在使用HashSet時，應注意：一、自己的類有屬性，就應當重寫方法equals和hashCode；二、寫方法時要遵守的原則，兩個類equals返回true時，hashCode一定相等（考慮一下雜湊表的衝突就可以理解）；三、在使用時不得隨意修改HashSet中的值，否定將會導致結果不準確性。

import java.util.*;

class R
{
	public int count;
	public R (int count)
	{
		this.count = count;
	}
	public String toString()
	{
		return "R(count " + count + " )";
	}
	public boolean equals(Object obj)
	{
		if(obj instanceof R)
		{
			R r = (R)obj;
			if( r.count == this.count)
				return true;
		}
		return false;
	}
	public int hashCode()
	{
		return this.count;
	}
}

public class TestHashSet
{
	public static void main(String[] args)
	{
		HashSet hs = new HashSet();
		hs.add(new R(5));
		hs.add(new R(-3));
		hs.add(new R(9));
		hs.add(new R(-2));
		System.out.println(hs);
		Iterator it = hs.iterator();
		R first = (R)it.next();
		first.count = -3;//直接改變，最好先刪除再插入
		System.out.println(hs);
		hs.remove(new R(-3));//只查詢到了原先的-3
		System.out.println(hs);
		System.out.println("has -3 " + hs.contains(new R(-3)));//false
		System.out.println("has -5 " + hs.contains(new R(-5)));//false
	}
}