HashSet存储元素equals() hashCode()方法重写

  set是类似于数学概念中“集合“的无顺序、不可重复的数据结构。

  其常用实现类是HashSet，HashSet在存储一般java定义的数据类型如String、Integer时，这些类型是已经重写了equals()、hashCode()方法的，所以存储时如果是相同内容的元素只会存储一遍，不会重复存储。但是用户自己定义的类就如要自己重写equals()和hasCode()方法。

  如例子：

user类:

public class User {

private String id;

private String userName;

private String password;

private String createTime;

private String roleId;

private String rolename;

private String isDel;

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((id == null) ? 0 : id.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;
User other = (User) obj;
if (id == null) {
if (other.id != null)
return false;
} else if (!id.equals(other.id))
return false;
return true;
}

public String getId() {
return id;
}

public void setId(String id) {
this.id = id;
}

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 String getCreateTime() {
return createTime;
}

public void setCreateTime(String createTime) {
this.createTime = createTime;
}

public String getRolename() {
return rolename;
}

public void setRolename(String rolename) {
this.rolename = rolename;
}

public String getIsDel() {
return isDel;
}

public void setIsDel(String isDel) {
this.isDel = isDel;
}

public String getRoleId() {
return roleId;
}

public void setRoleId(String roleId) {
this.roleId = roleId;
}

}

package com.zhu.test;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.Set;

import com.zhu.model.TSet;
import com.zhu.model.User;

public class TestSet {

public static void main(String[] args) {
Set<User> set = new HashSet<User>();
User user1 = new User();
User user2 = new User();
user1.setId("11");
user2.setId("11");
System.out.println("user1code:"+user1.hashCode()+"---user2code:"+user2.hashCode());
System.out.println(user1.equals(user2)?"yes":"no");
set.add(user1);
set.add(user2);
System.out.println(set.size());
}
}