0 :-1);
                           if(rulst==0)
                           {
                                   rulst=s1.name.compareTo(s2.name);
                           }                
                           return rulst;
                   }                
           }

//寫(xiě)具體的比較方法

           public int compareTo(Object o)                

           {
                   int result;
                   Students s=(Students)o;
                   result=num >s.num ? 1:(num==s.num ? 0 : -1);
                   if(result==0)
                   {
                           result=name.compareTo(s.name);
                   }
                   return result;
           }
           public String toString()
           {
                   return num+":"+name;
           }
}

HashSet是基于Hash算法實(shí)現(xiàn)的,其性能通常優(yōu)于TreeSet,我們通常都應(yīng)該使用HashSet,在我們需要排序的功能時(shí),我門(mén)才使用TreeSet;

說(shuō)了這么多廢話就是一點(diǎn)compareTo這個(gè)方法，當(dāng)?shù)扔诘臅r(shí)候就返回0，當(dāng)大于就返回1，當(dāng)小于就返回-1，別發(fā)呆，就這么簡(jiǎn)單，有幫你排一排和亂亂的而已；