重载equals方法就一定要要重载hashCode方法

转载于:http://songhongchen.iteye.com/blog/365029

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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

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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

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具体代码可以参照我的github，里面还带有TreeSet compare的方法
https://github.com/jinzhencs/MyJavaBasic/tree/master/src/com/myjavabasic/java/OverrideEqualsAndHashCode