重载equals方法就一定要要重载hashCode方法

    在进行对象之间的比较的时候，一般都会override equals方法，而hashCode的override很多时候都会被忽略。hashCode方法一般都被认为是用来判断两个对象是不是同一个引用的，它返回一串散列码，这个散列码的主要用途是进行Hash存储时，作为键用的，加快查找对象的速度。一般都会以31 29这样的素数作为基数，主要目的是为了减少在一个大的集合中键值的重复，导致冲突的情况发生，以提高性能。
有关深入HashCode方法详见：http://blog.csdn.net/axman/archive/2004/11/26/195159.aspx

下面有个Bean，在只重载了equals方法后，执行插入Set后的个数和重载了equals与hashCode方法两个方法后的Set集合中的对象个数：

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

public class ListSet
{
	public static void main(String... a)
	{
		Set<UserBean> set = new HashSet<UserBean>();
		Date date = new Date();
		UserBean ub;
		for (int i = 0; i < 20; i++)
		{
			ub = new UserBean();
			ub.setUsername("javaeye");
			ub.setPassword("javaeye");
			ub.setSex(i % 2);
			ub.setBirthDay(date);
			set.add(ub);
		}
		System.out.println("Set中的UserBean对象个数:" + set.size());
	}
}

class UserBean
{
	private String username;
	private String password;
	private int sex;
	private Date birthDay;

	@Override
	public boolean equals(Object obj)
	{
		if (this == obj) return true;
		if (obj == null) return false;
		if (getClass() != obj.getClass()) return false;
		final UserBean other = (UserBean) obj;
		if (birthDay == null)
		{
			if (other.birthDay != null) return false;
		}
		else if (!birthDay.equals(other.birthDay)) return false;
		if (password == null)
		{
			if (other.password != null) return false;
		}
		else if (!password.equals(other.password)) return false;
		if (sex != other.sex) return false;
		if (username == null)
		{
			if (other.username != null) return false;
		}
		else if (!username.equals(other.username)) return false;
		return true;
	}
	public String getUsername()
	{
		return username;
	}
	public void setUsername(String username)
	{
		this.username = username;
	}
	public String getPassword()
	{
		return password;
	}
	public void setPassword(String password)
	{
		this.password = password;
	}
	public int getSex()
	{
		return sex;
	}
	public void setSex(int sex)
	{
		this.sex = sex;
	}
	public Date getBirthDay()
	{
		return birthDay;
	}
	public void setBirthDay(Date birthDay)
	{
		this.birthDay = birthDay;
	}
}

执行结果是：Set中的UserBean对象个数:20
而加了hashCode方法后：

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

public class ListSet
{
	public static void main(String... a)
	{
		Set<UserBean> set = new HashSet<UserBean>();
		Date date = new Date();
		UserBean ub;
		for (int i = 0; i < 20; i++)
		{
			ub = new UserBean();
			ub.setUsername("javaeye");
			ub.setPassword("javaeye");
			ub.setSex(i % 2);
			ub.setBirthDay(date);
			set.add(ub);
		}
		System.out.println("Set中的UserBean对象个数:" + set.size());
	}
}

class UserBean
{
	private String username;
	private String password;
	private int sex;
	private Date birthDay;

	@Override
	public int hashCode()
	{
		final int prime = 31;
		int result = 1;
		result = prime * result + ((birthDay == null) ? 0 : birthDay.hashCode());
		result = prime * result + ((password == null) ? 0 : password.hashCode());
		result = prime * result + sex;
		result = prime * result + ((username == null) ? 0 : username.hashCode());
		return result;
	}
	@Override
	public boolean equals(Object obj)
	{
		if (this == obj) return true;
		if (obj == null) return false;
		if (getClass() != obj.getClass()) return false;
		final UserBean other = (UserBean) obj;
		if (birthDay == null)
		{
			if (other.birthDay != null) return false;
		}
		else if (!birthDay.equals(other.birthDay)) return false;
		if (password == null)
		{
			if (other.password != null) return false;
		}
		else if (!password.equals(other.password)) return false;
		if (sex != other.sex) return false;
		if (username == null)
		{
			if (other.username != null) return false;
		}
		else if (!username.equals(other.username)) return false;
		return true;
	}
	public String getUsername()
	{
		return username;
	}
	public void setUsername(String username)
	{
		this.username = username;
	}
	public String getPassword()
	{
		return password;
	}
	public void setPassword(String password)
	{
		this.password = password;
	}
	public int getSex()
	{
		return sex;
	}
	public void setSex(int sex)
	{
		this.sex = sex;
	}
	public Date getBirthDay()
	{
		return birthDay;
	}
	public void setBirthDay(Date birthDay)
	{
		this.birthDay = birthDay;
	}
}

执行结果是：Set中的UserBean对象个数:2