java list中的对象去重原理

/*******************************************************************************
 *
 * Copyright (c) Weaver Info Tech Co. Ltd
 *
 * SessionInfo
 *
 * app.backend.model.SessionInfo.java
 * TODO: File description or class description.
 *
 * @author: Administrator
 * @since:  2014-5-26
 * @version: 1.0.0
 *
 * @changeLogs:
 *     1.0.0: First created this class.
 *
 ******************************************************************************/  

import java.io.Serializable;  

/**
 * @author Administrator
 *
 */
@SuppressWarnings("serial")
public class SessionInfo implements Serializable{
    private int id;
    private String url;
    public int getId() {  

        return id;
    }
    public void setId(int id) {  

        this.id = id;
    }  

    public String getUrl() {  

        return url;
    }

    @Override
    public String toString() {
        return "SessionInfo{" +
                "id=" + id +
                ", url='" + url + '\'' +
                '}';
    }

    public void setUrl(String url) {

        this.url = url;
    }  

    /* (non-Javadoc)
     * @see java.lang.Object#hashCode()
     */
    @Override
 public int hashCode() {
        return id;
    }  

    /* (non-Javadoc)
     * @see java.lang.Object#equals(java.lang.Object)
     */
    @Override
 public boolean equals(Object o) {
        if (o == null) {
            return false;
        } else {
            if (o.getClass() != this.getClass()) {
                return false;
            } else {
                final SessionInfo s = (SessionInfo) o;
                return s.id == this.id;
            }
        }
    }
}  

import java.util.*;

public class HashCodeTest {  

    public static void main(String[] args){  

        SessionInfo sessionInfo1 = new SessionInfo();
        SessionInfo sessionInfo2 = new SessionInfo();
        sessionInfo1.setId(1);
        sessionInfo2.setId(1);
        sessionInfo1.setUrl("test");
        sessionInfo2.setUrl("test");

        SessionInfo mSessionInfo = null;                    //实体类对象
        Set<SessionInfo> sessionList = new HashSet<SessionInfo>();  //声明set集合对象
        final List<SessionInfo> list = new ArrayList<SessionInfo>();//声明list集合对象

        sessionList.add(sessionInfo1);//将对象添加到Set集合中
        sessionList.add(sessionInfo2);//将对象添加到Set集合中
        list.addAll(sessionList);//将数据添加到list集合对象

        for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i));
        }

    }

}