HashSet和TreeSet中最好只放入不可变对象

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class UN implements Comparable {     private final int count;     public UN()     {         this.count=0;     }     public UN(int count)     {         this.count=count;     }     public int getCount()     {         return this.count;     }     public String toString()     {         return "R[count:"+count+"]";     }     public boolean equals(Object obj)     {         if(this==obj)         {             return true;         }         if(obj!=null&&obj.getClass()==UN.class)         {             UN r=(UN)obj;             if(r.count==this.count)             {                 return true;             }         }         return false;     }     public int compareTo(Object obj)     {         UN r=(UN)obj;         return count>r.count? 1:count<r.count? -1:0;     } }