学步园

        在使用HashSet时，应注意：一、自己的类有属性，就应当重写方法equals和hashCode；二、写方法时要遵守的原则，两个类equals返回true时，hashCode一定相等（考虑一下哈希表的冲突就可以理解）；三、在使用时不得随意修改HashSet中的值，否定将会导致结果不准确性。

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;//直接改变，最好先删除再插入
		System.out.println(hs);
		hs.remove(new R(-3));//只查找到了原先的-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
	}
}