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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原文链接：http://blog.csdn.net/elim168/article/details/40586219