为什么要重写equals和hashCode方法

package com.wuyu.java;

	import java.util.HashMap;
	
	   class Key {
	       private Integer id;
	      public Integer getId()
	  {return id; }
	       public Key(Integer id)
	   {this.id = id;  }
	  // 故意先注释掉equals和hashCode方法
	    public boolean equals(Object o) {
	        if (o == null || !(o instanceof Key))
	       { return false; }
	        else
	       { return this.getId().equals(((Key) o).getId());}
	   }
	     
	    public int hashCode()
	  { return id.hashCode(); }
	  }	 
	  public class Demo01 {
	      public static void main(String[] args) {
	          Key k1 = new Key(1);
	         Key k2 = new Key(1);
	          HashMap<Key,String> hm = new HashMap<Key,String>();
	          hm.put(k1, "Key with id is 1");    
	          System.out.println(hm.get(k2));    
	      }
	  }