Java 从Set里面取出有序的记录详解及实例

java 从set里面取出有序的记录详解及实例

set里面的记录是无序的，如果想使用set，然后又想里面的记录是有序的，就可以使用treeset，而不是hashset，在使用treeset的时候，里面的元素必须是实现了comparable接口的，treeset在进行排序的时候就是通过比较它们的comparable接口的实现！ 

下面是hashset的无序和treeset的有序的比较： 

test类：

import java.util.hashset; 
import java.util.iterator; 
import java.util.set; 
import java.util.treeset; 
 
public class test { 
 
  public static void main(string args[]) { 
     
    usehashset(); 
    usetreeset(); 
     
  } 
 
  /** 
   * set默认是无序的，一般从set里面拿出来的数据每次的顺序都会是不一样的,如果想里面的顺序一样 
   * 就使用treeset 
   */ 
  public static void usehashset() { 
    system.out.println("-----------------hashset start------------------"); 
    set<user> set = new hashset<user>(); 
    for (int i = 0; i < 10; i++) { 
      user user = new user((i + 1), "uname" + (i + 1), "pswd" + (i + 1)); 
      set.add(user); 
    } 
    iterator<user> iter = set.iterator(); 
    while (iter.hasnext()) 
      system.out.println(iter.next()); 
    system.out.println("------------------hashset end----------------------"); 
  } 
 
  /** 
   * treeset是有序的，treeset在给里面的元素排序是通过它们的comparable接口的实现来比较的，所以， 
   * 如果里面的对象没有实现comparable接口，则treeset在运行时就会报错，所以如果想从set里面拿出来的数据是 
   * 有序的就得使里面的对象实现comparable接口,user2是实现了comparable接口的，并对它们的id进行比较，id大 
   * 的就会排在后面 
   */ 
  public static void usetreeset() { 
    system.out.println("-----------------treeset start------------------"); 
    set<user2> set = new treeset<user2>(); 
    for (int i = 0; i < 10; i++) { 
      user2 user = new user2((i + 1), "uname" + (i + 1), "pswd" + (i + 1)); 
      set.add(user); 
    } 
    iterator<user2> iter = set.iterator(); 
    while (iter.hasnext()) 
      system.out.println(iter.next()); 
    system.out.println("------------------treeset end----------------------"); 
  } 
 
} 

 user类：

public class user { 
 
  private int id; 
  private string username; 
  private string password; 
 
  public user() { 
  } 
 
  public user(int id, string username, string password) { 
    this.id = id; 
    this.username = username; 
    this.password = password; 
  } 
 
  public int getid() { 
    return id; 
  } 
 
  public void setid(int 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; 
  } 
 
  @override 
  public int hashcode() { 
    final int prime = 31; 
    int result = 1; 
    result = prime * result + id; 
    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 != other.id) 
      return false; 
    return true; 
  } 
 
  @override 
  public string tostring() { 
    return " id = " + id + ", \r\n username = " + username 
        + ", \r\n password = " + password; 
  } 
 
} 

user2类：

public class user2 implements comparable<user2> { 
 
  private int id; 
  private string username; 
  private string password; 
 
  public user2() { 
  } 
 
  public user2(int id, string username, string password) { 
    this.id = id; 
    this.username = username; 
    this.password = password; 
  } 
 
  public int getid() { 
    return id; 
  } 
 
  public void setid(int 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; 
  } 
 
  @override 
  public int hashcode() { 
    final int prime = 31; 
    int result = 1; 
    result = prime * result + id; 
    return result; 
  } 
 
  @override 
  public boolean equals(object obj) { 
    if (this == obj) 
      return true; 
    if (obj == null) 
      return false; 
    if (getclass() != obj.getclass()) 
      return false; 
    user2 other = (user2) obj; 
    if (id != other.id) 
      return false; 
    return true; 
  } 
 
  @override 
  public string tostring() { 
    return " id = " + id + ", \r\n username = " + username 
        + ", \r\n password = " + password; 
  } 
 
  @override 
  public int compareto(user2 user) { 
    // todo auto-generated method stub 
    //这里我的实现是按照id进行排序 
    if (user == null) 
      return 1; 
    if (id > user.getid()) 
      return 1; 
    else if (id == user.getid()) 
      return 0; 
    else  
      return -1; 
  } 
 
} 

运行结果：

-----------------hashset start------------------ 
 id = 3,  
 username = uname3,  
 password = pswd3 
 id = 4,  
 username = uname4,  
 password = pswd4 
 id = 1,  
 username = uname1,  
 password = pswd1 
 id = 2,  
 username = uname2,  
 password = pswd2 
 id = 7,  
 username = uname7,  
 password = pswd7 
 id = 8,  
 username = uname8,  
 password = pswd8 
 id = 5,  
 username = uname5,  
 password = pswd5 
 id = 6,  
 username = uname6,  
 password = pswd6 
 id = 9,  
 username = uname9,  
 password = pswd9 
 id = 10,  
 username = uname10,  
 password = pswd10 
------------------hashset end---------------------- 
-----------------treeset start------------------ 
 id = 1,  
 username = uname1,  
 password = pswd1 
 id = 2,  
 username = uname2,  
 password = pswd2 
 id = 3,  
 username = uname3,  
 password = pswd3 
 id = 4,  
 username = uname4,  
 password = pswd4 
 id = 5,  
 username = uname5,  
 password = pswd5 
 id = 6,  
 username = uname6,  
 password = pswd6 
 id = 7,  
 username = uname7,  
 password = pswd7 
 id = 8,  
 username = uname8,  
 password = pswd8 
 id = 9,  
 username = uname9,  
 password = pswd9 
 id = 10,  
 username = uname10,  
 password = pswd10 
------------------treeset end---------------------- 

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