如果加入TreeSet和TreeMap的元素没有实现comprable中的compareTo()方法，那么会报错“treeset cannot be cast to java.lang.Comparable”.
因此需要对元素类型实现comparable借口，并实现compareTo()方法，即告诉TreeSet和TreeMap你要怎么排列，它才能进行排序。
基础类：
package niukewang;
import java.util.Objects;
public class setClass implements Comparable<setClass>{
String a;
String b;
public setClass(String a, String b)
{
this.a=a;
this.b=b;
}
public int hashCode() {
return a.hashCode();
}
public boolean equals(Object obj)
{
if(obj==null) return false;
if(getClass() != obj.getClass()) return false;
final setClass s=(setClass)obj;
return Objects.equals(this.a, s.a);
}
@Override
public int compareTo(setClass o) {
int num=this.b.compareTo(o.b);
return num;
}
}
测试类：
package niukewang;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;
import java.util.TreeSet;
import java.util.Vector;
public class test1 {
public static void main(String args[])
{
setClass s1=new setClass("http://www.yjbys.com/", "same0");
setClass s2=new setClass("http://www.yjbys.com1/", "same0");
setClass s3=new setClass("http://www.yjbys.com2/", "same2");
setClass s4=new setClass("http://www.yjbys.com2/", "same3");
//hasSet
System.out.println("hasSet......");
Set<setClass> set=new HashSet<setClass>();
set.add(s1);
set.add(s2);
set.add(s3);
set.add(s4);
Iterator it= set.iterator();
while(it.hasNext())
{
setClass ob=(setClass)it.next();
System.out.println(ob.a);
}
//TreeSet
System.out.println("\nTreeSet........");
Set<setClass> tree=new TreeSet<setClass>();
tree.add(s1);
tree.add(s2);
tree.add(s3);
tree.add(s4);
Iterator treeit=tree.iterator();
while(treeit.hasNext())
{
setClass ob=(setClass) treeit.next();
System.out.println(ob.a);
}
//TreeMap
System.out.println("\nTreeMap.........");
Map<setClass,String> treemap=new TreeMap<setClass,String>();
treemap.put(s1, "TreeMap one");
treemap.put(s2, "TreeMap two");
treemap.put(s3, "TreeMap three");
treemap.put(s4, "TreeMap four");
for(Map.Entry<setClass, String> entry: treemap.entrySet())
{
System.out.println("The treemap key is "+entry.getKey().a+" The value is "+ entry.getValue());
}
//HasMap
System.out.println("\nHashMap......");
Map<setClass,String> hashmap=new HashMap<setClass, String>();
hashmap.put(s1, "HashMap one");
hashmap.put(s2, "HashMap two");
hashmap.put(s3, "HashMap three");
hashmap.put(s4, "HashMap four");
for(Map.Entry<setClass, String> entry : hashmap.entrySet())
{
System.out.println("The key is "+entry.getKey().a+" The value is "+entry.getValue());
}
//HasTable
System.out.println("\nHashTable......");
Map<setClass,String> hashtable=new Hashtable<setClass, String>();
hashtable.put(s1, "HashTable one");
hashtable.put(s2, "HashTable two");
hashtable.put(s3, "HashTable three");
hashtable.put(s4, "HashTable four");
for(Map.Entry<setClass, String> entry: hashtable.entrySet())
{
System.out.println("The HashTable key is "+entry.getKey().a+" The HashTable value is "+entry.getValue());
}
//LinkedList
System.out.println("\nLinkedList.....");
List list=new LinkedList();
list.add(s1);
list.add(s2);
list.add(s3);
list.add(s4);
Iterator listit=list.iterator();
while(listit.hasNext())
{
setClass ob=(setClass)listit.next();
System.out.println(ob.a);
}
//ArrayList
System.out.println("\nArrayList.....");
List array=new ArrayList();
array.add(s1);
array.add(s2);
array.add(s3);
array.add(s4);
Iterator arrayit=array.iterator();
while(arrayit.hasNext())
{
setClass ob=(setClass)arrayit.next();
System.out.println(ob.a);
}
//vector
System.out.println("\nvector.....");
Vector v=new Vector();
v.add(s1);
v.add(s2);
v.add(s3);
v.add(s4);
Iterator vit=v.iterator();
while(vit.hasNext())
{
setClass ob=(setClass)vit.next();
System.out.println(ob.a);
}
}
}
输出结果：
hasSet...... http://www.yjbys.com1/ http://www.yjbys.com/ http://www.yjbys.com2/ TreeSet........ http://www.yjbys.com/ http://www.yjbys.com2/ http://www.yjbys.com2/ TreeMap......... The treemap key is http://www.yjbys.com/ The value is TreeMap two The treemap key is http://www.yjbys.com2/ The value is TreeMap three The treemap key is http://www.yjbys.com2/ The value is TreeMap four HashMap...... The key is http://www.yjbys.com1/ The value is HashMap two The key is http://www.yjbys.com/ The value is HashMap one The key is http://www.yjbys.com2/ The value is HashMap four HashTable...... The HashTable key is http://www.yjbys.com1/ The HashTable value is HashTable two The HashTable key is http://www.yjbys.com/ The HashTable value is HashTable one The HashTable key is http://www.yjbys.com2/ The HashTable value is HashTable four LinkedList..... http://www.yjbys.com/ http://www.yjbys.com1/ http://www.yjbys.com2/ http://www.yjbys.com2/ ArrayList..... http://www.yjbys.com/ http://www.yjbys.com1/ http://www.yjbys.com2/ http://www.yjbys.com2/ vector..... http://www.yjbys.com/ http://www.yjbys.com1/ http://www.yjbys.com2/ http://www.yjbys.com2/
TreeSet并不像hashSet那样，用hashCode和equals取决于元素是否相等。
而是取决于compareTo()方法中指定的排序的方式；
setClass s1=new setClass("http://www.yjbys.com/", "same0");
setClass s2=new setClass("http://www.yjbys.com1/", "same0");
setClass s3=new setClass("http://www.yjbys.com2/", "same2");
setClass s4=new setClass("http://www.yjbys.com2/", "same3");
如果用hashCode和equals取决于元素是否相等，那么s3和s4是相等的；
而此时候，是根据compareTo()方法中的第二个字符串b来判断是否相等，则s1与s2相等，s3和s4不相等，所以才有这样的输出：
TreeSet........
http://www.yjbys.com/
http://www.yjbys.com2/
http://www.yjbys.com2/
时间： 2024-10-13 23:00:27