Java TreeSet插入对象疑问

Question

首先是Customer类定义：

package com.lzw;
import java.util.Comparator;
public class Customer implements Comparator<Object>{
private long id
private String name;
public Customer(String name,long id){
this.id=id;
this.name=name;
}
public int compare(Object o1,Object o2){
Customer c1=(Customer)o1;
Customer c2=(Customer)o2;
if(c1.getName().compareTo(c2.getName())>0) return -1;
if(c1.getName().compareTo(c2.getName())<0) return 1;
return 0;
}
public String getName(){
return name;
}
public long getId(){
return this.id;
}
}

然后是CustomerComparator类定义：

package com.lzw;
import com.lzw.Customer;
import java.util.Comparator;
import java.util.Set;
import java.util.TreeSet;
import java.util.Iterator;
public class CustomerComparator implements Comparator<Object>{
public int compare(Object o1,Object o2){
Customer c1=(Customer)o1;
Customer c2=(Customer)o2;
if(c1.getName().compareTo(c2.getName())>0) return -1;
if(c1.getName().compareTo(c2.getName()) return 0;
}
public static void main(String[] args) {
// TODO 自动生成的方法存根
TreeSet<Customer> set=new TreeSet<Customer>(new CustomerComparator());
Customer c1=new Customer("Andy",109201041);
Customer c2=new Customer("JACK",109207042);
Customer c3=new Customer("MIKE",109207043);
Customer c4=new Customer("Kath",109207044);
set.add(c1);
set.add(c2);
set.add(c3);
set.add(c4);
Iterator it=set.iterator();
while(it.hasNext()) {
Customer customer=(Customer)it.next();
System.out.println(customer.getName()+" "+customer.getId());
}
}
}

实例化对象：

  TreeSet<Customer> set=new TreeSet<Customer>(new CustomerComparator());

如果改成：

 TreeSet<customer> set=new TreeSet<customer>();

在 set.add(c1);语句时报错误：
Exception in thread "main" java.lang.ClassCastException: com.lzw.Customer cannot be cast to java.lang.Comparable
at java.util.TreeMap.compare(TreeMap.java:1290)
at java.util.TreeMap.put(TreeMap.java:538)
at java.util.TreeSet.add(TreeSet.java:255)
at com.lzw.CustomerComparator.main(CustomerComparator.java:24)
问题：1、请问为什么会这样？2、TreeSet的集合元素是Customer对象，我也在Customer类实现了compare的方法，为什么在add的时候不是调用这个方法而是在CustomerComparator实现的compare的方法？

Answer 1

 public class Customer implements Comparator
 改成
 public class Customer implements Comparable

 如果treeset中通过构造初始化的，Comparator。
 如果构造函数没有初始化Comparator，那么add进入的对象必须实现Comparable才行，不然无法转化对象。