从Set里面取出有序的记录

Set里面的记录是无序的。假设想使用Set，然后又想里面的记录是有序的，就能够使用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較它们的Comparable接口的实现。

 

以下是HashSet的无序和TreeSet的有序的比較：

 

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,假设想里面的顺序一样
     * 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接口的实现来比較的，所以，
     * 假设里面的对象没有实现Comparable接口，则TreeSet在执行时就会报错，所以假设想从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进行比較。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
        //这里我的实现是依照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;
    }

}

 

执行结果：

-----------------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----------------------